Analysis of the Vegetative Growth Development and Phenology of Hop Cultivars Grown in the Subtropics Under a Two-Crop-a-Year System

Abstract

The aim of this study was to characterize the vegetative growth development of hop plants grown in the subtropics under a two-crop-a-year system with artificial supplementation lighting. The development of ‘Mapuche’ and ‘Spalter’ hops was compared during the summer 2022–2023, fall 2023, summer 2023–2024 and fall 2024 harvest seasons, considering the effects of the air temperature on the vegetative growth of plants from thermal sums in a subtropical climate region. The experiment was conducted in Palotina, Paraná, Brazil (24°17′40.05″ S, 55°50′23.16″ W, at 332 m elevation). The hops were trained on a 5.5 m high vertical trellis, using a ‘V’-shaped training system. Vegetative growth was evaluated based on the plant height development (m), hop growth rate (HGR), and classification of four growth stages based on the HGR. The phenology of the hop cultivars was determined visually according to the duration in days of the phenological stages. The development of the plant height and HGR was analyzed by nonlinear regressions of the Gompertz model and Gaussian function, respectively. ‘Mapuche’ and ‘Spalter’ hops had complete vegetative growth and phenological phases in the summer and fall seasons, with greater precocity in plant development in the summer season. The growth model based on the air temperature demonstrated that under subtropical conditions, the growth was maximized in seasons with higher temperatures. The duration of the phenological phases and the complete cycle of the plants was influenced by the vegetative growth of each cultivar in each harvest season. Therefore, double annual crop production of the hop cultivars ‘Mapuche’ and ‘Spalter’ is possible in a subtropical climate with artificial light supplementation.

1. Introduction

Hops (Humulus lupulus L.) is a perennial climbing plant whose female inflorescences are used as a basic ingredient in beer production because they give bitterness, flavor and aroma to the drink [1], as well as for pharmaceutical products. Commercial hop production in the world is traditionally associated with temperate climate countries, between the latitudes of 35° and 55° north or south of the equator. Regions parallel to this latitude range were considered unsuitable for cultivation for many years [2]; however, this species is currently being cultivated in several tropical and subtropical regions [3].

Among the main climatic factors that limit the growth of hop plants are the photoperiod and air temperature [2]. Hops are short-day plants with a critical photoperiod of 16.5 h of light, i.e., during the early season, the photoperiod must be greater than 16.5 h to avoid early flowering until the vertical growth of the hop plants is complete [4]. Traditionally, hop cultivation is associated with cooler temperatures and the need for a dormant period during the winter, with temperatures between 13.2 °C and 20.5 °C considered ideal for plant growth [5,6].

The growth of the craft beer market around the world and the emergence of demand for a diverse range of quality hops has led to the expansion of hop cultivation in areas parallel to latitudes between 35° and 55° in both hemispheres [7], with investment in the adaptation of cultivation technologies and cultivars for these emerging areas [8]. In some subtropical climate regions, such as Florida, USA [9], and Paraná, BR [10], artificial lighting has been used to complement the daily photoperiod during plant growth in order to control the early flowering of hop plants. Furthermore, there are already reports of production in more than one cycle per year for these regions, which have mild winters that allow for complete plant growth with satisfactory quantitative and qualitative production aspects for a second annual harvest [11,12].

The first harvest season, also known as summer season, begins with the sprouting of pruned hops in mid-winter, and the cones are usually harvested in early summer. After harvesting and pruning, the plants are quickly induced to sprout due to the high summer temperatures, and a new cycle begins, known as the fall season. The air temperature remains within the ideal range for plant development during the fall, allowing the growth and production of cones by the plants [3,12]. This is interesting from the point of view of maximizing agricultural production in these regions, since in traditional growing regions, only one cycle is obtained per year [11].

The aim of this study was to characterize the vegetative development and phenology of ‘Mapuche’ and ‘Spalter’ hops cultivated under a two-crop annual system with artificial light supplementation, considering the effects of the air temperature and thermal requirements in a subtropical climate region.

2. Materials and Methods

2.1. Description of the Experimental Area and Experiment

In this study, the vegetative growth development of ‘Mapuche’ and ‘Spalter’ hops (Humulus lupulus L.), grown under artificial lighting supplementation, was assessed during the summer 2022–2023, fall 2023, summer 2023–2024 and fall 2024 harvest seasons. The experiment was carried out in Palotina, Paraná, Brazil (24°17′40.05″ S, 55°50′23.16″ W and altitude of 332 m), in an experimental area located at the Federal University of Paraná (UFPR). The local climate is subtropical (Cfa) humid, with an average annual temperature of 20.8 °C, average annual rainfall of 1508 mm, maximum photoperiod of 13.5 h, and minimum photoperiod of 10.5 h [10].

The experimental area had a double-tape drip irrigation system and a lighting supplementation system with light-emitting diode (LED) lamps. The artificial lighting was used during the vegetative growth phase of the cultivars, and after the plants reached the total trellis height of 5.5 m, the light supplementation was permanently turned off. The lamps used were Philips GreenPower DR/W 10W LED bulbs (Philips, Eindhoven, Netherlands) with a spectrum developed to inhibit the flowering of short-day plants, containing a photon flux of 25 mol s⁻¹, with a predominant light spectrum of 450 nm (blue) and 650 nm (red). The spacing between the lamps at the top of the trellises was 10 m in the planting row. The lamp activation system was controlled by an automatic timer, which activated the LEDs daily 30 min before sunset, keeping them on for the time necessary to supplement 17 h of photoperiod. The daily times for the automatic switching of light supplementation on and off were determined from the daily photoperiods calculated for the latitude of Palotina, PR, using the equation $\mathrm{N}=(-\tan \phi \times \tan \delta ),$ where N = length of day in hours; φ = geographic latitude (negative values for the southern hemisphere); and δ = solar declination, which was calculated using the equation $\delta =23.45 \sin [360(284+\mathrm{n})/365]$, where n is the Julian day. The sine and tangent values were transformed into radians for the sine and tangent [13].

The average maximum, mean and minimum air temperature (°C) and the accumulation of rainfall (mm) were determined from hourly measurements by the S824 meteorological station, located at the SIMEPAR entity in the municipality of Palotina (24°31′ S, 53°90′ W), PR, in 2022, 2023 and 2024 (Figure 1).

The thermal requirement (

Table 1. Number of days and accumulated degree days (GDAs) of growth of ‘Mapuche’ and ‘Spalter’ hop plants during the summer 2022–2023, fall 2023, summer 2023–2024 and fall 2024 seasons grown in a subtropical climate.

Harvest Seasons	Cultivars
	‘Mapuche’		‘Spalter’
	Growth Duration (Days)	GDA (°C)	Growth Duration (Days)	GDA (°C)
Summer 2022–2023	78	1840.6	78	1840.6
Fall 2023	112	2083.9	112	2083.9
Summer 2023–2024	77	1910.5	119	2856.0
Fall 2024	147	3036.4	140	2798.5

), in degree days (GDs), was calculated during the vegetative growth of the plants, based on the following equation: $\mathrm{GD}=\sum _{\mathrm{i}=1}^{\mathrm{k}}=({\mathrm{T}}_{\mathrm{avg}}-{\mathrm{T}}_{\mathrm{base}}{)}_{\mathrm{i}}$, where GD is the total accumulated degree day; i and k indicate the first and last day of measurement, respectively; T_avg is the average daily temperature; and T_base is the base temperature (i.e., the temperature value below which plant growth is considered zero), which for this experiment was considered 0 °C [15,16].

In the summer 2022–2023 and fall 2023 seasons, three-year-old plants were arranged in a randomized block design, with four replicates and four plants per plot. In September 2023, after the end of the fall season, the experimental area underwent a change in the experimental layout to include other experiments, in which the rhizomes of the 3-year-old plants were transplanted to another location within the same area and, to complete the experimental design, 1-year-old plants propagated in vitro were inserted. Therefore, in the summer 2023–2024 and fall 2024 seasons, the third- and first-year plants were arranged in a completely randomized design, with ten replicates (plants) per experimental plot.

In all the seasons, the hop bines were cut back close to the ground to stimulate bud development. In the summer 2022–2023 season, the cut back was carried out at the end of October. In the fall 2023 season, the plants were cut back at the beginning of April. In the summer 2023–2024 season, both cultivars were cut back in early October; however, due to uneven sprouting, the ‘Mapuche’ plants were re-cut back in early November. In the fall 2024 season, the plants were cut back at the end of March.

After pruning, the hop plants started to develop at the beginning of the growing season, and the initial vigorous growth of some plants tended to form bines with long internodes that did not lead to high yields; for this reason, a critical selection of 6 uniform bines per plant was carried out. The plants were trained on a vertical trellis system with ‘V’ tutoring in the row, with 3 bines on each coir twine, height of 5.5 m and spacing of 1.0 m between plants and 3.0 m between rows. Similar fertilization and weed and pest control protocols were followed in all the seasons. Fertilization was performed via split applications during vegetative development by applying 50 g of NPK 10-10-10 per plant at the beginning of sprouting, leaf development, and side branch formation. Weeds were mow-controlled in rows and emerald grass (Zoysia japonica) was present between rows. Pest control, especially for spotted mites, was conducted by sequential application of citrus essential oil extracts, neem oil, and cupric fungicides.

2.2. Analysis

2.2.1. Vegetative Growth

The vegetative growth development was assessed based on the plant height (m), hop growth rate (HGR) and classification of four growth stages [15].

The height of the plants was measured weekly until the plants reached the top of the training system. The development of the plant height (m) was analyzed by nonlinear regressions of the Gompertz model: $\mathrm{Y}=\mathrm{a} {\exp }^{-{\exp }^{-\mathrm{b} (\mathrm{x}-\mathrm{t})}}$, where y is height (i-th observation of the dependent variable), a is the asymptotic value of y, b is the integration parameter or intercept with the y-axis, x is the explanatory variable, and t is the number of days in the i-th observation of the independent variable.

The HGR rates were calculated from the plant height development according to the following formula: $\mathrm{HGR}=\mathrm{Ht}/\mathrm{GD},$ where Ht is the height value at a given time and GD is the total degree day accumulated at a given time. The HGR trends were fitted by nonlinear Gaussian function regressions: $\mathrm{Y}=\mathrm{a} {\exp }^{-\mathrm{b} (\mathrm{x}-\mathrm{t}{)}^2}$, where y is the growth rate (i-th observation of the dependent variable), a is the asymptotic value of y, b is the integration parameter or intercept with the y-axis, x is the explanatory variable, and t is the number of days in which the growth rate is maximized.

The plant growth was divided into four growth stages based on the values of the HGR rates and the peak function presented after applying the Gaussian function. Stage I represents the initial slow growth phase, characterized by HGR values below 3 mm GDD⁻¹. Stage II is the rapid growth phase, culminating in the peak growth rate. Stage III corresponds to the post-peak slow growth phase, and stage IV marks the late phase, where growth declines sharply and eventually ceases [15].

2.2.2. Phenology

The phenology of the hop cultivars was determined according to the duration in days of the following stages (BBCH) from cut back pruning: sprouting; development of leaves and elongation of bines; emergence of side branches; emergence of inflorescence; flowering; cone development; and cone maturation [17].

The determination of the phase change was carried out from visual observations made every three days, from pruning to cone harvesting. From these data, diagrams were constructed with the duration scale in days of each of the hops’ phenological phases.

2.2.3. Statistical Analysis

To apply the nonlinear regression models to the data set on the plant height development and growth rates, the statistical program R, version R 4.4.3, was used.

3. Results

3.1. Vegetative Growth Development

The nonlinear regression of the Gompertz model presented the three significant parameters (p < 0.01) for the two hop cultivars in all the harvest seasons, demonstrating that the data set fitted to the model (Table S1). The plant growth was different between the cultivars and among the harvest seasons in terms of the time required to reach the top of the trellis (5.5 m) (Figure 2). The hop plants had earlier growth in the summer seasons each year, reaching the top of the trellis at approximately 80 days after pruning in 2022–2023 and for ‘Mapuche’ in 2023–2024, and at 120 days after pruning for ‘Spalter’ in 2023–2024 seasons. For the fall seasons, the plants reached the top of the trellis height approximately 110 days after pruning in 2023, and 150 days after pruning in 2024.

The nonlinear Gaussian function regression had the three significant parameters (p < 0.01) for both cultivars in all the seasons, demonstrating that the data set fitted to the model (Table S2). The hop plant growth rates (HGRs) formed peak functions in both cultivars in all the seasons (Figure 3). The maximum HGR values were different between cultivars and seasons. For both cultivars, the summer 2022–2023 season had higher maximum HGR values than the fall 2023 season, while the summer 2023–2024 had lower maximum HGR values than the fall 2024 season. In the summer 2022–2023 and 2023–2024 seasons, and the fall 2024 season, ‘Mapuche’ had a higher maximum HGR value, while in the fall 2023 season, ‘Spalter’ had a higher maximum HGR value.

The classification of the plant growth into four stages based on the HGR values had distinct ranges of values and durations of each phase among the cultivars and seasons assessed (

Table 2. Growth stages according to the HGR intervals (mm GDDs⁻¹) and durations in days of each growth stage of ‘Mapuche’ and ‘Spalter’ hops during the summer 2022–2023, fall 2023, summer 2023–2024 and fall 2024 seasons grown in a subtropical climate.

Seasons	Growth Stages 1	HGR Values Range (mm GDDs−1)	Stages Duration in Days
‘Mapuche’
Summer 2022–2023	I	0.00–0.51	24
	II	4.33–7.27	18
	III	6.38–3.80	18
	IV	1.40–0.00	18
Fall 2023	I	0.00–2.11	35
	II	3.68–5.48	42
	III	4.83–2.22	21
	IV	1.39–0.00	14
Summer 2023–2024	I	0.00–2.99	21
	II	4.20–6.01	28
	III	4.44–1.70	14
	IV	0.97–0.00	14
Fall 2024	I	0.00–2.79	84
	II	3.37–7.40	21
	III	7.35–5.21	21
	IV	2.17–0.00	21
‘Spalter’
Summer 2022–2023	I	0.00–2.30	18
	II	3.17–7.11	24
	III	4.81–2.56	18
	IV	0.73–0.00	18
Fall 2023	I	0.00–0.03	35
	II	3.17–6.25	35
	III	5.18–2.82	28
	IV	1.56–0.00	14
Summer 2023–2024	I	0.00–2.62	49
	II	3.11–6.45	21
	III	4.45–2.42	21
	IV	1.62–0.00	28
Fall 2024	I	0.00–2.57	70
	II	3.68–9.23	42
	III	4.46–2.31	14
	IV	0.93–0.00	14

¹ Stage I—initial slow growth phase with HGR values below 3 mm GDD⁻¹; stage II—rapid growth phase culminating in peak growth rate; stage III—post-peak slow growth phase; stage IV—late phase where growth declines and ceases.

). The duration in days of stage I was greater for both cultivars in the fall seasons and for ‘Spalter’ in the summer 2023–2024 season. The duration in days of stage II was greater for ‘Mapuche’ in the fall 2023 season, and for ‘Spalter’ in the fall 2023 and 2024 seasons. The duration in days of stages III and IV were similar between cultivars and harvests.

3.2. Phenology

The duration of the main phenological phases was different between cultivars and seasons (Figure 4). The initial development of the plants was similar until the beginning of leaf development and the elongation of bines, when the cycles began to differ between cultivars and seasons. It was found that ‘Mapuche’ and ‘Spalter’ had greater leaf development and elongation of bines for the fall compared to the summer season in both years and for the summer 2023–2024 and fall 2024 season compared with the same seasons of the previous year, leading to a longer time taken to reach the reproductive development phases and, consequently, later cycles. The emergence of the side branches phase was shorter for ‘Mapuche’ compared with ‘Spalter’ in the summer 2023–2024 season, while the emergence of inflorescence was shorter for ‘Mapuche’ and ‘Spalter’ in the summer 2022–2023 and fall 2024 seasons. The flowering phase was longer for ‘Mapuche’ compared with ‘Spalter’ in the fall 2023 season. For both cultivars, the cone development phase until maturation was longer for the summer 2023–2024 and fall 2024 seasons compared with the same seasons in the previous year.

4. Discussion

4.1. Vegetative Growth

Hop plants present a complex interaction between plant genotype and environment [18]. Previous studies associated the heterogeneity of hop plant growth between the summer and fall seasons with distinct environmental conditions found in each season. The reduction in the day length from the summer to the fall season, in relation to the loss of sunlight quality and irradiance, was related to lower plant growth in the harvest fall, even with the use of artificial photoperiod supplementation [12]. Artificial lighting developed for flowering control, such as that used in this study, has a very low light intensity, approximately 25 μmol m⁻² s⁻¹, in relation to total sunlight, approximately 1900 μmol m⁻² s⁻¹ [9]; therefore, despite preventing early flowering and allowing vegetative growth of the bines, in seasons with a longer photoperiod, the plants may present a higher photosynthetic rate and, consequently, greater growth due to the greater irradiance exposure [12,19].

Another environmental factor that has been associated with the faster growth of hops is the air temperature [10]. Studies have shown that air temperatures between 21 and 39 °C increases the photosynthetic capacity of hops, as long as they receive sufficient water during plant growth development [20]. Therefore, the increasing temperatures during the summer season may result in earlier growth, while the decreasing temperatures in the fall may result in later plant growth [11,12].

The standardization of consecutive summer and fall harvests is only possible under controlled environmental conditions that minimize the environmental factors, consequently equalizing the vegetative and productive development of plants between seasons [21]. However, the control of the field environment is limited, and it is necessary to consider the influence of the climate of each season [11].

The plant growth development of ‘Mapuche’ and ‘Spalter’ was similar, except for the summer 2023–2024 season, when ‘Mapuche’ had earlier growth than ‘Spalter’. In previous studies, these cultivars had similar behavior when exposed to the same climatic conditions, considered the early cycle in a subtropical climate [10]. The summer 2023–2024 season corresponds to the first harvest after changing the layout of the experimental area, with the transplanting of the rhizomes of 3-year-old plants and planting of new 1-year-old plants, in which the ‘Mapuche’ plants showed a need for re-pruning due to uneven sprouting within the experimental plot. The later pruning of ‘Mapuche’ and, consequently, the initial growth of the plants in different environmental conditions, may justify its earliness in relation to ‘Spalter’.

Furthermore, it was observed that after changing the layout of the experimental area, the ‘Spalter’ cultivars in the summer 2023–2024 season and ‘Mapuche’ and ‘Spalter’ in the fall 2024 seasons took, respectively, 40 and 70 days after pruning to present uniform shoots and start growing on the coir twines, and they consecutively presented later growth when compared to the same seasons in the previous year. The use of rhizomes and rooted plants is recognized as a safe method for establishing new hop plants [22]. The rhizome is the perennial organ of the hop plant specialized in the accumulation of reserve substances and the formation of shoots [4,23]. During the removal and transplantation of rhizomes, it is necessary to clean the root system, and the loss of true roots can cause significant stress on the plant’s metabolism, which may be associated with slower plant growth [23,24]. When using new plants from in vitro propagation, an average time of three years is considered for stabilization and formation of the rhizome’s nutritional reserves [25]. Therefore, the initial development of plants during the summer 2023–2024 and fall 2024 seasons may have been affected by the stress of transplanting 3-year-old plants or by the low accumulation of reserves in 1-year-old plants with forming rhizomes.

The formation of a peak function across growth rates demonstrates that there is a stage at which plant growth is maximized and subsequently declines throughout the remaining growing season [15]. Hop growth is known to be fast due to the strong apical dominance of the bines, presenting a peak growth rate that can reach 25 cm per day [2,26].

The vegetative growth rate (HGR) of hops is completely influenced by the temperature, since it considers the total degree days accumulated in a given time to be calculated [15,16]. The average daily values of the degree days during plant growth of both cultivars were higher in the summer seasons, which justifies the higher maximum HGR values found during the summer 2022–2023 compared with the fall 2023 season [20].

An opposite behavior was found in the summer 2023–2024 season, which had the lowest maximum HGR values in relation to the fall 2024 season, that is, despite the greater accumulation of degree days, the plants presented lower growth rates. Considering that the summer 2023–2024 season was the first harvest after changing the layout of the experimental area, we can consider that the delay in the initial development of the plants and the ‘Mapuche’ plants requiring to be cut back may have significantly influenced the growth rates.

The differences between the HGR curves demonstrate that despite the similar behavior between the cultivars regarding the plant height development curves, ‘Mapuche’ most often resulted in a higher maximum HGR value than ‘Spalter’. The genotype is an important factor in the growth of hop plants, and each one must present its own behavior when interacting with the environment [10,27]. Given the higher peaks in the growth rates of ‘Mapuche’, it is assumed that this hop cultivar will show faster growth in subtropical climate conditions.

The model of the classification of plant growth into four stages based on the HGR values was adopted, aiming to integrate the leaf development and growth phases proposed by the BBCH phenology classification system for hop plants [15]. The BBCH scale describes hop phenology in nine macro stages from budding to plant senescence [17] and separates growth and development during the vegetative stages of plants [15]. Sprouting (0) and the development of leaves (1) are the first macro stages of phenology, and these are followed by the emergence of side branches (2) and by the elongation of bines (3) [17]. This classification ends up being confusing in the field, when the elongation of bines occurs simultaneously with the development of new pairs of leaves and sometimes together with the emergence of side branches [28,29].

From the classification of growth into four stages, it was found that the greatest differences are mainly associated with stages I and II, related to the phases of slow initial growth and rapid growth that culminates in the maximum point of the HGRs, which can be considered as critical phases for plant growth [15]. Therefore, the influence of variations in the environmental factors previously described can be decisive in the dynamics of vegetative development when imposed on these first two stages.

4.2. Phenology

The phenological classification of BBCH hops was developed considering the behavior of the plant under environmental conditions in temperate regions [17]. In subtropical climate regions such as Brazil, the photoperiod is always inductive, as the critical point for inducing hop flowering is 16.5 h and the maximum photoperiod found is 14 h of light; therefore, the plants can flower at any time of the year [4]. Studies indicate that under an inductive photoperiod, plants are generally induced to flower early, and this phase lasts for a period three times longer than the typical flowering period [5], with the production of scattered flowers that implies lower production [4]. Under early flowering, variations in the other stages are also observed, such as the elimination of the emergence of side branches, directly affecting the formation of hop cones in quantity [5,30,31], or the occurrence of distinct phenological stages simultaneously, such as the emergence of side branches, the development of new flowers and the senescence of cones [5].

The phenological behavior found in this study for both cultivars and seasons was similar to the classification described for a temperate climate [17] and can be clarified by the use of artificial photoperiod supplementation. Artificial control of the photoperiod allows the development of plants in ordered phenological phases, with the beginning of the season presenting only vegetative development, until they reach the maximum height of the support trellis, when, upon turning off the lighting, vegetative growth ceases and the first reproductive structures begin to develop, followed by the very rapid development of cone growth [10,17,32]. Flowering control by photoperiod supplementation allows us to consider that the longer duration in days taken to achieve complete vegetative growth is associated with the later cycle of these plants, as observed in this study.

In addition to the factors described above, the timing of flowering is also dependent on the genotype and may change the duration of the stages between different genetic materials after the reduction of the photoperiod. Therefore, the differences observed between cultivars regarding the duration in days for inflorescence emergence and flowering may be associated with the need of each genotype to perceive and respond to the inductive photoperiod [4].

Finally, our overall perception is that it is feasible to obtain two crop seasons per year for ‘Mapuche’ and ‘Spalter’ hops under subtropical soil and climate conditions with the use of supplemental lighting, provided that the plants have orderly and complete vegetative growth and phenological phases in both seasons. Differences associated with the harvests and cultivars occur, which require attention from the hop grower in adopting management practices, such as pruning and harvesting. In addition, it is worth noting that environmental variations are factors with great influence on the development of the plants in each season. Therefore, for all the emerging regions in terms of hop cultivation, it is recommended to develop basic research to assess the adaptation of cultivars to local edaphoclimatic conditions to generate reliable information on the appropriate agricultural practices to be adopted for the development of stable and efficient hop production in these new cultivation areas.

5. Conclusions

‘Mapuche’ and ‘Spalter’ hops had complete vegetative growth and phenological phases in the summer and fall seasons of each year evaluated, demonstrating that under artificial light supplementation, it is possible to grow hops under a two-crop-a-year system in a subtropical climate. The summer seasons had induced precocity in plant development compared to the fall seasons. The growth model based on the air temperature demonstrated that this variable is one of the factors influencing plant growth and that, in subtropical conditions, growth is maximized precisely in seasons with higher temperatures. The duration of the phenological phases and the complete cycle of the plants was influenced by the vegetative growth of each cultivar in each season.