Mulch and Organic Herbicide Combinations for In-Row Orchard Weed Suppression

Abstract

Trials were established at two locations to evaluate weed control provided by mulches and organic herbicides alone or in combination. Mulch treatments included barley straw, pine wood chips, paper, and no mulch (bare-ground control). Herbicide treatments included three organically certified herbicides: lemon grass oil (14% v/v), clove oil (8% v/v), and two rates of acetic acid (10 and 15% v/v). Other treatments included pelargonic acid (7% v/v), glyphosate (1.06 kg ai/ha), and no-herbicide (control). Individual herbicide treatments were applied according to weed pressure and were determined every 7 to 10 days. Herbicide applications were made two and three times in 2009 at Logan and Kaysville, respectively. Treatments were carried out for a second year at both locations with herbicide treatments applied a maximum of four times. A third trial was established in 2010 at Kaysville with four application times. Weed control evaluations were made once a month, and weed population densities were evaluated at the beginning and end of the season. Weed biomass and percent weed cover were collected at the end of the 2010 season. Mulches each provided significant weed suppression in the first year, but each mulch presented unique challenges over time. For example, paper mulch effectiveness decreased over time as cracks developed during wet-dry cycles allowing weed seedlings to emerge. Volunteer barley became problematic in straw mulch plots in the first year of each trial, and annual weed seed germination on top of the wood chip mulch became problematic in the second year. The straw mulch consistently provided among the highest weed control and the least end-of-season weed biomass. Glyphosate and pelargonic acid gave near 100% weed control among all mulch treatments, including the non-mulched plots. Lemon oil and clove oil displayed the same weed control ratings as the no-herbicide treatment in the three mulches (straw, wood chip, and paper), but displayed 41–95% weed control when applied without mulch. While organically certified herbicides generally did not provide sufficient weed control alone, some mulches and organic herbicide combinations provided weed suppression similar to conventional herbicide application.

Keywords:

• weed suppression
• clove oil
• lemon grass oil
• acetic acid
• straw mulch
• paper mulch
• integrated weed management

INTRODUCTION

Weed management in orchards prevents unnecessary competition with fruit-bearing trees. Greater yields and fruit quality are attainable when competition with weeds is eliminated (Merwin, 2004). Parker et al. (1993) demonstrated that peach tree growth, expressed as change in trunk cross-sectional area (TCSA), was greater in a weed-free environment than a weedy environment. Other research documented that weed pressure in a young peach orchard reduced TCSA compared to a weed-free environment (Belding et al., 2004). Competition was also detected between apple trees and slow growing living mulches as compared to a weed-free control (Hoagland et al., 2008). According to Merwin (2004), a single young apple tree needs at least 2 m² of bare ground to prevent negative impacts on yield and tree growth. Timing of weed control is also important to reduce weed competition during important growing stages of the tree. Al-Hinai and Roper (2001) showed that weed pressure during the spring and early summer reduced tart cherry tree growth, and post-harvest weed growth reduced yield the following year.

Typical weed control in conventional orchards of the Intermountain West region of the United States is comprised of a bare ground strip maintained in the tree row, and a grass alleyway between tree rows. Glyphosate and 2,4-D are typically applied three to four times during the season to control vegetation in the tree row. In some cases, pre-emergence herbicides are used instead of or in addition to the glyphosate and 2,4-D combinations. A glyphosate and 2,4-D tank mixture has been shown to be effective in controlling weeds and maximizing fruit yield (Parker et al., 1993; Sirrine et al., 2008). In organic orchards, cultivation in the tree row is a common weed control method (Granatstein and Sanchez, 2009). Cultivation is effective for weed control (Reighard and Newall, 1993), but can damage tree roots that are close to the soil surface, and reduce root number and root mass of peach trees (Parker et al., 1993).

In organic systems, a major limitation to successful weed control is that synthetic herbicides are not allowed (Baier and Gegner, 2008). However, several non-synthetic herbicides have been approved for use in organic systems. All the current organic certified herbicides kill weeds through membrane disruption, and provide no residual weed control (Dayan et al., 2009). An organic approved herbicide with clove oil as the active ingredient effectively controlled broadleaf weeds when applied at high concentrations, but did not effectively control some grass species (Boyd and Brennan, 2006). Acetic acid, the active ingredient in a recently approved organic herbicide (Weed Pharm, Port Townsend, WA, USA), has been used to control weeds in field crops (Young, 2004). However, acetic acid and clove oil are expensive to use because a large volume of the product is required to provide adequate control of small weeds (Dayan et al., 2009).

Mulches can also effectively control weeds by preventing light from reaching weed seeds and creating a physical barrier to germination. Compost mulch has been shown to suppress weeds in the tree row (Brown and Tworkoski, 2004). Mulches can also have positive secondary impacts, such as increasing the biodiversity of arthropods (Brown and Tworkoski, 2004), or decreasing evaporation from the soil surface (Monks et al., 1997). Wood chip mulch has been shown to improve tree growth during orchard establishment (Smith et al., 2000). Alfalfa hay mulch caused reduced tree growth but increased total nitrogen content in apple leaves. Alfalfa hay also increased the volumetric water content of the soil but proved ineffective at controlling quackgrass, a vigorous weed (Stefanelli et al., 2009). In a study conducted in a newly planted apple orchard in Washington state, the greatest tree growth as determined by TCSA occurred in a wood chip mulch treatment compared to living mulches in the tree row (Hoagland et al., 2008). However, weed control using mulches alone is not always effective, especially if the mulch itself contains weed seeds or a source of wind-blown seed is deposited on the surface of the mulch. Little is known about utilizing combinations of organic herbicides and mulches for in-row orchard weed suppression. Using a combination of mulch and organically approved herbicides may increase the efficacy of both treatments while reducing the cost of the weed control program. The objective of this research was to evaluate the effectiveness of organic herbicides and mulches alone and in combination for weed control.

MATERIALS AND METHODS

Replicated field trials were established to evaluate mulch and herbicide treatments in narrow strips similar to that found in orchard tree rows. Two research sites were located at the Kaysville Research Farm, Kaysville, UT (41.01 N latitude, 1330 m elevation; Kaysville #1 and Kaysville #2) and one research site at the Greenville Research Farm in North Logan, UT (41.46 N latitude, 1385 m elevation; Logan). The Kaysville site has a Kidman fine sandy loam soil with a pH of 7.5 and 1.5% organic matter. The Logan location has a Millville silt loam soil with a pH of 7.9 and 1.5% organic matter. Sites were prepared by disking the ground multiple times, followed 2 weeks later with roller packing to create a firm seed bed. Each trial was arranged in a split-plot design with mulch as the whole plot and herbicide as sub-plot treatments. All treatments were replicated in four blocks, with blocking by field location. Sub-plots measured 1.2 m × 12.2 m to simulate a typical weed-free tree row in an orchard environment.

Mulch treatments included barley (Hordeum vulgare) straw, pine wood chips, paper, and a non-mulched bare-ground control. The wood chips were not composted. Mulches were applied July 15, 2009 at Logan, July 22, 2009 at Kaysville #1, and April 19, 2010 for the Kaysville #2 experiment. Straw was applied at a 15 cm thickness and wood chips were applied to a thickness of 6 cm. Paper mulch was applied as slurry with hydro-seed equipment at 2.18 Mg·ha⁻¹ or ≈1 cm thickness, in ≈72,700 L·ha⁻¹ of water.

Herbicide treatments included three organically certified herbicides; lemon grass oil (GreenMatch, Marrone Bio Innovations, Davis, CA, USA), clove oil (Matran, Ecosmart Technologies Inc., Ames, IA, USA), glacial acetic acid (Fisher Scientific, Pittsburgh, PA, USA), and two additional herbicides; pelargonic acid (Scythe, Mycogen Corporation, San Diego, CA, USA); and glyphosate (Roundup Powermax, Monsanto, St. Louis, MO, USA). Glacial acetic acid was used because no local source of the acetic acid herbicide formulation could be found. Pelargonic acid was included as a treatment because it was being considered for organic certification at the time. A glyphosate treatment and a no-herbicide treatment were included to allow comparison to a conventional industry standard herbicide and a non-treated control. Herbicides were applied using a CO₂-pressurized backpack sprayer calibrated to deliver 561 L·ha⁻¹ at 278 kPa pressure through three flat fan nozzles (8004, Tee-jet, Wheaton, IL, USA). Lemon grass oil, clove oil, and pelargonic acid were applied at spray solution concentrations of 14, 8, and 7% v/v, respectively. Acetic acid was applied at 10 or 15% v/v spray solution concentration. Glyphosate applications were made at 5.67 g ai·L⁻¹ in 187 L·ha⁻¹ of spray solution (1.06 kg ai/ha), except for the first two application dates at Logan and Kaysville #1 in 2010 where it was applied at 1.89 g ai·L⁻¹ in 561 L·ha⁻¹ (1.06 kg ai/ha). Individual herbicide treatments were applied according to weed presence, and were determined every 7 to 10 days. Herbicide treatments were applied between 11 a.m. and 2 p.m. when wind speed was below 8 km·h⁻¹. Plots were irrigated with 4 cm of water at weekly intervals.

Plots were visually inspected for weed presence, and herbicide applications were omitted if there were no weeds present or a few small seedlings were present that could be controlled by a later application. Application dates are listed in Table 1. Weed control was evaluated at the end of the establishment year. Visual evaluations of control of each weed species were determined by comparing treated plots to the untreated control (no-mulch and no-herbicide).

TABLE 1 Herbicide Application Dates ^z

Logan	Kaysville #1	Kaysville #2
First year
Aug. 13 ^y	Aug. 10 ^y	June 3 ^x
Sept. 3	Aug. 25	June 22
	Sept. 11	July 13
		Aug. 11
Second year
May 26	Apr. 9 ^y
June 15	May 19
June 29	June 8
July 21	June 23
^zLogan and Kaysville #1 had herbicide treatments implemented in 2009. Kaysville #2 was initiated in 2010. Second year treatments were later at Logan because of a shorter growing season.
^yGlyphosate treatment was not applied because of sufficient weed control.
^xHerbicide treatments were not applied to the wood chip mulch because of sufficient weed control.

Year 2

Mulch and herbicide treatments were continued a second year at the Logan and Kaysville #1 sites. Paper mulch was reapplied at Kaysville but not at the Logan site. No additional material was applied for any of the other mulch treatments. Herbicide treatments were applied a total of four times in Logan and Kaysville #1 (Table 1), with application conditions as described for the establishment year. Weed densities, percent cover, and destructive biomass samples were collected at the end of the season. Weed population densities were determined by randomly sub-sampling the plot with a 0.25 m² quadrat. Biomass samples were collected from the same random sub-sampled 0.25 m² as the weed densities. Percent weed cover was visually evaluated by the same person for the entire experimental plot.

Cost

Treatment costs were calculated for each herbicide, mulch, and combination to determine cost effectiveness. Material costs were gathered from local farm co-ops and suppliers. Labor costs for applying mulches were not included because of the potential for using many different application methods.

Statistical Analysis

Data were analyzed using the GLM procedure in SAS (version 9.2, SAS Institute Inc., Cary, NC, USA). Data that did not meet the statistical assumptions of normality and homoscedasticity were transformed using the log, arcsine, logit, or square-root transformations. For clarity, non-transformed data are presented in the results. Due to large numbers of 100% ratings, data normality could not be achieved with transformations of the Kaysville #2 end-of-season control data, but results are still included.

RESULTS AND DISCUSSION

Analysis of the first year data revealed significant site-by-treatment interactions, so locations were analyzed separately. During trial establishment, summer annuals dominated the experimental plots (data not shown). Common lambsquarters (Chenopodium album L.) was the most prevalent summer annual weed in all three locations, and was used as an indicator of weed control. Significant herbicide-by-mulch interactions were found in all three locations for season-end common lambsquarters control (Table 2).

TABLE 2 Common Lambsquarters Control (%) at the End of Establishment Year in Response to Mulch and Sequential Herbicide Treatments; Common Lambsquarters Were Selected for Comparison of Treatment Effects Because of Uniform Presence in All Locations ^z

Mulch	Herbicide	Concentration^y	Logan	Kaysville #1	Kaysville #2
No-mulch	No-herbicide	n.a.	0d ^x	0c	0h
	Acetic acid	10% v/v	78bc	51b	61fg
	Acetic acid	15% v/v	85abc	80ab	71def
	Lemon grass oil	14% v/v	68c	95ab	41g
	Clove oil	8% v/v	65c	56b	75cdef
	Pelargonic acid	7% v/v	100a	100a	99a
	Glyphosate	5.67g ai/L	100a	89a	95abc
Straw	No-herbicide	n.a.	100a	78ab	64ef
	Acetic acid	10% v/v	100a	100a	99a
	Acetic acid	15% v/v	100a	100a	99abc
	Lemon grass oil	14% v/v	100a	100a	93abc
	Clove oil	8% v/v	100a	100a	99a
	Pelargonic acid	7% v/v	100a	100a	100a
	Glyphosate	5.67g ai/L	100a	100a	100a
Wood chips	No-herbicide	n.a.	70c	96a	84abcde
	Acetic acid	10% v/v	98ab	100a	87abcd
	Acetic acid	15% v/v	100a	100a	90abcd
	Lemon grass oil	14% v/v	96ab	100a	95abc
	Clove oil	8% v/v	93ab	91a	95abc
	Pelargonic acid	7% v/v	100a	100a	100a
	Glyphosate	5.67 g ai/L	100a	98a	99ab
Paper	No-herbicide	n.a.	98ab	58b	79bcdef
	Acetic acid	10% v/v	86ab	100a	96ab
	Acetic acid	15% v/v	100a	96a	89abcd
	Lemon grass oil	14% v/v	99a	96a	83abcde
	Clove oil	8% v/v	100a	98a	99ab
	Pelargonic acid	7% v/v	100a	100a	98a
	Glyphosate	5.67g ai/L	100a	99a	100a
Analysis of variance			(P)
Mulch			<0.0001	<0.0001	<0.0001
Herbicide			<0.0001	<0.0001	<0.0001
Mulch × Herbicide			<0.0001	<0.0016	<0.0001
^zMeans within a column followed by the same letter are not significantly different (P > 0.05) according to LSD.
^xNo-herbicide no-mulch combination was defined as 0% weed control in visual ratings.
^yHerbicide concentrations (v/v) were delivered in 561 L·ha^−1, except glyphosate at 187 L·ha^−1.

At the Logan site, glyphosate and pelargonic acid provided complete lambsquarter control across all mulch treatments. Among the remaining herbicides, acetic acid provided the highest weed control, where the high rate of acetic acid provided control that was statistically similar to that of glyphosate across all mulch treatments. Lambsquarter control with the low concentration acetic acid only differed from glyphosate in the absence of mulch. Lemon grass oil, clove oil, and low concentration acetic acid did not differ from glyphosate treatments, except in the case of bare ground where control was 68, 65, and 78%, respectively. In the absence of herbicides, straw and paper mulch provided significantly higher common lambsquarters control compared to wood chip mulch or no-mulch (Table 2).

At the Kaysville #1 site, first-year results were similar to the Logan site, with a few notable exceptions. In the absence of herbicides, wood chip mulch provided the best lambsquarter control (96%), whereas paper mulch was significantly less effective. In the absence of mulch, lemon grass oil and the high rate of acetic acid provided lambsquarter control similar to conventional herbicides, whereas low concentration acetic acid and clove oil provided lower control at 51 and 56%, respectively (Table 2). Kaysville #2 showed similar results to Kaysville #1, where in the absence of herbicides, wood chip mulch provided the best weed control, followed by paper mulch and straw mulch. In the absence of mulch, lemon grass oil herbicide provided significantly less weed control than the clove oil or high concentration acetic acid.

At all three sites, volunteer barley (Hordeum vulgare L.) was problematic in the straw mulch plots. Control of volunteer barley varied among herbicides with glyphosate providing the greatest control at Logan and glyphosate or pelargonic acid providing among the greatest control at Kaysville #1 and Kaysville #2 (Fig. 1). At Logan, acetic acid treatment and pelargonic acid provided greater barley control than lemon grass or clove oil treatments. At Kaysville #1, acetic acid at 15% was better than acetic acid at 10% or lemon grass oil, but similar to clove oil.

FIGURE 1 Volunteer barley control in response to multiple applications of organic and conventional herbicides during the establishment year at Logan, Kaysville #1, and Kaysville #2 sites. Data were collected on Sept. 22, 2009, Aug. 31, 2010, and Sept. 17, 2009 at Kaysville #1, Kaysville #2, and Logan sites, respectively. Letters designate within-site differences (P = 0.05) with means separated by LSD.

For first-year weed control, glyphosate was the most consistent herbicide across all locations and among all mulch treatments, followed by pelargonic acid. Among organic herbicides, high concentration acetic acid was the most consistent across location and mulch treatments. In combination with wood chip and paper mulches, clove oil and lemon grass oil provided weed control that was statistically similar to glyphosate. In the absence of mulch, however, weed control by clove oil and lemon grass oil was inconsistent across locations.

Year 2

Treatments were continued for a second year at the Logan and Kaysville #1 sites. In the second year of the trial, there was a species shift from summer annuals to winter annuals with prickly lettuce the prevalent weed in both locations. Total weed control, total weed densities, total percent weed cover, and total weed biomass were evaluated to include minor populations of field bindweed (Convolvulus arvensis L.), common lambsquarters, and volunteer barley. The Logan site also included kochia (Kochia scoparia L.).

Weed control was generally higher where the organic herbicides were used in combination with mulch (Table 3). In the absence of mulch, glyphosate and pelargonic acid provided the highest level of weed control. The high concentration of acetic acid provided weed control that was numerically lower but statistically similar to glyphosate. There were no significant differences among three of the four organic herbicide treatments (acetic acid 10%, lemon grass oil, and clove oil). Straw mulch generally increased the total weed control of all the organic herbicides. In the absence of herbicide, wood chip mulch showed lower total weed control ratings than straw mulch at both locations. At Logan, the combination of wood chip mulch and either clove oil or lemon grass oil provided weed control ratings ≤25%.

TABLE 3 Total Weed Control (%) at Logan and Kaysville #1 in Response to Mulch and Sequential Herbicide Treatments Evaluated During the Second Year of the Trial ^z

Mulch	Herbicide	Concentration^y	Logan	Kaysville #1
No-mulch	No-herbicide	n.a.	0l ^x	0i
	Acetic acid	10% v/v	44ghijk	44h
	Acetic acid	15% v/v	64defg	60fgh
	Lemon grass oil	14% v/v	46ghijk	46h
	Clove oil	8% v/v	26jk	53gh
	Pelargonic acid	7% v/v	87abcd	87abcd
	Glyphosate	5.67 g ai/L	96abcd	74cdefg
Straw	No-herbicide	n.a.	62efg	83abcde
	Acetic acid	10% v/v	75bcdef	84abcde
	Acetic acid	15% v/v	89abcd	80abcdef
	Lemon grass oil	14% v/v	81abcde	66feg
	Clove oil	8% v/v	79abcde	81abcdef
	Pelargonic acid	7% v/v	96abc	71defg
	Glyphosate	5.67g ai/L	99a	89abc
Wood chips	No-herbicide	n.a.	0l	53gh
	Acetic acid	10% v/v	52fghi	84abcd
	Acetic acid	15% v/v	55fghi	78bcdef
	Lemon grass oil	14% v/v	22kl	67efg
	Clove oil	8% v/v	25jk	72defg
	Pelargonic acid	7% v/v	91abc	81abcdef
	Glyphosate	5.67g ai/L	96abc	84abcde
Paper	No-herbicide	n.a.	32ijk	77bcdef
	Acetic acid	10% v/v	73bcdef	91abc
	Acetic acid	15% v/v	59efgh	90abc
	Lemon grass oil	14% v/v	59efgh	83abcde
	Clove oil	8% v/v	33ijk	65fgh
	Pelargonic acid	7% v/v	86abcd	92ab
	Glyphosate	5.67g ai/L	95abc	95a
Analysis of variance			(P)
Mulch			<0.0001	<0.0001
Herbicide			<0.0001	<0.0001
Mulch × Herbicide			<0.0039	<0.0001
^zMeans within a column followed by the same letter are not significantly different (P > 0.05) according to LSD.
^xNo-herbicide no-mulch combination was defined as 0% weed control in visual ratings.
^yHerbicide concentrations (v/v) were delivered in 561 L·ha^−1, except glyphosate at 187 L·ha^−1.

The effectiveness of mulch treatments differed among locations. Wood chip mulch alone was effective at Kaysville #1, but not at Logan. A very large prickly lettuce (Lactuca serriola L.) seed source was adjacent to the experimental plots at the Logan site, and prickly lettuce seeds seemed to germinate well on top of the wood chip mulch. The wood chip mulch treatment combinations showed similar trends at Kaysville #1, where the lemon grass oil was significantly less effective than the 10% acetic acid and was not significantly different from the no-herbicide control. In the paper mulch treatments at both locations, clove oil herbicide did not significantly increase weed control compared to paper mulch alone. When combined with any mulch, acetic acid at either rate provided results similar to glyphosate and pelargonic acid at Kaysville #1. In the absence of mulch, however, acetic acid treatments resulted in lower weed control than glyphosate or pelargonic acid (Table 3).

Data for second-year total weed density and total weed cover did not show significant mulch-by-herbicide interactions, and main effect means are presented. In Logan, weed densities were lowest in the straw mulch treatments compared to no-mulch, wood chips, and paper mulch (Table 4). Glyphosate and pelargonic acid had the lowest weed densities, while no-herbicide, lemon oil, and clove oil treatments had among the highest densities. In Kaysville, straw and paper mulch lowered weed densities compared to no-mulch and wood chip mulch. Pelargonic acid and 15% acetic acid resulted in the lowest weed densities compared to all other treatments.

TABLE 4 The Effect of Mulch and Herbicide Treatments at the Kaysville #1 and Logan Sites on 2nd-Year Weed Densities and Weed Cover; the Mulch × Herbicide Interactions Were Not Significant, and Main Effect Means Are Presented ^z

		Total weed densities (no./m^2)		Total weed cover (%)
Treatment	Conc. ^y	Logan	Kaysville #1	Logan	Kaysville #1
Mulch main effects
No-mulch	—	36.6a	55.9a	35.8a	22.8a
Straw	—	9.3b	16.7c	5.9b	8.1bc
Wood	—	27.1a	30.3b	38.0a	12.1bc
Paper	—	25.1a	11.0c	23.5a	4.1c
Herbicide main effects
No-herbicide	—	40.5a	39.8a	55.4a	19.6a
Acetic acid	10% v/v	25.0b	28.3ab	25.2b	9.8b
Acetic acid	15% v/v	24.5b	20.5bc	14.2bc	8.0b
Lemon oil	14% v/v	29.9ab	37.3ab	27.3b	19.2a
Clove oil	8% v/v	28.8ab	32.0ab	36.5b	11.4a
Pelargonic acid	7% v/v	13.3c	12.5c	11.5c	9.3b
Glyphosate	5.67 g ai/L	5.3c	30.3ab	7.2d	5.3b
^zMeans within a column and main effect grouping followed by the same letter are not significantly different according to the LSD test at p = 0.05.
^yHerbicide concentrations are on a volume basis and were applied in 561 L·ha^−1, except glyphosate, which was applied in 187 L·ha^−1.

Results of total weed biomass were similar for the two locations. In Logan, straw mulch had the lowest weed biomass compared to bare ground, wood chip, and paper mulch treatments (Fig. 2). Among herbicide treatments, weed biomass was the least for glyphosate and pelargonic acid at Logan. Similar results were found at Kaysville #1 except that paper mulch reduced weed biomass similar to the straw mulch treatment. Differences in the paper mulch response between the two locations were likely due to a second application of paper mulch at the beginning of the 2010 growing season at Kaysville. In general, clove oil and lemon grass oil herbicide treatments resulted in the highest weed biomass in both locations, but results were statistically similar to the acetic acid treatment.

FIGURE 2 Main effects of mulch or sequential herbicide applications on weed dry biomass after two seasons. Biomass was collected on July 20, 2010 and Aug. 3, 2010 at Kaysville #1 and Logan sites, respectively. Within a main effect, bars labeled with different letters are significantly different according to LSD at P = 0.05.

Averaged over mulches, the organic herbicide treatments in Logan had higher weed biomass (Fig. 2), weed densities, and weed cover (Table 4) than the conventional herbicides. While some of these trends were also true at the Kaysville #1 site, there were fewer significant differences there between organic and conventional herbicide effects. Pelargonic acid provided higher weed control as measured by weed biomass compared to all other treatments, except for glyphosate. Weed densities in pelargonic acid-treated plots were lower than all other treatments except for acetic acid at the high rate. Glyphosate, pelargonic acid, and both rates of acetic acid reduced weed cover compared to the other treatments.

In most cases, organic herbicides increased the level of weed control provided by the mulches. In the Kaysville #1 trial, straw mulch alone suppressed weeds as well as the straw mulch + glyphosate combination. With straw mulch providing high levels of weed suppression, herbicides may not be needed during subsequent years. Other researchers have found that straw mulch effectively controlled weeds without supplemental herbicides. In an apple orchard in India, rice straw mulch effectively controlled weeds compared to no-mulch treatment (Ramakrishna et al., 2006). Straw mulch effectively controlled weeds in potatoes and watermelons in Georgia, USA when applied at planting, but when applied 4 weeks after planting, weeds were not suppressed (Johnson et al., 2004). Hay mulch used in tomato plots in Virginia, USA had lower weed biomass compared to plastic and paper mulch (Schonbeck, 1999).

Effective weed control was also attained using the paper mulch without added herbicides in Kaysville #1. Paper mulch was successfully used to suppress weeds in annual vegetable crops during 4 consecutive years, by reapplying the mulch each year (Runham et al., 1998). The best weed control in the paper mulch was attained at the Kaysville #1 site, where mulch was reapplied at the beginning of the second year. It was also observed in these trials that volunteer barley introduced with the straw mulch was more difficult to control than small annual weeds. Others have observed that grass species are difficult to control with organic herbicides (Boyd and Brennan, 2006), which illustrates the importance of using straw mulch that is free of viable seed.

Weed Control Costs

Based on costs from local sources, material costs were calculated for orchard situations common to the region. Material costs would differ according to the treated area in the orchard, and the treated area would be related to orchard row spacing. Tart cherry (Prunus cerasus L.) is the most common orchard crop in Utah, and typical row spacing is 5.5 m. Material costs for a tart cherry orchard ranged from $597 to $1771 per ha for straw and wood chip mulch, respectively (Table 5). Wood chips were the most expensive mulch due to a limited local supply. In production areas where there is continual removal and disposal of large trees, wood chips are a waste product and can be more readily available at little to no cost (Granatstein, personal communication). Paper mulch cost was similar to wood chip mulch, also related to limited local supply. A limitation to the use of mulches is that application requires specialized equipment that may not be readily available to many fruit growers. Wood chip and straw mulches would best be applied with a side delivery applicator while paper mulch is best applied in a slurry, requiring specialized equipment. The economic viability of mulch use will depend on price and local availability of both the mulch material and application equipment.

TABLE 5 Estimated Materials Costs for Mulches in Typical Tart Cherry, Peach, and High Density Apple Orchards ^z

	Material amount (Mg·ha^−1)			Material cost ($/ha)
Mulch	Tart cherry	Peach	Apple	Tart cherry	Peach	Apple
Straw	7.5	8.4	11.2	597.94	672.75	896.91
Wood chip	32.1	36.2	48.2	1,771.67	1,993.33	2,657.50
Paper	1.5	1.6	2.2	1,441.46	1,621.80	2,162.19
^zCosts were based on local sources, assuming a 1.8 m vegetation-free strip, with row spacing of 5.5 m for tart cherry, 4.9 m for peach, and 3.7 m for apple. Labor cost was not included because of differing application methods.

Organic herbicides were extremely expensive at the rates used, compared to conventional herbicides (Table 6). Lemon grass oil was the least expensive organic herbicide treatment at $321 per ha, but provided the least consistent weed control. Acetic acid at 10% concentration was the next least expensive organic herbicide treatment at $356 per ha. Organic herbicide costs added significantly to treatment costs when used in combination with mulch. Careful weed monitoring and spot applications of organic herbicides could significantly reduce material cost (Buhler, 2002). For these studies, herbicides were banded over the treatment area and not spot applied.

TABLE 6 Estimated Cost of Herbicides Applied Four Times During The Growing Season ^z

			Cost per application ($/ha)
Treatment	Conc. ^y	Trade-name	Tart cherry	Peach	Apple
Acetic acid	10% v/v	Weed Pharm	356.40	405.00	540.00
Acetic acid	15% v/v	Weed Pharm	534.60	607.50	810.00
Lemon grass oil	14% v/v	GreenMatch	321.55	365.40	487.20
Clove oil	8% v/v	Matran	432.43	491.40	655.20
Pelargonic acid	7% v/v	Scythe	227.30	258.30	344.40
Glyphosate	5.67 g ai/L	Roundup	3.17	3.60	4.80
^zCosts were based on prices at local suppliers, assuming a 1.8 m vegetation-free strip with row spacing of 5.5 m for tart cherry, 4.9 m for peach, and 3.7 m for apple.
^yHerbicide concentrations are on a volume basis, and were delivered in 561 L·ha^−1, except glyphosate, which was delivered in 187 L·ha^−1.

The combination of mulches and organic herbicides was particularly expensive. In contrast to the expense of organic approaches, weed control in conventional orchards is one of the least expensive orchard maintenance inputs (Seavert et al., 2007). According to results presented here, the cost of some organic herbicides may not be justified as the herbicides did not contribute to significant increases in weed control. More organically approved herbicides are becoming available and independent research is required to test the effectiveness of these products in varied geographical locations and crops.

Organically registered herbicides have the potential to provide moderate weed control. However, when used in combination with mulch their weed control potential significantly increases. Total weed control was not attained even with the combinations of mulch and organic herbicides after the second year. In addition, mulches need to be reapplied every 1 to 2 years for adequate control, and this could lead to reduced N availability due to the high carbon-to-nitrogen ratio of the materials. Mulches control weeds best with good site preparation, and when applied before significant weed emergence. If this window of weed control is missed, then it may be too late to control weeds using mulches and an alternative control method such as tillage would be required. Competition during fruit crop establishment cannot be tolerated, as reduced tree growth during establishment can affect fruit production throughout the life of that tree. In addition, yield losses can be expected the following year if weed management is not adequate (Al-Hinai and Roper, 2001). Mulches may also have adverse effects on the trees, increasing winter vole damage, or resulting in increased Phytophthora root rot (Merwin, 2004; Wiman et al., 2009). Mechanization of mulch application is also required for feasibility on commercial scale operations. Implementation of this type of extensive weed control requires extra planning, extensive weed scouting, and the time to carry out the treatments (Granatstein and Sanchez, 2009). Other mulches may be available in different regions. In the south, for example, there is a large supply of cotton gin by-products, which may prove effective mulch materials in orchards. In areas where there is a large amount of recyclable material, it could potentially be used. Organic weed control is a dynamic topic that requires growers and researchers to think creatively and adapt their farming systems to local conditions and available resources.

ACKNOWLEDGMENTS

This article is a portion of a M.S. Thesis submitted by M. A. Rowley. The authors gratefully acknowledge the technical assistance of Kyle Roerig, James Frisby, and Thor Lindstrom. Funding for this project was provided by the USDA—Organic Research and Extension Initiative, the Utah State Horticulture Association, the Utah Department of Agriculture and Food Specialty Crop Block Grant program, and the Utah Agricultural Experiment Station—Utah State University (Journal paper number 8328).

Notes

Use of trade names does not imply an endorsement of the products named or criticism of similar ones not named.