Evaluation of Wood Vinegar as an Herbicide for Weed Control

Abstract

Wood vinegar, a by-product of wood pyrolysis, is mostly discarded. Wood vinegar has a phytotoxic effect and could be potentially used as a naturally derived herbicide for weed control. The objective of this research was to evaluate the efficacy of wood vinegar from the pyrolysis of apple (Malus × domestica Borkh.) tree branch wastes to control weeds. The wood vinegar concentrations required to inhibit 50% motherwort (Leonurus cardiaca L.), redroot pigweed (Amaranthus retroflexus L.), Spanish needles (Bidens pilosa L.), and tall fescue (Festuca arundinacea L.) seed germination measured 0.51%, 0.48%, 0.16%, and 1.1%, respectively. The wood vinegar application rates (spray volume) required to provide 50% control of motherwort and Spanish needles measured 1911 L ha⁻¹ and 653 L ha⁻¹, respectively, while the highest evaluated rate at 4000 L ha⁻¹ controlled 35% tall fescue by 10 days after treatment (DAT). Common purslane (Portulaca oleracea L.) control increased as the wood vinegar application rate increased from 500 L ha⁻¹ to 2000 L ha⁻¹. Wood vinegar was more effective in dark than light conditions for controlling common purslane. By 5 DAT, averaged over application rates, wood vinegar provided 95% and 87% control of common purslane in dark and light conditions, respectively. These findings suggest that wood vinegar obtained from the pyrolysis of apple tree branches could be used for weed management.

1. Introduction

Many synthetic herbicides used in agricultural production are suspected of being problematic and could potentially pollute the environment [1,2]. For example, organic arsenic herbicides, such as monosodium methyl arsenate, are previously registered for weed control in cotton (Gossyptium hirsutum L.), turfgrass landscape, and nut trees, but more strict restrictions have been imposed in many regions in the world, including China, Europe, and the United States due to the pollution associated with arsenic in underground water [3,4,5,6,7]. Atrazine, a photosystem II inhibitor, is extensively used for weed control in various cropping systems [8,9], but it is one of the most frequently detected synthetic herbicides in underground water [9,10,11]. The European Union entirely bans the use of atrazine [12]. The United States lists atrazine as a restricted-use pesticide, prohibits aerial applications of all formulations, restricts the annual application rate up to 2.24 kg ha⁻¹, and prohibits application during rain, storm, or when soils are saturated or above field capacity [13]. Nevertheless, no such legal restriction is being imposed for using atrazine in China. Consequently, a critical need exists to develop environmentally friendly naturally derived herbicides for weed management.

Over-dependence on synthetic herbicides, especially when repeated using a single mechanism of action successively over the years, could increase biological pressure for selecting herbicide-resistant weed populations [14,15]. The prevalence of herbicide-resistant weeds poses severe challenges to sustainable weed management [15,16,17,18]. Using herbicides with new modes of action could alleviate the problem and reduce herbicide-resistant evolution [15]. Unfortunately, since 1985, only one synthetic herbicide, cinmethylin, was developed with a new mode of action of inhibiting cell membrane production, according to the Herbicide Resistance Action Committee [19,20]. Acetic acid could cause a rapid phytotoxic effect [21] and is presently being listed as a natural herbicide in the United States Department of Agriculture National Organic Program for weed management in organic crops [5]. The concentration, application volume, and adjuvant of acetic acid for weed control have been well documented [21,22,23,24]. Unfortunately, using horticultural grade acetic acid for weed control in large fields is prohibitively expensive. Therefore, exploring alternative products containing organic acids for weed management is needed.

In China, the total apple planting area and yield are far more than other deciduous fruit trees such as apricot (Prunus armeniaca L.), peach (Prunus persica (L.) Batsch), and pear (Pyus communis L.) [25]. Apple trees need to be pruned to improve tree structure, photosynthetic rate, and yield [26]. Each year, a considerable amount of pruned apple branches is produced and primarily disposed into landfill or burned to provide thermal energy [27]. Open-burning agricultural waste is a straightforward approach of waste disposal [28,29]; however, this practice is currently being banned in China due to its adverse impacts on air quality and human health. Previous studies explored the feasibility of using pruning wastes for producing organic fertilizer and generating electricity [30,31,32]. However, exploring additional options to effectively utilize pruning wastes remains necessary.

Pyrolysis thermally decomposes lignocellulosic materials into several products, including charcoal, flammable gases, tar, and wood vinegar, under inert conditions in the deficiency of oxygen [30,33,34,35]. Wood vinegar primarily contains water and also consists of various organic compounds, such as acids, alcohols, aldehydes, carbohydrates, esters, nitrides, ketones, and phenols, depending on the feedstock [33]. At low concentrations and application volumes, wood vinegar could be used as a foliar-applied fertilizer [36,37]. For example, it was recently reported that the foliar application of wood vinegar boosts chickpea (Cicer arietinum L.) and lettuce (Lactuca Sativa L.) yield [38,39]; however, at high concentrations and application volumes, wood vinegar exhibits herbicidal properties [33,34,40]. For example, wood vinegar produced from elm (Ulmus sp. pl.) tree branches controlled several broadleaf weed species, including Carolina geranium (Geranium carolinianum L.), creeping woodsorrel (Oxalis corniculata L.), and perilla mint (Perilla frutescens (L.) Britt.) [34]. Undiluted wood vinegar from pruned apple tree branches applied at a rate of 2800 L ha⁻¹ caused 95% annual bluegrass (Poa annua L.) visual injury [34]. However, the efficacy of wood vinegar from the pruned apple tree branches for the control of more weed species remains needs to be explored to effectively use it as a nonsynthetic herbicide.

Temperature affects wood vinegar for weed control [34,40]. In previous research, wood vinegar at 1000 L ha⁻¹, 2000 L ha⁻¹, or 3000 L ha⁻¹ exhibited greater efficacy for controlling white clover (Trifolium repens L.) at 10 °C than 30 °C temperature conditions [40]. Similarly, Liu et al. [35] found that wood vinegar induced greater visual injury and reduced more aboveground biomass on perilla mint at 10 °C than 20 °C and 30 °C temperature conditions. However, the impact of light versus dark on wood vinegar for weed control has never been previously reported. Common purslane (Portulaca oleracea L.), redroot pigweed (Amaranthus retroflexus L.), Spanish needles (Bidens Pilosa L.), and motherwort (Leonurus cardiaca L.) (these common names were acquired from Weed Science Society of America Composite List of Weeds) are common weed species found throughout North and East China. In a preliminary study, wood vinegar obtained from pruned apple branches caused greater injury on motherwort, redroot pigweed, and Spanish needles than tall fescue (Festuca arundinacea L.); however, the control efficacy of wood vinegar on these plants is yet to be verified.

The objectives of this research were to (1) identify effective wood vinegar concentrations for inhibiting motherwort, Spanish needles, redroot pigweed, and tall fescue seed germination, (2) evaluate the effect of various wood vinegar concentrations on newly emerged motherwort, Spanish needles, redroot pigweed, and tall fescue seedlings, (3) evaluate the impact of the light conditions and wood vinegar concentrations on common purslane seed germination, and (4) investigate the impact of light conditions on wood vinegar application rates for controlling common purslane.

2. Materials and Methods

2.1. Wood Vinegar

Wood vinegar was produced from the apple tree branches at the Nanjing Forestry University (NFU), Jiangsu, China, in November 2018 using a custom-built pyrolysis reactor previously described by Liu et al. [34]. Various chemicals, including alcohols, acids, esters, phenols, and ketones, were identified using gas chromatography–mass spectrometry; acetic acid exhibited the largest chromatographic peak area of 16.63%. Wood vinegar was sealed in a glass tank and filtered through filter papers (Whatman-Xinghua Filter Paper Co., Ltd., Huangzhou, China) (Figure 1) prior to conducting the following experiments. Tall fescue seeds used in this study were a turf cultivar (Leonardo) purchased from a seed vendor. Common purslane, motherwort, redroot pigweed, and Spanish needle seeds used in this research were collected during September and October 2020 at roadsides in Shuyang, Jiangsu, China (47.16° N, 36.31° E).

2.2. Impact of Wood Vinegar Concentrations on Seed Germination

The experiments were performed in April 2021 at NFU to evaluate the impact of wood vinegar on motherwort, redroot pigweed, Spanish needles, and tall fescue seed germination. A single layer of filter paper was placed inside the Petri dish. For each weed species, a total of 25 seeds were evenly placed onto the filter paper inside the Petri dish. Then, 4 mL of wood vinegar at a concentration of 0%, 0.05%, 0.1%, 0.25%, 0.5%, 1%, 5%, or 10% was transferred into the Petri dishes. The Petri dishes were wrapped with polyethylene plastic film and placed in a growth chamber at a constant temperature of 26 °C under a light intensity of 300 µmol m⁻² s⁻¹. Seed germination was counted weekly for 4 weeks. Seed germination data at 14 DAT were used for analysis because no new germination was observed after that date.

The experiments were designed as a randomized complete block with four replications. The experiments were repeated twice over time. Seed germination data were converted to the percentage of nontreated control. Data were subjected to analysis of variance (ANOVA) in SAS (version 9.4, SAS Institute, Cary, NC, USA). Levene’s test was performed to examine the homogeneity of equal variances prior to combining the data from two experimental runs. Data were regressed with a three-parameter regression Equation:

y = β₀ + β₁ ∗ exp(−β₂ ∗ x)

(1)

where y is seed germination inhibition (%), x is wood vinegar concentration (%), β₀ is the intercept, β₁ is the asymptote, and β₁ is the slope estimate. The Equation was selected because it best described the relationship of seed germination with wood vinegar concentrations. The effective wood vinegar concentration that causes 50% seed germination inhibition (C₅₀) was determined from the regression Equation.

2.3. Impact of Wood Vinegar Concentrations on Newly Germinated Seedlings

Experiments were conducted in April and May at NFU to evaluate the impact of wood vinegar on newly germinated motherwort, Spanish needles, redroot pigweed, and tall fescue. A single layer of filter paper was placed inside the Petri dishes. A total of 25 seeds of each weed species were evenly scattered onto the filter paper. To each Petri dish was added 4 mL of deionized water. The Petri dishes were wrapped with polyethylene plastic film and placed in a growth chamber set at 26 °C under a light intensity of 300 µmol m⁻² s⁻¹. When the seeds were germinated and the seedlings reached approximately 2 cm in length, the seedlings were removed to the new Petri dishes with a single layer of filter paper that contained 4 mL of wood vinegar at a concentration of 0%, 0.05%, 0.1%, 0.25%, 0.5%, 1%, 5%, or 10%. Plant injury was visually measured based on plant tissue discoloration, chlorosis, and necrosis and evaluated on a percent scale where 0 is no visual injury and 100 is complete desiccation.

Experiments were a randomized complete block with 4 replications. Data were subjected to ANOVA in SAS. Levene’s test for homogeneity of equal variance was performed prior to combining data from two experimental runs. Data were regressed with a three-parameter quadratic nonlinear regression Equation:

y = β₀ + β₁ ∗ (1 − exp(−β₂ ∗ x))

(2)

where y is plant visual injury (%), x is wood vinegar concentration (%), β₀ is the intercept, β₁ is the asymptote, and β₂ is the slope estimate. The Equation was selected that best described the relationship of plant visual injury with wood vinegar concentration. The C₅₀ values were computed from the regression Equation.

2.4. Impact of Wood Vinegar Application Rate on Motherwort, Spanish Needles, and Tall Fescue

Experiments were conducted at NFU in May and June 2021 to evaluate the herbicidal efficacy of the wood vinegar application rate (spray volume) on motherwort, Spanish needles, and tall fescue. The seeds were planted in plastic pots measuring 81 cm² surface area and 10 cm in depth filled with commercial potting soil (Pindstrup Hortculture Co., Ltd., Pujin Road Minhang District Shanghai, Shanghai, China). The pots were placed in a growth chamber set at 26 °C under a light intensity of 300 µmol m⁻² s⁻¹ with a 12 h photoperiod. The plants were watered as needed to prevent soil moisture deficiency. The plants were sprayed with 500 L ha⁻¹, 1000 L ha⁻¹, 2000 L ha⁻¹, 3000 L ha⁻¹, and 4000 L ha⁻¹ wood vinegar when motherwort, Spanish needles, and tall fescue were 12 (±1.3) cm, 16 (±1.7) cm, and 11 (±0.8) cm in height. The wood vinegar treatments were sprayed with a handheld CO₂ sprayer with a single 8002EV nozzle (Teejet Spraying System, Co., Wheaton, IL, USA). Plant injury was visually evaluated at 10 DAT on a percent scale, where 0 represents no visual injury, and 100 represents complete desiccation. Data were subjected to ANOVA in SAS. Levene’s test was conducted to test homogeneity of equal variance prior to combining data from the experimental runs. Data were subjected to the following nonlinear regression Equation:

y = β₀ ∗ (1−exp(−β₁ ∗ x))

(3)

where y is plant visual injury (%), x is the wood vinegar application rate (L ha⁻¹), β₀ is the asymptote, and β₁ is the slope. The Equation that best described the relationship between plant visual injury and wood vinegar application rate was used for regression analysis. The wood vinegar rate required to induce 50% visual injury (I₅₀) was determined from the regression Equation.

2.5. Impact of Light and Wood Vinegar Concentrations on Common Purslane Seed Germination

Experiments were carried out twice over time to evaluate the impact of wood vinegar concentrations on common purslane seed germination under dark and light conditions. Experiments were conducted at the Peking University Institute of Advanced Agricultural Sciences (PKU-IAAS) in Weifang, Shandong, China, in October 2022. A single layer of filter paper was placed onto the Petri dish. For each weed species, a total of 25 seeds were evenly scattered onto the filter paper inside the Petri dish. To the Petri dish was added a total of 4 mL of wood vinegar solutions at a concentration of 0%, 0.05%, 0.1%, 0.25%, 0.5%, 1%, 5%, or 10%. All Petri dishes were covered with polyethylene plastic film. The Petri dishes were wrapped with aluminum foil for dark treatment to prevent light penetration. The Petri dishes were not wrapped with aluminum foil for light treatment. The Petri dishes were placed in a growth chamber set for an 18 h photoperiod with a constant temperature of 24 °C under a light intensity of 104 µmol m⁻² s⁻¹. Seed germination data were counted at 10 DAT after no new germination for 3 successive days.

The experiments were carried out as a completely randomized design with four replications. Data were subjected to ANOVA in SAS. Levene’s test was used to examine the homogeneity of equal variance before combining data from the experimental runs. The seed germination data were subjected to quadratic nonlinear regression analysis using the following Equation:

y = β₀ + β₁ ∗ (1 − exp(−β₂ ∗ x))

(4)

where y is seed germination inhibition (%), x is wood vinegar concentration (%), β₀ is the intercept, β₁ is the asymptote, and β₂ is the slope estimate. The Equation was selected that best describes the relationship between seed germination inhibition and wood vinegar concentration. The C₅₀ values were determined from the regression Equation.

2.6. Impact of Light Condition and Wood Vinegar Application Rate on Common Purslane

The experiments were conducted at PKU-IAAS in November 2022 to evaluate the impact of light conditions and wood vinegar application rate on common purslane. A total of 8 common purslane seeds were planted in each plastic pot measuring 81 cm² surface area and 10 cm depth filled with the commercial potting soil. The pots were placed in a greenhouse set at 25/17 °C day/night temperatures. The plants were watered as needed to prevent soil moisture deficiency. The plants were thinned down to 4–5 plants per pot after emergence. When the plants reached 8 cm (±1.2 cm) height, 500 L ha⁻¹, 1000 L ha⁻¹, or 2000 L ha⁻¹ wood vinegar were sprayed with a handheld CO₂-pressured sprayer equipped with a single 8002EV nozzle (Teejet Spraying System, Co., Wheaton, IL, USA).

For the dark treatment, the pots were covered with aluminum foil immediately following the treatment of wood vinegar. Plant injury was visually evaluated 1, 3, and 5 DAT on a percent scale where 0 is no injury and 100 is complete desiccation. Shoots were harvested 5 DAT, and fresh weight was determined.

Two experiments were performed over time and established as a split-plot design with the light condition as the main plot factor and the wood vinegar rate as the sub-plot factor. Each treatment was replicated four times. Data were subjected to ANOVA in SAS utilizing the PROC MIXED procedure, with the experimental run and replication regarded as the random factor, while the light condition and wood vinegar rate were regarded as the fixed factor. Levene’s test and Shapiro–Wilk were used for examining homogeneity of equal variance and normality, respectively. The treatment means were separated with the Tukey adjustment mean comparison statement.

3. Results

3.1. Impact of Wood Vinegar Concentrations on Weed Seed Germination

Wood vinegar at low concentrations exhibited an inhibitory effect on seed germination. For each plant species, the percentage of seed germination of the nontreated plants, with standard errors of the means, is presented in Figure 2. The C₅₀ values measured 0.51%, 0.16%, and 0.48% for motherwort, redroot pigweed, and Spanish needles, respectively, while the C₅₀ value measured 1.1% for tall fescue (

Table 1. Regression parameters for data presented in figures ^a.

Figure	Plant Species	Measurement	Equation b	β0	β1	β2	R2	SME c	p-Value	C50 or I50
Figure 2	Motherwort	Germination inhibition	(1)	77.02 (±7.75)	0.822 (±0.25)	−0.72 (±6.31)	0.77	16.75	<0.0001	0.51
Figure 2	Redroot pigweed	Germination inhibition	(1)	11.92 (±1.61)	−0.33 (±0.08)	−11.72 (±4.26)	0.87	14.73	<0.0001	0.16
Figure 2	Spanish needles	Germination inhibition	(1)	92.19 (±5.24)	1.20 (±0.18)	−1.38 (±4.03)	0.91	11.33	<0.0001	0.48
Figure 2	Tall fescue	Germination inhibition	(1)	81.42 (±8.28)	0.31 (±0.10)	−8.09 (±8.77)	0.86	11.78	<0.0001	1.1
Figure 3	Motherwort	Visual injury	(2)	−2.90 (±1.67)	124.99 (±9.85)	0.17 (±0.03)	0.99	2.96	<0.0001	3.33
Figure 3	Redroot pigweed	Visual injury	(2)	−15.82 (±10.38)	114.71 (±11.77)	1.89 (±0.50)	0.94	10.66	0.0014	0.47
Figure 3	Spanish needles	Visual injury	(2)	−19.83 (±8.80)	119.91 (±9.02)	3.04 (±0.57)	0.97	7.25	0.0003	0.42
Figure 3	Tall fescue	Visual injury	(2)	−1.27 (±5.65)	111.38 (±12.93)	0.30 (±0.12)	0.96	9.12	0.0009	2.00
Figure 4	Motherwort	Visual injury	(3)	73.28 (±7.22)	0.0006 (±0.0001)	-	0.88	9.18	<0.0001	1911
Figure 4	Spanish needles	Visual injury	(3)	83.42 (±3.76)	0.0014 (±0.0002)	-	0.90	10.53	<0.0001	653
Figure 4	Tall fescue	Visual injury	(3)	20.51 (±0.47)	0.0017 (±0.0002)	-	0.96	1.46	<0.0001	NA
Figure 5	Common purslane	Germination under dark	(4)	−5.45 (±3.22)	108.80 (±4.35)	1.72 (±0.18)	0.93	11.54	<0.0001	0.49
Figure 5	Common purslane	Germination under light	(4)	−11.28 (±3.63)	114.83 (±4.94)	1.65 (±0.19)	0.92	13.11	<0.0001	0.61

^a Numbers in parentheses are standard error of the estimate. ^b For Equation (1), y = β₀ + β₁ ∗ exp(−β₂ ∗ x), y is seed germination inhibition (%), x is wood vinegar concentration (%), β₀ is the asymptote, and β₁ is the slope estimate. For Equation (2), y = β₀ + β₁ ∗ (1 − exp(−β₂ ∗ x)), y is plant visual injury (%), x is wood vinegar concentration (%), β₀ is the intercept, β₁ is the asymptote, and β₂ is the slope estimate. For Equation (3), y = β₀ ∗ (1 − exp(−β₁ ∗ x)), y is plant visual injury (%), x is wood vinegar application rate (L ha⁻¹), β₀ is the asymptote, and β₁ is the slope. For Equation (4), y = β₀ + β₁ ∗ (1 − exp(−β₂ ∗ x)), y is seed germination inhibition (%), x is wood vinegar concentration (%), β₀ is the intercept, β₁ is the asymptote, and β₂ is the slope estimate. ^c Abbreviations: SME, standard error of the mean; C₅₀, the wood vinegar concentrations required to induce 50% seed germination or visual injury; and I₅₀, the wood vinegar rate (spray volume) required to induce 50% visual plant injury.

). When the tested wood vinegar concentration was 5%, no seeds were germinated, regardless of species. Redroot pigweed exhibited the greatest susceptibility to wood vinegar. Wood vinegar concentration of 1% completely inhibited redroot pigweed germination.

3.2. Impact of Wood Vinegar Concentrations on Newly Emerged Seedlings

Increased wood vinegar concentrations increased the mortality rate for all weed species (Figure 3). The C₅₀ values measured 0.42% and 0.47% for redroot pigweed and Spanish needles, respectively, while the C₅₀ values measured 3.33% and 2.00% for motherwort and tall fescue, respectively (Table 1). No weeds survived when the wood vinegar concentration was 10% (Figure 3).

Figure 3. The impact of various wood vinegar concentrations on motherwort, Spanish needles, redroot pigweed, and tall fescue. Vertical bars represent standard errors of the mean (n = 8).

Figure 3. The impact of various wood vinegar concentrations on motherwort, Spanish needles, redroot pigweed, and tall fescue. Vertical bars represent standard errors of the mean (n = 8).

3.3. Impact of Wood Vinegar Application Rate on Motherwort, Spanish Needles, and Tall Fescue

An increased wood vinegar application rate increased the control of motherwort, Spanish needles, and tall fescue, as shown in Figure 4. According to the regression analysis, the I₅₀ values measured 1911 L ha⁻¹ and 653 L ha⁻¹ for motherwort and Spanish needles, respectively, while the highest evaluated application rate at 4000 L ha⁻¹ only induced 32% tall fescue visual injury.

Figure 4. Motherwort, Spanish needles, and tall fescue injury at 10 days after wood vinegar treatments. Vertical bars represent standard errors of the mean (n = 8).

Figure 4. Motherwort, Spanish needles, and tall fescue injury at 10 days after wood vinegar treatments. Vertical bars represent standard errors of the mean (n = 8).

3.4. Impact of Light and Wood Vinegar Concentrations on Common Purslane Seed Germination

As shown in Figure 5, under both dark and light conditions, the seed germination rates decreased as wood vinegar concentrations increased. Based on the regression analysis, the C₅₀ values measured 0.49% (±0.8% for lower and upper 95% confidence interval bounds) and 0.61% (±1.2% for lower and upper 95% confidence interval bounds) under dark and light conditions, respectively. When wood vinegar was 1%, no seeds were germinated under either dark or light conditions.

Figure 5. Impact of light and wood vinegar concentrations on common purslane seed germination.

Figure 5. Impact of light and wood vinegar concentrations on common purslane seed germination.

3.5. Impact of Light Condition and Wood Vinegar Application Rate for Control of Common Purslane

Rapid foliage wilting was observed following the treatment of wood vinegar. Under the light condition, an increased application rate increased the control of common purslane. Wood vinegar at 500 L ha⁻¹ provided 51%, 68%, and 83% control of common purslane, while 1000 L ha⁻¹ provided 78%, 95%, and 99% control at 1, 3, and 5 DAT, respectively (

Table 2. Common purslane control following the wood vinegar application at 1 and 3 DAT under light condition.

Wood Vinegar Rate (L ha−1)	Common Purslane Control
	1 DAT	3 DAT
	%
500	51 c	68 c
1000	70 b	79 b
2000	78 a	94 a
p-value	<0.0001	<0.0001

Different letters within a column indicate significant difference at the 0.05 significance level. Abbreviation: DAT, days after treatment.

and

Table 3. Common purslane control following wood vinegar application at 5 DAT.

		Weed Control	Aboveground Biomass
		%	% of nontreated control
Light condition	Dark	95 a	57 a
	Light	87 b	79 b
Wood vinegar	500 L ha−1	83 a	62
	1000 L ha−1	91 b	69
	2000 L ha−1	99 c	75
Light condition		<0.0001	<0.0001
Wood vinegar		<0.0001	0.9382
Light condition × wood vinegar		0.0612	0.4622

Different letters within a column indicate significant difference at the 0.05 significance level. Abbreviation: DAT, days after treatment.

). Light condition by wood vinegar rate interaction was not detected for visual injury and shoot biomass reduction data, and thus, only the main effects are presented (Table 3). Wood vinegar was more effective under dark than light conditions (Figure 6). At 5 DAT, wood vinegar caused 95% and 87% visual injury under dark and light conditions, respectively; shoot biomass was 57% of the nontreated control under dark compared to 79% under light conditions.

4. Discussion

Developing environmentally friendly and naturally derived pesticides are currently being encouraged in China [41]. The results of the present study confirmed the effectiveness of using wood vinegar as an herbicide for the control of weeds. Acetic acid is consistently noted as the largest proportion of organic compound in wood vinegar [33,34,35,42], along with minor proportions of other organic acids, including butanoic and propanoic acids [33,34]. Therefore, acetic acid likely is the primary active ingredient causing the phytotoxic effect.

This study suggests that low concentrations of wood vinegar could effectively inhibit weed seed germination. This is the first report that confirmed wood vinegar could inhibit seed germination. Although wood vinegar had an inhibitory effect on germinating seeds, it may rapidly dissipate in the soil and therefore does not have residual activity for weed control; however, this assumption needs to be further verified in field conditions. An additional study is required to evaluate wood vinegar efficacy on seed viability when it is applied to dormant seeds.

Previous research focused on using wood vinegar as a foliar-applied herbicide for controlling weeds [33,34,35,40]. Aguirre et al. [33] applied a 25% concentration of wood vinegar onto plant foliage, which did not effectively control white mustard (Sinapis alba L.), while a 50% or higher concentration provided excellent control. In the present study, wood vinegar at low concentrations substantially damaged newly germinated seedlings. This finding is inconsistent with Liu et al. [34], who reported that when wood vinegar was drenched in soil, a substantial control reduction was observed for controlling matured annual bluegrass at 3–5 tillers. The greater weed control observed in the present study compared to previous observations reported by Liu et al. [34] is likely due to the following reasons: (1) soil particles, such as sand, silt, clay, and/or organic matter, may decrease the herbicidal activity of wood vinegar, and (2) wood vinegar is more effective on newly germinated seedlings compared to matured weeds. However, these assumptions need to be further verified.

Interestingly, in the present study, wood vinegar was more effective under dark than light condition for controlling common purslane. However, we cannot provide an adequate explanation for this observation. In fact, synthetic nonselective contact herbicides, such as diquat, glufosinate, and paraquat, are light-dependent and exhibit higher efficacy in light than dark. These herbicides often provided erratic weed control due, in part, to the influence of climate factors, including light, and as a result, the time of day of spraying these herbicides is important [43,44,45]. Overall, our results showed that wood vinegar acted as a light-independent herbicide and was more effective under dark than light conditions, suggesting that wood vinegar application timing is flexible and could be sprayed at night for weed control.

Acetic acid is expensive for weed control. For example, in the United States, although acetic acid could be used as an organic herbicide for the management of weeds [5,23], the application of 5% or 20% of acetic acid would cost 1107 USD and 4426 USD per hectare, respectively [23]. The present study suggests that wood vinegar may be an alternative to acetic acid to control weeds. However, it should be noted that wood vinegar is presently not registered as an herbicide in China. Before legal use, comprehensive research is needed to examine the utilities of wood vinegar, including the application rate, method, and adjuvant for weed control in various cropping systems. In addition, a careful review of the current literature suggests that the impact of wood vinegar on soil fertility, chemistry, microbial community, and non-target organisms is yet to be investigated and warrants further investigation.

Previous research indicates that application timing and wood vinegar rate are important factors for weed control, and plant species demonstrated a varying degree of tolerance to wood vinegar. For example, Aguirre et al. [33] reported that wood vinegar at a concentration ranging from 25% to 100% only caused transient injury on silver wattle (Acacia dealbata Link), a woody perennial plant, whereas white mustard, an annual broadleaf weed species was completely desiccated. Liu et al. [34] found that perilla mint exhibited greater susceptibility to wood vinegar than creeping woodsorrel. However, in another study, the application of 0.2% and 0.5% wood vinegar promoted shepherd’s-needle (Scandix pecten veneris L.) seedling emergence rate and growth, while inhibiting large venus’s-looking-glass (Legousia speculum-veneris L. Chaix) growth [46]. In this study, at all evaluated application rates, tall fescue demonstrated a substantially greater tolerance level than motherwort and Spanish needles; however, the application rate that provided adequate control of motherwort and Spanish needles also significantly injured tall fescue turf. An additional study is needed to evaluate the control of annual weeds in woody perennial crops.

Wood vinegar might be utilized to control problematic and herbicide-resistant weed species. For example, herbicide-resistant Amaranthus species, including redroot pigweed evaluated in this study, have been frequently documented in the literature [47,48,49,50,51]. In China, enhanced herbicide metabolism and target-site mutation caused redroot pigweed population’s resistance to multiple herbicide modes of action [52,53]. Therefore, wood vinegar may fit into a niche because of the lack of effective synthetic herbicides for controlling herbicide-resistant weed populations.

Overall, this research demonstrated that wood vinegar could be used as an herbicide for controlling weeds. These findings have a significant implication for managing pruned fruit tree branch wastes. Each year, a vast amount of pruned branch waste is produced in China. Using wood vinegar as an herbicide may offer a solution to dealing with pruned wood waste. Further research is needed to evaluate the efficacy of wood vinegar for controlling difficult-to-control and herbicide-resistant weed populations.