Use of Plant Growth Regulators for Sustainable Management of Vegetation in Highway

Abstract

Plant growth regulators (PGRs) are natural or synthetic substances that control and manipulate plant physiological processes, controlling branching and vegetative growth. Maintaining roadside vegetation through frequent mowing is costly, dangerous, and unsustainable. This narrative literature review proposes a revolution in this management by conducting a systematic literature review on the strategic application of PGRs on roadsides. Practices such as the application of plant growth regulators, the use of native cover crops, and bioengineering techniques with stabilizing species were analyzed. Previous studies have shown that the use of regulators such as mepiquat chloride and paclobutrazol reduces plant height and aboveground biomass, favoring growth control and compacting the plant architecture. The environmental and operational impacts related to vegetation control on roadside strips were also considered. Integrated with LiDAR technology for precise monitoring, this model establishes a new paradigm: smart, safe, and sustainable. Therefore, it is hoped that this compendium will fill a gap in national guidelines by offering an evidence-based protocol guideline for the use of PGR as an alternative to traditional management methods, thus reducing the number of mowing and weeding operations in highway right-of-way areas.

1. Introduction

Roadside vegetation plays a key role in environmental conservation, contributing to slope stabilization, reducing erosion, and maintaining biodiversity. According to the Highway Vegetation Manual [1], perennial grasses are recommended for their dense, superficial root systems, which contribute to slope stabilization and erosion control, while low-lying legumes can be used to enrich the soil with nitrogen and expand green cover.

The types of existing species and management practices have a direct influence on road safety. A study by the National Council of Urban Development [2] showed that stretches with tall vegetation obstructing the visibility of road signs cause accidents with fatality rates up to 60.8% higher than in areas with full visibility, highlighting the importance of vegetation control. This clearly demonstrates that tall grasses along highways, by hindering the visibility of road signs and combined with excessive speed, are directly associated with an increase in serious accidents, sometimes resulting in fatalities. Studies show that the presence of excessive vegetation, among safety and signaling devices, reduces drivers’ reaction time and impairs their perception of road conditions, contributing to collisions, departure from the route, and run-overs.

On slopes made of unstable materials, vegetation helps prevent landslides and erosion, especially during rainy periods. Among the solutions, root systems mechanically reinforce the soil, increasing its cohesion, contributing to stress dissipation, and controlling runoff [3]. According to Kang et al. [4], the density of very fine roots (diameter < 0.25 mm) can continuously improve mechanical stability, while coarser roots (diameter > 1 mm) can even have adverse effects after a certain limit. Roots in each diameter range improve the water stability of aggregates, but the correlation between root density and stability indices decreases as diameter increases.

Intensive cutting practices in vegetated areas of highways, although common, increase energy consumption, carbon emissions, and logistical costs [5]. The selection of moderate-growth species, combined with the judicious use of plant growth regulators (PGRs) and slow-release nutrient technologies, can balance ecological functionality and operational viability [4]. Such practices, may be incorporated into the guidelines of the National Department of Transport Infrastructure (DNIT).

A chemical alternative to mechanical management is the use of Plant Growth Regulators (PGRs). Classical PGRs, such as paclobutrazol, are synthetic compounds designed specifically to modulate plant physiology without causing plant death. Furthermore, a distinct strategy involves the application of herbicides, such as glyphosate and 2,4-D, at sub-lethal doses, exploiting their growth-inhibiting hormetic effects for vegetation control [6,7]. The effect can vary depending on the species and plant and the dosage applied, with a view to more pronounced growth inhibition [7]. Paclobutrazol, a compound from the triazole group, acts by inhibiting the biosynthesis of gibberellic acid, a plant hormone essential for cell elongation. The result is reduced vegetative growth, shorter internodes, and more compact plant development, representing an ideal solution to the problem in question [8].

Commercial products based on paclobutrazol already have established use in fruit growing, such as in mango management for flower induction and vigor control. However, the application of PGRs to native species or grasses lacks defined protocols. The dose and method of application can vary dramatically between species. Suboptimal application results in ineffectiveness, while overdose can cause phytotoxicity, in addition to representing a waste of resources and an environmental risk [9].

Managing marginal vegetative growth is, therefore, a challenge that involves safety, maintenance, and sustainability. The use of PGRs emerges as a high-potential alternative to control growth, improve plant resilience, and minimize the need for maintenance/conservation interventions, such as frequent cutting and pruning, which is crucial for the sustainability of the system [10]. Furthermore, modern monitoring technologies, such as LiDAR remote sensing, allow for accurate quantification of associated risks, particularly for the emergence and growth of invasive plant species, paving the way for more efficient management strategies based on appropriate data. In this context, this study aims to synthesize and critically analyze the national and international technical-scientific literature on the application of PGR as an alternative to reducing traditional vegetation management (mowing) on highway margins, aiming at the development of a technical-scientific protocol for possible adoption in the Highway Infrastructure Departments of this country. Thus, identifying the main synthetic plant growth regulators (e.g., mepiquat chloride, trinexapac-ethyl) and the potential use of herbicides at sub-lethal doses (e.g., 2,4-D, glyphosate) with proven effect in inhibiting the vegetative growth of cover species is relevant to the Brazilian scenario.

Our aim is to evaluate the effects of PGRs on plant morphophysiological characteristics (height, aboveground biomass, root architecture, stress resistance) and their implications for slope stability and road safety. We also propose future studies and evaluations for PGR selection, respective dosages, application timings, and methods, and post-application monitoring indicators, targeting transportation infrastructure management agencies.

2. Materials and Methods

2.1. Bibliographic Review

The manuscript consists of a systematic literature review. The workflow was guided by the principles of the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) protocol, ensuring transparency and reproducibility [11].

The bibliographic search will be conducted in the following electronic databases, considered the most comprehensive in the fields of agricultural, environmental, and biological sciences: Web of Science (Core Collection), Scopus, SciELO (Scientific Electronic Library Online), and Google Scholar (for capturing the gray literature, such as theses, dissertations, and technical documents).

Search terms (keywords) will be combined using Boolean operators (and, OR) and adapted to the syntax of each database. The strategy will use the following descriptors:

• “plant growth regulator” OR PGR OR “mepiquat chloride” OR glyphosate OR “2,4-D” OR “trinexapac-ethyl”.
• “roadside vegetation” OR “turfgrass management” OR “growth inhibition” OR “vegetation control” OR “cover crops”.
• “slope stability” OR erosion OR “maintenance cost” OR NERp OR “soil bioengineering”.

The search period will be from January 2009 to 2025, to ensure the capture of the most recent and relevant scientific evidence.

Eligibility Criteria (Inclusion and Exclusion): The identified studies will be filtered based on the following criteria: Inclusion Criteria: (i) Original articles, review articles, theses, and dissertations. (ii) Studies evaluating the effect of PGR on the growth, morphophysiological development (aboveground and root parts), or stress resistance of plants with potential application on roadsides. (iii) Studies addressing the economic viability of plant management techniques. (iv) Official documents and technical manuals from regulatory agencies (DNIT).

Exclusion Criteria: (i) Studies focusing exclusively on agricultural crops without extrapolation to the context of perennial vegetation. (ii) Opinion articles or news articles. (iii) Studies not available in full. (iv) Studies in languages that are not easily translated (the search will be limited to Portuguese, English, and Spanish).

2.2. Study Selection and Data Extraction

The selection process will be conducted in two stages by a single evaluator (given the scope of the master’s project): Title and Abstract Screening: Records identified in the searches will be initially screened based on their title and abstract. Full-Text Review:

Studies deemed potentially relevant in the initial screening will have their full text analyzed for final eligibility verification. Data from the selected studies will be extracted and organized into a Microsoft Excel^® spreadsheet containing the following information: (a) Author and year of publication. (b) Study objective. (c) Plant species studied. (d) PGR, dosage, and application method used. (e) Main results (percentage of height and biomass reduction, effects on the root system). (f) Main conclusions. This will be available on the website of the journal where the article will be published. It will also be available in the Google Scholar database.

The results of the systematic review and economic analysis will be summarized in a narrative format. This summary will serve as the basis for developing a technical protocol that integrates scientific evidence into practical guidelines. The protocol will include sections on: selection of appropriate PGRs, recommended dosages, application timings and methods, safety measures, and indicators for monitoring efficacy and post-application environmental impacts.

3. Results

Hormonal control of plant development using the active ingredients “chlormequat chloride”, “mepiquat chloride”, and “paclobutrazol” is rarely reported worldwide. A previous search of the Scopus and Web of Science databases revealed over 2500 publications related to the use of chlormequat chloride, mepiquat chloride, and paclobutrazol. Mepiquat chloride’s use in plants had only four publications, chlormequat chloride had 338 publications, and paclobutrazol had over 2800 publications in the Web of Science database.

Table 1. Title and objectives of 20 evaluated articles.

Authors	Article Title	Objectives	Main Results
DNIT 182/2018-PRO [12]	National Department of Transportation Infrastructure (2018): Maintenance of right-of-way with vegetation cover.	This standard establishes the methodology for determining the level of mowing effort on highways, based on rainfall data, aiming at the standardization of conservation services.	The method calculates the point’s Mowing Effort Level (NERp) based on the rainfall intensity factor (nd) and distances to nearby rainfall stations. The result is an integer value, which can be adjusted by ±1 depending on soil fertility and vegetation type, ensuring flexibility in maintenance service scheduling.
DNIT, [1]	National Department of Transport Infrastructure (2009). Road Vegetation Manual—Volume 1. Road Research Institute.	Its objective is to guide the implementation, recovery and maintenance of vegetation on highways, aiming at erosion control, landscape integration, road safety and environmental compliance, with sustainable techniques and judicious use of resources.	The manual establishes detailed methodologies for revegetation of degraded areas, including planting techniques, recommended species by biome, and maintenance procedures. It also offers appendices with native and exotic tree species, as well as examples of practical applications, such as the use of vetiver grass for erosion control. It serves as a tool for harmonizing road infrastructure with environmental conservation.
Lima et al. [13]	Analysis of the use of vegetation in slope containment.	This article aims to gather technical information on the use of vegetation in slope stabilization, aiming to raise awareness among technicians and academics about its importance in preventing erosion.	Vegetation has proven to be an effective and affordable method for slope containment, increasing soil shear strength through the root system. Vegetation cover reduces erosion, prevents landslides, and improves water infiltration, providing a sustainable and low-cost solution for slope stabilization.
Embrapa, [14]	Irrigation and plant hormones increase coffee productivity and quality, reveals a study with 3D modeling.	The research evaluated how punctual irrigation, applied for six weeks during the critical ripening phase, and the use of plant hormones gibberellic acid and ethylene (in the form of Ethephon) directly impact the physiology, architecture and quality of coffee.	The results show that irrigation, even in the short term, preserves leaf area and maintains high levels of photosynthesis, ensuring greater carbon assimilation and, consequently, more energy for grain filling, explained Miroslava. Without water, plants experience a decline in leaf area, photosynthetic activity, and the number of ripe fruits—the red grains ideal for harvesting.
Dinalli et al. [15]	Nitrogen fertilization and glyphosate as a growth regulator: Effects on the nutritional efficiency and nutrient balance in emerald grass.	The article aims to evaluate the effects of nitrogen and glyphosate doses on the growth, aesthetic quality and nutritional efficiency of emerald grass, aiming to recommend management practices that reduce maintenance.	An annual dose of 15 g m−2 of N combined with 400 g ha−1 of glyphosate reduced growth (leaf area, height, and dry matter) without compromising green color. This combination promoted greater nutritional stability, reducing nutrient export through clipping removal and reducing the need for replenishment via fertilization.
Martins & Silva [16]	Evaluation of yield and boll weight with different doses of the regulator mepiquat chloride in cotton crops.	To evaluate the effect of different doses of the regulator mepiquat chloride on average boll weight and yield in cotton crops.	The application of the regulator did not significantly influence the average boll weight. However, the doses of 750 and 1500 mL ha−1 provided a significant increase in productivity, reaching 483.5 and 470.6 kg ha−1, respectively, due to the better establishment of reproductive structures.
Vieira et al. [17]	Use of 2,4-D as a growth regulator and foliar fertilizer with amino acids in common bean crops.	To evaluate the influence of applying foliar fertilizer with amino acids and the herbicide 2,4-D at sub-dose as a growth regulator in carioca bean crops, aiming at productivity gains.	Amino acid foliar fertilizer increased plant height, pod number, and yield by 19.5%. 2,4-D reduced plant height but did not increase grain yield.
Hatta Antah et al. [18]	A. Perceived Usefulness of Airborne LiDAR Technology in Road Design and Management:	The paper reviews the application and perceived utility of airborne LiDAR technology in sustainable road planning, design and management.	The results demonstrate that LiDAR offers significant advantages in accuracy, efficiency, and safety in complex terrain, surpassing traditional methods. Furthermore, it identifies key factors for its acceptance, such as information quality and management support, and highlights future applications in as-built documentation and road inspection, while also highlighting its current limitations.
Carnot et al. [19]	Enhancing Roadway Safety: LiDAR-based Tree Clearance Analysis	Develop an automatic algorithm based on LiDAR point clouds to identify vegetation that invades the regulatory free height above public roads.	The proposed system demonstrated effectiveness in detecting invasive vegetation using semantic segmentation and a novel 2D-to-3D contouring algorithm. Parameters such as concatenation step, neighborhood radius, and angular threshold were optimized to balance accuracy and runtime. The identified points were projected onto images, assisting urban managers in preventive road safety maintenance.
Comesana-Cebral et al. [20]	Transport Infrastructure Management Based on LiDAR Synthetic Data: A Deep Learning Approach with a Road Sense Simulator.	Develop a 3D simulator (Road Sense) to generate synthetic and labeled point clouds to train deep learning models in road and forest scenarios.	Road Sense generated synthetic data that, when trained on the PointNet++ model, achieved MIoU of up to 92.8% in forests and 71.3% on roads. The results were comparable to those of the HELIOS++ simulator, demonstrating that synthetic data can replace real measurements with high accuracy, reducing costs and time in road infrastructure management.
Glab et al. [21]	Response of Kentucky bluegrass Turfgrass to Plant Growth Regulators	To evaluate the effect of six plant growth regulators, applied at five different doses, on the visual quality and color characteristics of Poa pratensis L. (Kentucky bluegrass) cultivars, using spectrophotometric methods and visual evaluation.	Paclobutrazol (PBZ) improved the color evaluation of Poa pratensis L. grass, but decreased its overall appearance. PBZ treatment resulted in darker leaves with a lower green and reddish hue. Notably, the browning effect was most pronounced at the R5 rate (0.96 kg PBZ ha−1). Furthermore, high PBZ rates decreased the overall appearance of the grass and reduced the ∆b parameter to −0.82 at the R5 rate.
Santos Filho et al. [22]	Paclobutrazol reduces growth and increases chlorophyll indices and gas exchanges of basil (Ocimum basilicum var. Cinnamon)	To evaluate the effect of paclobutrazol on the growth regulation and gas exchange of basil (Ocimum basilicum var. Cinnamon), aiming at its ornamental use.	Paclobutrazol (PBZ) significantly reduced the growth of basil (Ocimum basilicum var. Cinnamon) plants, with the 10 mg L−1 dose resulting in the lowest height (28.14 cm). PBZ increased chlorophyll indices (a, b, and total), stomatal conductance, net photosynthesis, and instantaneous water use efficiency. The 5 mg L−1 PBZ dose was considered optimal for growth regulation.
Fazeli et al. [23]	Effect of paclobutrazol on the growth characteristics of two turfgrasses (Festuca aranudinaceae Scherb.) and (Poa pratensis L.)	To evaluate the effect of different concentrations of paclobutrazol on the growth control, density and visual characteristics of two grass species (Poa pratensis L. and Festuca arundinacea Schreb.), aiming to reduce the frequency of cutting and maintenance costs.	Paclobutrazol (PBZ) significantly reduced the height of Poa pratensis and Festuca arundinacea Schreb. plants, with 45 mg L−1 being the most effective concentration. PBZ increased plant density, notably in F. arundinacea, and the chlorophyll index in both species at a concentration of 30 mg L−1. Furthermore, PBZ decreased leaf blade length and width and fresh weight of cuttings, with an optimal effect lasting up to 30 days after treatment.
Desta & Amare [8]	Paclobutrazol as a plant growth regulator	To review the current knowledge about paclobutrazol (PBZ) as a plant growth regulator, its role in protecting against abiotic stresses and its effects on crop physiology, productivity and quality.	Paclobutrazol (PBZ) acts as a growth regulator, altering the levels of gibberellins (GAs), abscisic acid (ABA), and cytokinins, which inhibits GA synthesis and reduces stem elongation. It is more effective through soil application than foliar application, as it provides longer duration and absorption of the active ingredient. It reduces plant height, prevents lodging, and increases fruit number, weight, and quality (higher carbohydrates, TTS, and lower acidity). It also reduces evapotranspiration, increases resistance to biotic/abiotic stresses, and acts as a fungicide.
Khaleel & Ahmed [24]	Genetic Potential Of Bermuda Grass (Cynodon dactylon L.) In Response To Foliar Application Of Organic Fertilizer (Libro) And Paclobutrazol	To evaluate the effect of foliar application of organic fertilizer (Libro) and paclobutrazol on vegetative growth, phenotypic and chemical characteristics of Bermuda grass (Cynodon dactylon L.), aiming to reduce the frequency of cutting and improve lawn quality.	The application of Paclobutrazol (PBZ) at 0.05 g L−1 decreased the internode length, reaching 5840 mm, and the number of cuts in Bermuda grass. Contradictorily, PBZ at 0.05 g L−1 increased the average number of branches per plant (19,377 branches plant−1), total chlorophyll content (29,988 mg L−1) and the percentage of carbohydrates in the shoots (15,467%). In addition, it reduced the degree of acceptability of quality and homogeneity to 2000.
Görüm, T. [25]	Landslide recognition and mapping in a mixed forest environment from airborne LiDAR data.	This article evaluates the use of LiDAR data to map landslides in forested areas, comparing it with traditional photointerpretation methods. The results show that LiDAR identifies significantly more landslides, especially smaller ones and those under dense vegetation.	The study demonstrated the superiority of LiDAR in identifying landslides in areas of dense forest cover. The LiDAR-based inventory revealed 902 landslides, compared with only 67 in the traditional photointerpretation inventory (PII). The mapping error index (E = 0.55) and the degree of correspondence (M = 0.45) highlighted significant discrepancies between the methods. LiDAR enabled the detection of landslides with areas of at least 100 m2, while the PII required slope height differences > 25 m and forest cover < 60% for recognition. The results highlight the need for LiDAR data for complete and reliable landslide susceptibility inventories.
Lindsay & Dhun [26]	Modelling surface drainage patterns in altered landscapes using LiDAR.	This paper presents a new DEM preprocessing algorithm to remove artificial dams created by infrastructure at embankment underpass locations, as well as to enforce flow along drainage ditches.	It demonstrated that the least-cost violation method used by the algorithm could reliably enforce drainage paths while minimizing the impact on the original DEM.
Saito et al. [27]	Study of automatic forest road design model considering shallow landslides with lidar data of funyu experimental forest.	Develop an automatic forest road design model that avoids areas at risk of shallow landslides, using LiDAR data to minimize earthwork costs and increase safety.	They developed an automatic forest road design model that considers shallow landslide risks using LiDAR. The model minimizes earthwork costs and avoids risk areas, with estimated earthwork volumes close to actual ones (14,162 m3 vs. 13,487 m3). The program proved effective in planning low-volume roads with reduced environmental impact.
Roelens et al. [28]	Drainage ditch extraction from airborne LiDAR point clouds.	This paper proposes an automated method to extract drainage ditches from LiDAR point clouds, combining geometric and radiometric features with a Random Forest classifier, aiming to accurately map artificial hydrographic networks.	They proposed a method to extract drainage ditches from LiDAR point clouds using Random Forest classification. They achieved high accuracy (kappa = 0.77 in a pasture area and 0.73 in a peri-urban area). Geometric features were the most influential, and geometric reconstruction of dropouts improved detection, with omission and commission errors below 0.15 for ditch centerlines.
Abdulkareem & Abdulrahman [29]	Influence of Shading and Paclobutrazol Concentrations on Growth and Quality Characters of Three Different Turf Grasses Genera.	To investigate the effect of shading levels and paclobutrazol concentrations on the growth and quality of three grass genera, aiming to identify ideal conditions for commercial cultivation in regions with variable climates.	Zero-shade conditions significantly increased the number of cuts, cumulative dry weight of the cut, root dry weight, total chlorophyll content, and carbohydrate percentage. Foliar spraying with PBZ influenced these characteristics; without PBZ, plant density and cumulative dry weight of the cut were higher. With 1500 and 750 mg L−1 of PBZ, there were improvements in color, coverage, total chlorophyll, and carbohydrates. The Festuca genus demonstrated overall superiority.

Source: Prepared by the author (2025).

presents the main information from the 20 studies selected through database searches. Of these, two were from DNIT, one from Embrapa, and 17 other published articles, seven in Portuguese and 10 in English. Articles from the last 16 years (2009–2025) that addressed the proposed topic were included.

4. Discussion

4.1. Regulatory Context and Challenges of Vegetation Management on Highways

The operational cost of vegetation maintenance on highways is directly influenced by the Mowing Effort Level (NERp), as established by DNIT standard 182/2018-PRO [12]. This index quantifies the annual frequency of mowing required on a road stretch, calculated based on the weighted average of the nearest rainfall stations, considering rainfall intensity and geographic distances. High NERp values as observed at the Cunani-AP station (NER = 12) indicate greater resource demands, reflecting the logistical costs, fuel consumption, and CO₂ emissions associated with frequent interventions.

Vegetation control on highway edges is a fundamental aspect of road safety and infrastructure preservation, especially in areas where mechanical mowing is impractical and manual weed control (weed removal) becomes costly. In these cases, the use of herbicides, whether selective, soil sterilizing, or growth inhibitors, is a viable alternative, provided that essential factors such as safety, efficacy, and economic viability are observed. It is recommended that the vegetation cover not exceed 30 cm in height around devices such as signs, guardrails, and posts, maintaining a control strip between 0.60 m and 1.00 m. The timing of application, the type of herbicide, and climatic conditions, such as temperature and rainfall, are crucial for the success of the operation [1].

Furthermore, rigorous care must be taken when preparing the solution, cleaning it, and disposing of the residue, avoiding contamination and harm to fauna, flora, and road users. The entire process must be conducted by specialized companies with proven experience and under strict supervision, ensuring that chemical management is carried out safely, efficiently, and environmentally responsibly [1].

Given this complex regulatory landscape and the high operational costs associated with a high NERp, the adoption of innovative strategies that transcend conventional chemical management with herbicides becomes urgent. The judicious application of Plant Growth Regulators (PGRs) in sub-doses emerges as a technical and sustainable alternative, with the potential to modify vegetation growth habits, inducing a more compact size and reducing the need for frequent interventions. By acting as physiological modulators rather than lethal agents, PGRs can offer a long-term solution for controlling aboveground biomass, aligning with DNIT’s stringent operational and environmental safety requirements, while directly addressing the main economic challenge: reducing NERp and its associated financial burden.

4.2. Composition and Zoning of Roadside Vegetation

Highway rights-of-way support a mosaic of plant communities, whose composition is influenced by the intensity of disturbance and, crucially, by the regional biome. As outlined in the Road Vegetation Manual [1], the vegetation is strategically zoned. The area immediately adjacent to the pavement is typically dominated by low-growing herbaceous species, including stress-tolerant grasses and forbs, which withstand high disturbance and provide initial soil cover. Progressing away from the road, a transitional zone emerges, characterized by a greater diversity of life forms, which may include a mix of grasses, legumes, and shrubs. The outermost zone, furthest from the traffic influence, is designed to resemble the natural landscape [30], featuring a composition of native perennials, shrubs, and trees specific to the region (e.g., Cerrado, Atlantic Forest, or Caatinga in Brazil).

In the context of the Brazilian Cerrado, where soils are often acidic and of low fertility, the use of a diverse mix of cover species becomes even more critical. Legumes play a key role in enriching the soil with nitrogen, while a variety of native grasses and forbs contribute to a dense root mat. This intercropping, or use of species mixes, combines plants with contrasting characteristics to maximize ecosystem benefits [31,32]. It improves soil physical, chemical, and biological properties, controls erosion, and enhances the soil’s capacity to retain water [13,33]. The root systems from this diverse plant community create a dense network that mechanically reinforces the soil, increasing its shear strength and dissipating stress, which is fundamental for slope stabilization [3,13].

4.3. Plant Growth Regulators (PGRs): Mechanisms and Applications

Plant growth regulators act as fundamental tools in modulating crop development, influencing physiological and morphological processes through hormonal mechanisms [34,35]. The use of plant growth regulators should consider not only the reduction in aboveground biomass but also the impacts on root development, especially when applied to road slopes where soil stability depends directly on root anchorage. Below, we will discuss the efficacy of two distinct approaches: the application of dedicated synthetic PGRs (e.g., mepiquat chloride and paclobutrazol) and the use of herbicides at sub-lethal doses (e.g., glyphosate and 2,4-D).

In this context, a study by Baio et al. [36] demonstrated that variable rate application (VRT) of the growth regulator mepiquat chloride, via an electronic flow controller, reduced cotton plant height variability by up to 50%, uniformizing the vegetative canopy. The method was based on vegetation index readings (Red Edge) to define application zones, with doses ranging from 0 to 60 mL ha⁻¹ of the commercial product, applied according to the daily growth rate of the plants. This approach demonstrated that it is possible to precisely control growth even using lower-cost equipment, as long as the dose variation does not exceed 20% in relation to the average dose, ensuring spray quality.

The judicious use of plant growth regulators, combined with sustainable agricultural practices such as controlled irrigation, can increase productivity without negatively impacting the environment. An Embrapa study on Arabica coffee cultivation demonstrated that the strategic application of plant hormones, such as gibberellic acid and ethephon, combined with appropriate water management, resulted in a significant improvement in bean quality and productivity, with no evidence of environmental harm. These results reinforce that the technical and informed management of regulators can be an effective tool for balancing production performance and sustainability [14].

4.4. Glyphosate

In a strategy distinct from the use of dedicated PGRs, glyphosate, widely recognized as a broad-spectrum herbicide, has been investigated for its growth-regulating potential when applied at sub-doses (hormetic effect). Studies demonstrate that doses between 200–600 g a.i. ha⁻¹ inhibits the development of turfgrasses such as Zoysia japonica Steud. without compromising their vitality or aesthetic quality. As demonstrated by Dinalli et al. [15], applications of 400 g a.i. ha⁻¹ significantly reduces leaf area (18.3%), plant height (14.7%), and total dry biomass (8.1%) by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), crucial for the synthesis of aromatic amino acids and plant hormones. This inhibition modulates nitrogen metabolism and auxin production, limiting cell elongation and mitotic division. Efficiency is optimized when associated with fractional nitrogen fertilization (15 g N m⁻² year⁻¹), which maintains nutritional balance and reduces nutrient export via cuts, minimizing maintenance costs in street lawns.

Complementarily, in another example of dicotyledonous systems such as soybean (Glycine max L.), glyphosate at sub-doses (0.5 mM ≈ 84 g a.i. ha⁻¹) demonstrated selective inhibition of vertical growth, with reductions of up to 99.1% in epicotyl elongation under light conditions [37]. This effect was partially reversed by the combined application of gibberellic acid (GA), which restored 93% of hypocotyl elongation and mitigated pigmentation losses (chlorophyll and anthocyanins), without compromising the basal regulatory action of the herbicide. The light-dependent modulation of phenylalanine ammonia-lyase (PAL) activity—a key enzyme in the synthesis of defense compounds—under glyphosate + GA treatment reinforces the importance of environmental conditions in the metabolic efficacy of growth regulators, suggesting that roadside applications should consider factors such as irradiance and circadian rhythm to optimize control of vertical development.

The regulatory effects of glyphosate at sub-dose doses transcend merely morphogenetic control, manifesting as crucial modulators of hormonal homeostasis and plant-environment interactions. As demonstrated by Fuchs et al. [38], glyphosate-based herbicide (GBH) residues in soil differentially alter hormonal profiles in non-target species, with cascading consequences for plant performance and herbivore defense. In oats (Avena sativa L.), the reduction in auxins (IAA, PAA) and benzoic acid (precursor of salicylic acid) via inhibition of the shikimate pathway resulted in reduced herbivore damage, suggesting an indirect reinforcement of defenses mediated by secondary metabolites (e.g., condensed tannins). Also in potatoes (Solanum tuberosum L.), the increase in stress hormones (ABA, JA) and aboveground biomass under GBH + phosphate revealed an adaptive response that balances resource allocation and defensive signaling. In strawberries (Fragaria × ananassa Duchesne.), sensitivity to hormonal changes was more discreet, reinforcing the species-dependent specificity of these effects.

In summary, the strategic use of glyphosate sub-doses emerges not only as a tool for growth control but as a systemic modulator that integrates plant development, biochemical defense, and ecological resilience, provided it is applied with agronomic precision and continuous monitoring of its impacts on the agroecosystem.

4.5. Considerations on the Risks of Glyphosate at Herbicidal Doses

While this study proposes the use of sub-dose glyphosate for sustainable road vegetation management, it is crucial to acknowledge the well-documented risks associated with its application at conventional herbicidal doses. Environmental contamination by glyphosate and its primary metabolite, aminomethylphosphonic acid (AMPA), constitutes a substantial ecotoxicological risk, given their ubiquitous detection in soil, surface water, and sediments [39]. Extensive application can lead to significant disturbances in the biota, manifested by the disruption of soil microbiota and the suppression of crucial enzymatic activities. In aquatic environments, the presence of residues, even at low concentrations, can induce neurotoxic effects in fish and cause malformations and oxidative stress in amphibians, with indirect impacts on biodiversity and ecosystem functions [39,40].

From a public health perspective, chronic human exposure, often associated with occupational settings or the consumption of contaminated water and food, raises critical concerns. The International Agency for Research on Cancer (IARC) has classified glyphosate as probably carcinogenic to humans (Group 2A), based on strong evidence for these mechanisms [40]. Studies on occupationally exposed populations report redox imbalance, evidenced by reduced activity of antioxidant enzymes and increased markers of lipid peroxidation. Furthermore, chronic exposure is implicated in systemic effects affecting the endocrine, immune, and neurological systems, in addition to potentially contributing to renal and hepatic dysfunctions [40,41].

Beyond ecotoxicological and public health concerns, the intensive and repeated use of glyphosate at herbicidal doses carries a significant agronomic risk: the selection of resistant weed biotypes [42]. The selection pressure exerted by this widespread herbicide has led to the documented emergence of glyphosate-resistant species globally, a number that continues to grow [43,44]. The development of resistance compromises the long-term efficacy of this tool for vegetation control, potentially leading to management failures [44] and increased costs for roadside maintenance.

It is crucial to emphasize that the proposed strategy in this study utilizes sub-lethal doses for growth regulation, a context fundamentally distinct from conventional weed control [45]. This approach, by applying significantly lower concentrations, aims to induce a hormetic response for growth suppression without causing plant death [46]. While the conventional use of high doses strongly selects resistance-conferring mutations, the inherently weaker selective pressure of sub-doses may not favor the same rapid evolution of resistance [44,46]. Furthermore, the target species for this application are often turfgrasses and cover crops, not the dicotyledonous weeds typically associated with resistance reports.

Therefore, integrating the use of PGRs, including sub-lethal glyphosate, into a broader Integrated Vegetation Management framework, which combines chemical, mechanical, and biological control methods, is essential to ensure sustainability and resilience of the proposed protocol.

Therefore, the strategy of using sub-dose glyphosate for growth regulation in this context is strictly founded on the hormesis principle, employing doses significantly lower than those used for herbicidal purposes. This approach aims to exploit beneficial regulatory effects, while the mitigation of the risks described herein is achieved by drastically reducing the total chemical load introduced into the environment, coupled with localized and monitored applications. This distinction is crucial for the acceptability and sustainability of the proposed protocol.

4.6. Paclobutrazol

Paclobutrazol (PBZ) is a plant growth regulator (PGR) widely used in agriculture, characterized by its high efficiency and low toxicity. It is a triazole derivative. Its primary action is to inhibit the biosynthesis of gibberellins (GA), hormones crucial for cell elongation and stem growth, acting as an inhibitor of cell elongation and internode extension [47]. Concomitantly, PBZ alters hormonal balance, promoting the synthesis of abscisic acid (ABA) and cytokinins (CTK), which results in the inhibition of shoot elongation and, in many cases, stimulation of root growth. Additionally, triazoles such as PBZ have systemic fungicidal activity, inhibiting sterol biosynthesis and conferring protection against several economically important fungal diseases [8]. PBZ can be applied via foliar spray or soil drench, the latter being generally more effective due to its absorption by roots and translocation predominantly via the xylem to the growing points [22]. Drench application allows for more uniform and long-lasting control of plant height with lower doses, since roots are sites of significant GA synthesis, where PBZ can act directly. PBZ persists in soil, with a half-life ranging from 6 to 12 months, depending on soil characteristics and environmental conditions [8]. Optimal PBZ concentrations vary considerably between species and objectives. In floriculture studies, typical soil drench application rates can range from 1 to 90 mg L⁻¹. Although low to moderate concentrations may be beneficial, the use of excessive doses is detrimental to root development and yield, and may lead to undesirable effects such as restricted growth and deformities [47].

The efficacy of PBZ is modulated by several environmental variables and chemical interactions. Under shaded conditions (75%), a reduction in root dry weight is observed compared to cultivation in full sun (0% shade), where PBZ can optimize root development [29]. PBZ also increases plant tolerance to abiotic stresses, such as drought, high and low temperatures, by increasing antioxidants, chlorophyll, and activating antioxidant enzymes. Soil type and irrigation system influence PBZ activity; for example, in bark-based media, adsorption can reduce the chemical’s availability to the plant [8]. Furthermore, PBZ interacts synergistically with other compounds; the application of organic fertilizers, such as Libro, together with PBZ in Cynodon dactylon L. (Bermuda grass), has been shown to increase the percentage of chlorophyll and carbohydrates, suggesting an optimization of plant metabolism [24].

In turfgrasses, PBZ is used to control vertical growth, resulting in a reduction in plant height and, consequently, a decrease in the number of cuts and the cumulative dry weight of clippings (clipping yield) [23]. Specifically, in Bermudagrass (Cynodon dactylon L.), the application of PBZ at a concentration of 0.05 g L⁻¹ reduced the internode length to approximately 5840 mm, compared to 9253 mm in the control, representing a reduction of approximately 36.9%. Additionally, the number of cuts was reduced from 4167 to 2000 with this same concentration, which corresponds to a decrease of approximately 52.09% [24]. PBZ also improves turfgrass visual quality by increasing plant density and chlorophyll concentration, which confers a darker color and delays leaf senescence, contributing to the “stay-green” effect [21]. In Festuca arundinacea Schreb. and Poa pratensis L., 45 mg L⁻¹ of PBZ was the most effective concentration in reducing height. These characteristics make PBZ a valuable tool for turfgrass maintenance, especially in low-input systems, by reducing operating costs [23].

In summary, paclobutrazol (PBZ) has established itself as a promising tool for the sustainable management of roadside vegetation, acting as an effective modulator of vegetative growth by inhibiting gibberellin biosynthesis and stimulating root development. Its soil application (drench) demonstrates superior uniformity and persistent height control, with residual effects that can last up to 12 months, significantly reducing mowing frequency and associated operating costs. In the context of roadside turf, concentrations of around 0.05 g L⁻¹ have been shown to reduce vertical growth by up to 36.9% and reduce the need for mowing by more than 50%, in addition to promoting improvements in visual quality and stress resistance. However, the effectiveness of PBZ depends on environmental factors, such as irradiance and soil type, and on dosage accuracy, as excessive concentrations can induce phytotoxicity and compromise root development, a critical aspect for slope stability. Therefore, incorporating PBZ into vegetation management protocols requires validation under specific soil and climate conditions and continuous monitoring to maximize its operational and ecological benefits in the context of Brazilian road infrastructure.

4.7. Mepiquat Chloride

Mepiquat chloride acts as an inhibitor of the biosynthesis of gibberellins, key hormones in cell elongation. By suppressing the activity of ent-kaurene oxidase (the rate-limiting enzyme in the gibberellin pathway), it drastically reduces the elongation of internodes and lateral branches, promoting a compact architectural profile [16].

The compactness induced by the regulator enables more efficient agricultural operations: (i) a 30–50% reduction in plant height improves insecticide/fungicide penetration into the canopy, reducing failures in the control of pests such as Anthonomus grandis; (ii) the uniformity of architecture minimizes mechanical losses during harvest, especially in tall varieties; and (iii) the greater light incidence in the basal third reduces boll abscission. In irrigated crops in the Cerrado, doses of 750 mL ha⁻¹ are considered optimal for balancing height control and maintaining productivity [16].

Under stress conditions, doses of 250 ppm of Mepiquat Chloride (MC) demonstrated efficacy in compacting plant architecture (23% reduction in corn height) and enhancing environmental tolerance, with an increase in proline (+358%) and K/Na ratio (+34%) [48]. These responses suggest potential for minimizing functional losses in roadside vegetation exposed to degraded soils or seasonal droughts. However, the lack of root assessment in this study reinforces the need for specific investigations into slope-stabilizing species.

Despite the operational benefits, the application of Plant Growth Regulators (PGRs) in roadside right-of-way management must be weighed against evidence of their potential adverse effects. Toxicological studies indicate that several PGRs, such as chlormequat chloride, paclobutrazol (PBZ), and ethephon, can act as endocrine-disrupting chemicals, interfering with the synthesis and secretion of sex hormones in mammals [49]. These same authors highlight that some PGRs, like chlormequat chloride, have been shown to cause adverse effects on the reproductive capacity of animals at doses below the Acceptable Daily Intake, indicating a need for caution in defining safe exposure levels. Therefore, the adoption of PGRs in roadside management requires a rigorous risk assessment, considering not only their efficacy for growth control but also their potential toxicity to non-target organisms and adjacent ecosystems.

4.8. 2,4-D

Similarly to glyphosate, the herbicide 2,4-D can be used as a growth-suppressing agent when applied at sub-lethal doses. Applications of 2,4-D at 10 mL ha⁻¹ demonstrate efficacy as a selective plant growth reducer, reducing bean plant height by 6.5% (84.06 cm vs. 89.86 cm for the control) by inducing oxidative stress and ethylene synthesis via the action of synthetic auxin [17]. Critically, its application in pre-flowering (reproductive stage) has proven to be strategic for architectural modulation without compromising key development phases, a principle adaptable to highway vegetation management, where interventions at the peak of vegetative growth can maximize plant height control. In contrast to regulators such as mepiquat chloride (which acts to inhibit gibberellins), 2,4-D operates through hormonal imbalance, promoting distinct morphological effects: while the former compacts the plant via suppression of cell elongation, 2,4-D induces apical decapitation and stimulation of lateral branches, making it more suitable for species with a shrubby habit.

Experience with growth regulators in road management reinforces the critical importance of dosage in biotechnological systems. Just as glyphosate at sub-doses (200–600 g a.i. ha⁻¹) modulates grass growth without compromising vital functions [15], the balance between 2,4-D (3.0 mg L⁻¹) and BAP (0.1 mg L⁻¹) in Crinum americanum L. (Amaryllidaceae) cultivation demonstrates that precise concentrations are crucial for the proliferation of friable calluses [50]. This synergy between controlled suppression and metabolic stimulation, observed both at the field scale and in vitro, highlights that hormonal modulation transcends applied contexts. The side effects of regulators on root systems, such as WRKY29 inhibition in Arabidopsis thaliana (L.) Heynh. [51], highlight the risks involved in biotechnological processes that depend on structural integrity. In the case of Crinum americanum L. (Amaryllidaceae), where roots and embryos are the primary sources of explants, the choice of PGR should prioritize compounds with selective aerial action (e.g., sub-dose 2,4-D) to avoid compromising organogenesis and metabolite synthesis [50]. Furthermore, the efficacy of Phytagel as a vehicle (superior to agar in delivering nutrients and regulators) echoes previous findings on the influence of adjuvants on PGR bioavailability. This convergence suggests that culture media formulations that include synergies between solidifying agents and chemical carriers deserve further investigation to maximize the production of pharmacological compounds in cell cultures.

4.9. Implications of PGRs on Root Architecture

A critical appraisal of the literature on PGRs reveals that the majority of efficacy and dosage data are derived from agricultural or horticultural systems, such as cotton, coffee, and basil cultivation. While these studies provide invaluable insights into the physiological mechanisms and growth suppression potential of these compounds, their direct applicability to roadside ecosystems is not straightforward. The management objectives, target species, and environmental conditions in highway rights-of-way, which prioritize erosion control, slope stabilization via root reinforcement, and low-input maintenance, differ fundamentally from those in production agriculture, which focus on maximizing yield and quality. This review has therefore incorporated supplementary evidence from turfgrass studies, which, while a closer analogue, still often represent managed systems rather than native roadside vegetation. This acknowledged limitation underscores the pioneering nature of proposing PGRs for large-scale road infrastructure management.

The discussion thus far has drawn upon studies from agricultural and horticultural systems to infer the potential efficacy of PGRs for roadside management. While this provides a foundational understanding of their physiological mechanisms, a critical appraisal of their applicability to roadside ecosystems is necessary. The primary objectives in agriculture, such as maximizing yield and fruit quality [52,53], differ fundamentally from the goals of roadside vegetation management, which prioritize erosion control [54], slope stabilization via root reinforcement [55], and the minimization of aboveground biomass for safety, all within a low-input regime. This distinction becomes particularly evident when examining the underexplored impact of PGRs on root systems, a factor crucial for slope stability but often secondary in agricultural contexts.

A comparative assessment of synthetic PGRs in grasses and related species reveals a complex, species-dependent effect on root architecture [56]. For instance, in warm-season grasses like “Fine Dacca”, paclobutrazol (PBZ) at 0.04% induced a pronounced 54% reduction in root dry weight, a suppression even greater than that observed in the shoot (45%) [57] Conversely, in a study on Salvia officinalis L. “Icterina”, PBZ reduced shoot dry weight by 69% while only reducing root biomass by 25%, leading to a significant decrease in the shoot-to-root ratio (from 2.52 to 1.02) [56]. This shift in biomass partitioning, favoring root retention, suggests a potential morphological advantage for stress tolerance. In contrast, mepiquat chloride demonstrated a distinct profile in the same study, reducing shoot biomass by 25% without significantly altering root dry weight, resulting in a more moderate shift in the shoot-to-root ratio [56]. Furthermore, mepiquat chloride has been reported to promote root development and lateral root formation in some crops. These findings underscore that PGRs can significantly alter root density and architecture.

For roadside management, these differential effects carry major implications. A PGR like MC, which can suppress vertical growth [58] while maintaining or even stimulating root development, may be ideal for slope stabilization, as a robust root system is essential for soil cohesion [57]. However, a compound like PBZ, which can severely restrict root biomass in certain species, might compromise the very root network critical for preventing erosion, despite its superior aerial growth control. Therefore, the selection of a PGR for highway applications must transcend the goal of simply reducing mowing frequency [56]. It requires a species-specific evaluation of its impact on the root system to ensure that the ecological function of vegetation in slope stabilization is not inadvertently undermined [59]. Future research must prioritize field studies on native or common roadside grass species to directly quantify these trade-offs between growth inhibition, root architecture, and soil stability.

The critical importance of root systems for slope stabilization underscores the need for PGR selection that does not compromise below-ground biomass and architecture. The efficacy of PGRs must be evaluated against key erosion-control species used in Brazilian roadside management. Grasses are widely utilized for revesting declives and protecting against erosion [13], with species like Vetiver (Chrysopogon zizanioides) being fundamental due to its deep, robust root system that provides superior soil shear strength reinforcement [60]. Legumes, such as forage peanut (Arachis pintoi) and jack bean (Canavalia ensiformes), contribute through rapid ground cover and nitrogen fixation, which improves soil structure and aggregate stability [61]. The practice of using species mixes is also highly effective, as demonstrated by a consortium of oat, common vetch, and forage radish, which achieved complete soil coverage and complementary nutrient accumulation [62]. Therefore, future research on PGRs for roadside vegetation must prioritize these and other locally relevant species to ensure that growth regulation strategies do not inadvertently undermine the root development that is fundamental to their anti-erosion function and overall ecological integrity.

4.10. Monitoring and Diagnostic Technologies for Precise Management of Road Vegetation

In addition to the use of PGR, remote sensing technologies such as LiDAR (Light Detection and Ranging) have been employed for precise monitoring of road vegetation. As reviewed by Hatta Antah et al. [18], LiDAR enables the generation of three-dimensional vegetation models, identification of critical areas, and evaluation of the effectiveness of management interventions. This technology can be integrated with the use of PGR to create “prescription maps” that guide localized applications, maximizing operational efficiency and reducing the Level of Mowing Effort (NERp).

Yu et al. [63] aimed to provide references to existing work in 3D road modeling based on LiDAR point clouds, critically discuss them, and present challenges for future studies. The review helps researchers improve existing approaches and develop new techniques for road modeling based on LiDAR point clouds. Figure 1A,B present a visualization of how LiDAR technology can be implemented.

The integration of Plant Growth Regulators (PGRs) with remote sensing monitoring systems is fundamental for establishing a precise and efficient management cycle. This synergy enables a targeted, data-driven approach where technologies like LiDAR identify critical vegetation zones for intervention [19], allowing for the localized application of PGRs and maximizing resource efficiency. By preventing blanket applications, this strategy inherently mitigates the risk of overuse and subsequent environmental contamination, minimizing the chemical load on non-target areas and adjacent ecosystems. Furthermore, continuous monitoring provides quantifiable feedback on PGR efficacy, enabling the optimization of application rates and timings [36]. This shift from a reactive, calendar-based mowing schedule to a proactive, prescription-based system, guided by the Mowing Effort Level (NERp) concept [12], directly reduces operational frequency and costs while enhancing both road safety and environmental sustainability.

Figure 1A shows the LiDAR point cloud, captured by remote sensing. Each point reflects the 3D position of surface elements, such as vegetation, buildings, and terrain. The data allows preliminary analyses of the structure and spatial distribution of objects before any filtering or classification.

Figure 1B shows that the LiDAR data have been processed to generate a height map relative to the ground. The colors indicate elevation: blue tones represent areas closer to ground level (0 m), while red tones indicate higher elements, such as tree canopies or buildings. This product is essential for applications such as vegetation monitoring, urban planning, and terrain modeling.

Manual inspection of road vegetation, in addition to being costly and subjective, proves incapable of providing an accurate and auditable measurement of the problem. It is in this context that mobile LiDAR (Light Detection and Ranging) technology emerges as a revolutionary diagnostic tool, providing an accurate three-dimensional representation of the road environment. As demonstrated by Carnot et al. [19], vehicle-mounted LiDAR sensors allow the capture of georeferenced point clouds, which, when processed by semantic segmentation algorithms such as RandLANet, automatically classify points belonging to the road and vegetation. The processing pipeline, which includes the concatenation of multiple scans for complete coverage and an innovative algorithm for detecting road contours, allows the generation of a virtual volume that precisely defines the vertical height to be kept free of obstructions, automating and objectively identifying invasive trees and branches.

The true synergy of this monitoring system with the proposed use of Plant Growth Regulators (PGR) is achieved by creating a “prescription map” for intervention. The output of the Carnot et al. [19] algorithm, which projects problem points onto 3D images, can intelligently and locally direct PGR application, prioritizing only those areas where vegetation effectively violates the standard, optimizing resources, and enhancing the reduction in the Mowing Effort Level (NERp). Despite the unquestionable advantages in terms of accuracy, efficiency, and data generation for decision-making, large-scale implementation faces challenges such as the initial cost of acquiring the sensors, the need for technical training to operate the system, and the computational demand for processing the large volumes of data generated. Overcoming these barriers is essential to integrating the full cycle of smart management: monitoring (with LiDAR) to accurately identify critical areas, intervening (with PGR) in a sustainable and localized manner, and verifying (with a new LiDAR scan) the effectiveness of the intervention, completing a cycle of proactive, data-driven, and economically viable management.

In addition to the technological and operational challenges, the scarcity of large, labeled datasets for training semantic segmentation models constitutes a significant barrier to the large-scale implementation of AI-based monitoring systems. Innovative approaches, such as the use of simulators to generate synthetic data (Roadsense), emerge as a promising solution to circumvent this limitation, enabling the creation of arbitrary volumes of robust, labeled training data for deep learning algorithms [20].

The integration of mobile LiDAR sensors, AI processing, and the precise application of PGR represents the future of vegetation management. The development of tools such as synthetic data simulators is a crucial step in maturing these technologies, drastically reducing the cost and time of developing detection models specific to Brazilian road vegetation, paving the way for the full automation of diagnosis and intervention [20].

Beyond vegetation control, LiDAR demonstrates direct relevance in identifying geotechnical risks and mapping drainage infrastructure on roadsides. As reviewed by Hatta Antah et al. [18], models derived from LiDAR data, such as the Shallow Landslide Risk Map [27] and the Landslide Inventory [25], enable early detection of areas susceptible to shallow landslides, optimizing the allocation of resources for slope stabilization. Additionally, techniques such as Least-Cost Breaching [26] and LiDAR Dropout Modeling [28] enable the precise mapping of drainage channels under bridges and culverts, essential for avoiding obstructions and ensuring the hydrological efficiency of roads. These applications reinforce the role of LiDAR as an integrative tool in the sustainable management of road infrastructure, complementing PGR-based interventions by providing robust, spatially explicit environmental diagnostics.

Figure 2 illustrates the proposed integrated methodological flow for intelligent vegetation management in road infrastructure, combining remote monitoring technologies with data-driven interventions. The process begins with the acquisition of geospatial data using LiDAR, followed by the generation of 3D models and processing using AI algorithms for semantic segmentation and identification of critical areas such as invasive vegetation, slope instability, and drainage obstructions. This information supports the generation of a prescription map for localized application of plant growth regulators (PGRs), whose effectiveness is verified by a new LiDAR scanning cycle, completing a continuous cycle of improvement and resulting in reduced NERp, resource savings, and improved road and environmental safety.

As illustrated in Figure 2, the integration of remote sensing technologies (LiDAR) and physiological modulators (PGR) establishes a virtuous cycle of vegetation management. This cycle allows us to transition from a reactive model of frequent mowing to a proactive, data-driven model where interventions are targeted, their effectiveness measured, and protocols are continually improved, culminating in a systematic reduction in NERp and the promotion of road and environmental safety.

5. Conclusions

Vegetation along roadsides plays an essential role in environmental conservation, acting as a barrier against erosion and mechanically reinforcing slope stability. Proper maintenance of this vegetation enhances its effectiveness in water infiltration, slope retention, and biodiversity promotion, with particularly significant benefits in sensitive biomes and regions with fragile soils.

The strategic application of plant growth regulators (PGRs) emerges as a technically robust solution for reducing the Mowing Effort Level (NERp) by minimizing the frequency of mechanical interventions required. As demonstrated in the literature, the controlled suppression of vertical vegetation growth through synthetic PGRs (e.g., paclobutrazol and mepiquat chloride) and the application of herbicides at sub-lethal doses (e.g., glyphosate and 2,4-D) significantly extends maintenance intervals. This approach not only generates substantial savings in operational and logistical costs but also reduces carbon emissions associated with mowing operations, thus aligning with global sustainability and efficiency goals in road infrastructure management.

Integrated with precision monitoring technologies, such as LiDAR remote sensing for diagnosis and localized application, the use of PGRs enables the transition from a reactive management model to a proactive, data-driven, and economically viable paradigm. The expected results go beyond simply reducing plant height and mowing frequency, contributing to the continuous improvement of visibility, road safety, and environmental resilience along roadsides.

However, the successful implementation of this strategy on a global scale requires the validation of dosage protocols, application timing, and post-application monitoring adapted to the diverse soil and climate conditions and predominant plant species in each region. Therefore, additional studies and field tests in different geographic contexts are recommended to consolidate universally applicable and environmentally sound technical recommendations.

The results of this systematic review may inform the development of innovative technical guidelines and public policies, harmoniously integrating the pillars of road safety, operational efficiency, and environmental conservation. It is hoped that this compendium will serve as a catalyst for future research and the adoption of smart and sustainable vegetation management practices in transportation infrastructures around the world.