Effects of Vegetation-Free Strip Width and Irrigation on Newly Planted Peach

ABSTRACT

Field experiments were conducted at two locations (Clayton and Jackson Springs, NC) to determine the influence of vegetation-free strip width (VFSW) and irrigation on newly planted peach growth and yield in a low-density orchard with a volunteer weedy ground cover. The experiments included VFSW of 0, 0.6, 1.2, 2.4, 3, or 3.6 m under irrigated or nonirrigated conditions. Seasonal variation in the orchard floor vegetation was observed as different weed species reported in summer and winter. However, this difference was not apparent with respect to VFSF and irrigation. At Jackson Springs, NC, the predicted irrigated VFSW which would produce the same trunk cross-sectional area (TCSA) as the grower standard (3-m nonirrigated) was 1.5, 1.3, and 0.8 m for one-, two-, and three-year-old trees, respectively. The predicted irrigated VFSW which would produce the same yield as the grower standard was 1.16 m. At Clayton, TCSA and fruit yield were not different by irrigation, but did increase linearly with VFSW. At both locations, leaf nitrogen (N) concentration was lower in irrigated trees than nonirrigated trees. Leaf N, leaf area, and SPAD were positively related to VFSW at Jackson Springs. In contrast, leaf N concentration was not different by VFSW at Clayton. However, leaf area and SPAD were positively related to VFSW at Clayton. These results suggest that a 1.5 m VFSW combined with proper irrigation and fertilization will produce tree growth and yield in newly planted orchard with volunteer weedy vegetation similar to the current grower standard in the southeastern USA.

KEYWORDS:

• Competition
• fruit tree
• orchard
• floor-management
• weed-control

Peach (Prunus persica L.) production in the United States totaled 850,000 tons of fruit and $600 million in crop value in 2015 on 40,400 ha (USDA-NASS, 2016). A hindrance to peach production in the southeastern USA is orchard floor vegetation in which weeds compete with peach trees for water and nutrients (Glenn and Welker, 1991). Removal of all vegetation in the orchard floor or only in tree rows with herbicides reduces vole and northern pocket gopher (Thomomys talpoides) populations and their damage (Sullivan and Hogue, 1987). The nonmanaged vegetation can also harbor spider mites (Tetranychus urticae) and insects such as tarnished plant bug (Lygus lineolaris) and stink bug (Halyomorpha halys) (Killian and Meyer, 1984; Meagher and Meyer, 1990; Meyer, 1984). Previous studies found that peach trees grown in orchards with weedy or sod floor have fewer total and deep roots and limited lateral root spread compared to trees grown in vegetation-free orchards (Parker et al., 1993; Parker and Meyer, 1996). Glenn et al. (1996) found that a sod barrier restricted the growth of peach trees by reducing the size of the root system. Weller et al. (1985) found fewer fibrous peach tree roots in the top 15 cm of soil and reduced fibrous roots 15–30 cm deep when subjected to bermudagrass (Cynodon dactylon L.) competition, compared to vegetation-free conditions.

Orchard floor management strategies for peach production include first establishing a permanent sod or allowing naturally occurring vegetation to establish (as is the current practice in southeastern USA) in the orchard floor to prevent erosion, maintain soil structure, and facilitate equipment movement during wet weather, and then establishing a vegetation-free strip in the tree row (Lane and Tan, 1988; Layne et al., 1994; Mitchem, 2005). Herbicides, living or straw mulch, tillage, weed fabric, and mowing are commonly used practices to maintain vegetation-free strip in orchards (Aldridge et al., 2018; Buckelew et al., 2018; Tebeau et al., 2017). Vegetation-free strip width (VFSW) has been reported to influence development and yield of tree and small fruits. In Cabernet Franc winegrape (Vitis vinifera L.), VFSW ≥ 0.6 m resulted in increased wine growth and fruit compared to VFSW < 0.6 m (Basinger et al., 2018b). Meyers et al. (2014) reported that yield in newly planted blackberry (Rubus spp. ‘Navaho’) increased from 718 to 1015 kg ha⁻¹ when VFSW increased from 0 to 2.4 m. During further extension of this study Basinger et al. (2018a) reported that in 5-yr-old blackberry widening the VFSW from the current recommendation of 1.2–1.8 m could provide growers with increased plant growth, berry biomass, and yield. Welker and Glenn (1985, 1989)) reported peach tree growth and fruit yield was proportional to the size of VFSW under low-density production system. The proximity of sod to peach tree root systems likely limited nitrogen (N) uptake and reduced fruit number and yield (Tworkoski et al., 1997). Glenn and Newell (2008) reported greater peach fruit number and yield with a 2.4 m VFSW compared with 0.6 m. Williamson and Coston (1990) found that reducing the herbicide strip from 1 to 0.5 m reduced yield in peach. However, most of these studies were conducted in low-density peach production system. So, these study conclusions could be differed under high-density peach production systems.

The advantage of supplemental irrigation in peach orchards has been observed during previous research (Glenn et al., 1996; Lane and Tan, 1988; Layne et al., 1994). Trickle irrigation was effective in overcoming the competition imposed by permanent creeping red fescue (Festuca rubra L.) sod strips in row middles (Lane and Tan, 1988). Irrigation of newly planted peach trees growing in an orchard floor of annual ryegrass (Lolium multiforum Lam.) with 3 m VFSW had larger trunk cross-sectional area (TCSA) by the second growing season than nonirrigated trees (Huslig et al., 1993). Glenn et al. (1996) reported that applying supplemental irrigation during fruit development stage increased yield of large fruit and leaf N concentration in peach. Photosynthetic rate and stomatal conductance were enhanced, and leaf defoliation delayed by irrigation in peach trees (Layne et al., 1994). They also reported that the best soil-management system in peach orchards was a combination of permanent creeping red fescue sod strips between rows and trickle irrigation in the tree row when TCSA, marketable yield, and tree survival were considered.

Competition studies involving VFSW have primarily used well maintained grass sod as an orchard ground cover (Belding et al., 2004; Glenn et al., 1996; Welker and Glenn, 1985, 1989) likely because these studies were located in climates conducive for sod growth. In South Carolina, Georgia, and North Carolina, the orchard floor is often covered by volunteer weedy vegetation as the climate is not conducive for sod growth. The preferred orchard floor management system by growers in the southeastern USA is a 3.0–3.6 m VFSW in the peach tree rows (Mitchem, 2005). However, little is known about how VFSW and irrigation may influence peach growth, yield, and fruit quality in an orchard consisting of a volunteer weedy ground cover between tree rows. Thus, the objectives of this study were to determine the influence of VFSW and irrigation on growth and yield of newly (one to three year-old) planted peach growing in an orchard consisting solely of a volunteer weedy ground cover between tree rows and to compare the results with current standard 3.0 m VFSW under nonirrigated condition used by growers in the southeastern USA.

Materials and Methods

Field experiments were conducted from 2006 to 2008 at Central Crops Research Station, Clayton, NC (35.65 °N, 78.46 °W) and Sandhills Research Station, Jackson Springs, NC (35.21 °N, 79.63 °W). The soil was Norfolk loamy sand (fine-loamy, Kaolinitic, thermic Typic Kandiudults) with pH 6.1 and 0.41% humic matter at Clayton, and Candor sand (sandy, Kaolinitic, thermic Grossarenic Kandiudults) with pH 5.8 and 0.60% humic matter at Jackson Springs. Dolomitic lime was applied at both locations before tree planting at 1123 kg ha⁻¹. At Jackson Springs, the field was fumigated with 331 kg ha⁻¹ of 1,3-dichloropropene. At both sites, the tree row and swathes 90° to the tree row where the trees would be planted were chisel plowed and then disked. Bare-root grown ‘Contender’ (freestone, fresh-market type) peach trees on ‘Guardian’ rootstock (Vaughn Nursery, McMinnville, TN), each 61 to 76 cm tall were hand planted at 5.5 m in-row and 6.1 m between-row spacing (298 trees ha⁻¹; low-density system) during April 2005 at Clayton and January 2006 at Jackson Springs. Clayton trees were at least nine months older than trees at Jackson Springs. At planting, the bottom 30 cm of each tree was painted with white latex paint for protection from contact herbicide. Trees were pruned each spring in an open-center form (Lockwood and Myers, 2005). Fertilization schedules for both studies were followed as recommended by Georgia Cooperative Extension Service for newly planted trees (Lockwood et al., 2005).

There were two experiments conducted, one each at Clayton and Jackson Springs. The design of each experiment was a randomized complete block with a split plot arrangement. Four and six replications were used at Clayton and Jackson Springs, respectively. The VFSW (0, 0.6, 1.2, 2.4, 3, and 3.6 m with half of each VFSW distributed on either side of the planted row) was considered as the main plot and two type of irrigation (irrigated or nonirrigated) as the subplot. Six VFSW were assigned to main plots (eight trees) that were selected randomly in each block. Each main plot was divided into two subplots (four trees) to apply irrigation treatment. Randomly selected, one subplot received irrigation while other subplot did not receive irrigation. The middle two trees in each subplot were used for data measurements and the outer two on each end were buffer trees. There were no tree rows used as a buffer between treated tree rows. The VFSW were maintained weed-free using 213.3 g ai (active ingredient) ha⁻¹ flumioxazin (Chateau® 51DWG, Valent USA Corp., Walnut Creek, CA) for preemergence weed control, and paraquat (Gramoxone Inteon®, Syngenta Crop Protection, Inc., Greensboro, NC) at 0.67 to 1.0 kg ai ha⁻¹ plus 0.25% v/v nonionic surfactant (X-77, Loveland Industries, Greely, CO) for postemergence weed control. When perennial grasses escaped control by flumioxazin or paraquat, either fluazifop (Fusilade DX, Syngenta Crop Protection, Inc., Greensboro, NC), sethoxydim (Poast, BASF Corp., Research Triangle Park, NC) or clethodim (Select, Valent USA Corp., Walnut Creek, CA) plus adjuvant (1% v/v crop oil concentrate) was used as needed for control (Mitchem, 2005). Rates of each herbicide was specific to the guidelines for control of perennial grasses listed on each label. All herbicides were applied with a CO₂-pressurized backpack sprayer at 220 to 234 kPa using a flat fan 8002XR nozzle (Teejet 8002XR, Teejet® Technologies) to apply a spray volume of 187 L/ha. Single, double, and triple nozzle booms were used according to the strip width being sprayed. Row middles were allowed to populate with naturally occurring weed species and were maintained by mowing to a height of 10–13 cm tall.

Soil moisture was measured in all plots twice weekly May through August with Watermark™ granular matrix sensors (Irrometer Company, Inc., Riverside, CA) set at 30.5 cm soil depth. These sensors were set at the second tree in each plot 46 cm from the tree trunk. Dan Modular microsprinkler (Jain Irrigation Inc., Fresno, CA) were placed 15 cm on the other side of the tree trunk on soil surface and 76–91 cm from the sensor, so that the sensor, trunk and microsprinkler were in a triangular arrangement. The microsprinklers delivered 70 L/h with a surface wetting pattern of approximately 6.0 m diameter. Irrigation was applied approximately for 12 h to deliver 2.5 cm of water at each irrigation event. The optimum soil water tension for a particular crop depends primarily on soil texture. Field capacity is 10 kPa or less for a sand (Gary Grabow, personal communication) and the irrigation range for peach trees in the southeastern USA is 20–60 kPa (Taylor and Rieger, 2005). Therefore, plots were watered when the soil moisture readings of the 3.0 m VFSW treatment were greater than or equal to 20 kPa. Irrigation at both locations began during early summer 2006 and applied each year from March to October as required. The precipitation and irrigation data is presented in Table 1.

Table 1. Precipitation and irrigation data for Clayton and Jackson Springs, NC from 2006 to 2008.

	Clayton^a						Jackson Springs^b
	Precipitation			Irrigation^c			Precipitation			Irrigation^c
	2006	2007	2008	2006	2007	2008	2006	2007	2008	2006	2007	2008
Month	^___________________________________________________________ cm ^_________________________________________________________
March	3.2	8.1	8.02	0	0	0	4.97	7.49	8.76	0	0	0
April	9.67	8.4	14.42	0	0	0	8.48	9.14	11.96	0	0	0
May	11.17	2.31	7.34	0	5.08	0	8.28	3.47	5.33	0	5.08	0
June	21.92	6.5	6.78	0	5.08	7.62	29.38	8.53	4.08	2.54	10.16	10.16
July	5.38	10.38	12.44	5.08	2.54	5.08	3.3	6.37	13.23	8.89	7.62	5.08
August	8.35	1.8	4.95	7.62	12.7	2.54	14.75	1.04	22.27	7.62	7.62	2.54
September	11.02	8	4.26	0	0	0	9.98	4.24	14.98	0	0	0
October	9.09	7.79	3.98	0	0	0	7.79	12.92	4.14	0	0	0

^aPrecipitation data were obtained from the weather station located at Central Crops Research Station in 2006 and 2008 and State Climate Office of North Carolina ECOnet Database in 2007.

^bPrecipitation data were obtained from the weather station located at the Sandhills Research Station, Jackson Springs, NC.

^cIrrigation was applied to irrigated plots only and was 2.54 cm of water at each time.

Data recorded included TCSA, winter prunings, SPAD, N concentration in tree leaves, and fruit yield, individual fruit biomass, soluble solids content (SSC), and firmness. The TCSA was measured 30 cm above the soil line during the dormant season. Winter prunings from two middle trees per plot were cut and fresh biomass measured during the first week of March 2007 and 2008, and represented tree growth from 2006 and 2007, respectively. Leaf area of ten randomly selected leaves per tree was measured with a LI-COR LI-3100 area meter (LI-COR, Inc., Lincoln, NE) on 8 May 2008 at Clayton and 29 Apr. 2008 at Jackson Springs. The SPAD measurements were recorded using a SPAD 502 m (Konica Minolta Business Solutions USA Inc., Ramsey, NJ) on 5 July 2007 and 4 July and 22 Aug. 2008 at Clayton and 5 July 2007 and 3 July and 20 Aug. 2008 at Jackson Springs. The average of five SPAD readings per tree was recorded from new, fully expanded leaves, approximately the sixth to eighth leaf from each shoot tip. SPAD measurements reflect leaf chlorophyll, and are used as a measure of relative leaf nitrogen content because leaf chlorophyll content and leaf nitrogen content are closely linked (Bullock and Anderson, 1998). Peach leaves were sampled in late July of every year for determination of N content.

Fruit were thinned on 9 May 2008 at Clayton and 13 May 2008 at Jackson Springs to one fruit per 18 cm of shoot. Fruits with a yellow or gold background color were hand harvested for four dates at both locations in 2008. The final harvest consisted of all fruit regardless of color. At every harvest, five random fruit from each tree were weighed to obtain average biomass per fruit. During third harvest, fruit firmness and SSC (°Brix) of fruits were measured with a digital refractometer (Atago U.S.A, Inc., Bellevue, WA).

To measure the seasonal diversity in weed species, orchard floor vegetation was harvested in early summer and early Fall 2007 and 2008 at both locations to measure dry biomass and N concentration (only from early Fall harvest). One 0.37 m² quadrat was randomly sampled around one of each plot’s buffer trees, and samples were only collected from three replications. Total vegetation per tree per 3.6 m swathe was extrapolated from these sample quadrates, with 3.6 m VFSWs having values of zero, and therefore were excluded from statistical analyses. In plots with 0- and 0.6-m VFSW, mowing left approximately a 0.6 m nonmowed swathe on each side of the tree, so these nonmowed areas were sampled additionally, and taken into account when calculating the total vegetation. From each quadrate, species were separated and dried at 55°C until dry biomass was constant.

Data were subjected to ANOVA using SAS PROC GLM (SAS 9.2, SAS Institute, Cary, NC) to test for the treatment effects and interactions. Year, date, and location were tested using appropriate error terms for significant interaction with VFSW and irrigation to determine if data could be combined where appropriate. Data was not combined over year or locations, if the any of the two or three-way interaction between year, location, or treatments (VFSW or irrigation) was reported significant at α = 0.05. The dependent variable was regressed separately on VFSW for each irrigation level, if the interaction between VFSW and irrigation was significant at α = 0.05. However, when this interaction was not significant and effect of VFSW was significant at α = 0.05, combined means over both irrigation levels were regressed to describe trends. The ANOVA for testing of linear, lack of fit to linear, and quadratic relationships determined the type of regression or trend line. When linear relationships were significant, dependent variables as a function of VFSW were described through regression with the REG procedure of SAS.

(1) $\mathrm{Y}=\mathrm{a}+\mathrm{b}\mathrm{w}$(1)

When quadratic relationship was significant, relationships were described with the NLIN procedure of SAS to fit a quadratic-plateau model of the form:

(2) $\begin{array}{cc}\mathrm{Y}=\mathrm{a}+\mathrm{b}\mathrm{w}+\mathrm{c}{\mathrm{w}}^2& \mathrm{i}\mathrm{f}\mathrm{w}<\mathrm{v}\\ \mathrm{o}\mathrm{r}\mathrm{Y}=\mathrm{P}& \mathrm{i}\mathrm{f}\mathrm{w}\ge \mathrm{v}\end{array}$(2)

where Y is the dependent variable, a is the y intercept, b and c are coefficients defining the slope of the line, and w = the width (m) of the vegetation-free strip. The constants a, b, and c are constrained so that the entire function is unique at all rates of w. The constant P is the plateau value of the dependent value. The model is often used for growth responses to single nutrient fertilization doses (Bullock and Bullock, 1994). When the estimate of v is higher than 3.6 m, the response is similar to linear because it has not leveled off at the highest VFSW of 3.6 m. In this case, the v estimate is not reliable. When the estimate of v is lower than 3.6 m, the quadratic-plateau has a better fit (Cavell Brownie, personal communication). For TCSA, winter pruning, and harvest data, irrigated VFSWs were computed that were equivalent to the nonirrigated 3.0 m VFSW.

Results

Orchard Floor Vegetation

A difference for weed species with respect to irrigation or VFSW treatment was not apparent for any sampling date at both locations (data not shown). However, seasonal variation in the orchard floor vegetation was observed in terms of different weed species in summer and winter (data not shown). At both locations, the main effect of VFSW for orchard floor vegetation dry biomass was significant (P < 0.05) and displayed a negative linear response (Table 2). However, irrigation type was only significant for summer of 2007 at Clayton (P = 0.0006). This sample date coincided with a statewide drought and translated into reduced dry biomass in the nonirrigated plots. For this sample date, the irrigated plots contained on average 739.0 kg ha⁻¹ dry biomass while nonirrigated plots contained on average 368.2 kg ha⁻¹.

Table 2. Regression analyses of total dry biomass (kg ha⁻¹) of herbaceous vegetation as influenced by vegetation-free strip width (VFSW) for various sampling dates combined over irrigation type.^a

Location	Sample Date	VFSW P Value	Equation	R^2
Clayton	21 May 2007	0.0002	Y = 402.0–121.7w	0.87
Clayton	13 Aug 2007	0.0008	Y = 873.4–222.1w	0.57
Clayton	22 April 2008	0.0002	Y = 760.5–215.8w	0.80
Clayton	08 Sept. 2008	< 0.0001	Y = 2927–789.6w	0.90
Jackson Springs	16 May 2007	< 0.0001	Y = 929.8–286.3w	0.95
Jackson Springs	10 Aug. 2007	0.0142	Y = 181.8–49.1w	0.57
Jackson Springs	22 May 2008	0.0003	Y = 1950.3–511.3w	0.61
Jackson Springs	15 Sept. 2008	< 0.0001	Y = 2159.5–658.2w	0.94

^aVFSWs of 3.6 m were not included in the analysis.

At Jackson Springs, forbs made up 83–100% of winter vegetation dry biomass, while 0–17% consisted of grasses and sedges. However, in summer vegetation the forbs percentage decreased (17–66%), while grasses and sedges increased (34–83%). At Clayton, forbs made up 98–100% of winter vegetation dry biomass, while 0–2% consisted of grasses and sedges. Forbs made up 0–29% of summer vegetation dry biomass, while 71–100% consisted of grasses and sedges.

At Jackson Springs, nitrogen concentration in orchard floor vegetation displayed a negative linear response (Y = 30.3–8.6w, R² = 0.92) with increasing VFSW. The N concentration in irrigated vegetation (15.9 kg ha⁻¹) was lower than nonirrigated (19.8 kg ha⁻¹). Similarly at Clayton, nitrogen concentration in orchard floor vegetation displayed a negative linear response (Y = 36.6–10 w, R² = 0.85) with VFSW. The irrigated vegetation had lower N (18.7 kg ha⁻¹) than nonirrigated vegetation (25.5 kg ha⁻¹).

Trunk Cross-Sectional Area

The interaction of VFSW × irrigation type was significant (P ≤ 0.05) at both locations, with the exception of Clayton 2008. In both irrigated and nonirrigated, TCSA displayed a positive quadratic-plateau response to VFSW, except in 2007 and 2008 at Clayton where the relationship was linear (Figures 1 and 2). At both locations, TCSA was not different between irrigated and nonirrigated plots in the 0 VFSW (Figures 1 and 2), indicating that irrigation did not lessen the impact of weed competition when VFSW was 0 m. At Jackson Springs in the latter two years, irrigated trees had greater TCSA than their nonirrigated counterparts of the same VFSW when VFSWs was 0.6 m and higher. However, at Clayton, irrigated trees were not different from their nonirrigated counterpart of the same VFSW, except 3.0 m VFSW in 2007. At Jackson Springs, the predicted irrigated VFSW which would produce the same TCSA as the grower standard was 1.5, 1.3, and 0.8 m for one-, two-, and three-years-old trees, respectively. However, at Clayton, the predicted irrigated VFSW which would produce the same TCSA as the grower standard was not calculated due to the linear increase of TCSA with VFSW.

Figure 1. Effect of irrigation and VFSW on peach tree trunk cross-sectional area (TCSA), Clayton, NC, from 2006 to 2008. The value of VFSW at which the response plateau is represented by v.

Figure 2. Effect of irrigation and VFSW on peach tree trunk cross-sectional area (TCSA), Jackson Springs, NC, from 2006 to 2008. The value of VFSW at which the response plateau is represented by v.

Winter Prunings

The interaction of VFSW × irrigation type was significant (P < 0.05), with the exception of Clayton 2008. In both irrigated and nonirrigated, winter prunings displayed a positive quadratic-plateau response to VFSW, except at Clayton in 2007 for nonirrigated and 2008 where the relationship was linear (Table 3).

Table 3. Parameter estimates and equivalent irrigated vegetation-free strip width (VFSW) to 3.0 m nonirrigated for peach tree winter pruning fresh biomass as influenced by VFSW and irrigation, at Clayton and Jackson Springs, NC, 2007 and 2008. (All pruning were cut and recorded in March of the year listed, therefore reflect previous growing season).

		Irrigated				Nonirrigated					Equivalent irrigated
Location	Year	a	b	c v (m)^a	R^2	a	b	c	v (m)	R^2	VFSW (m)^b
Clayton	2007	0.86	1.61	−0.30 2.72	0.75	1.11	0.62	-	-	0.88	2.17
	2008^c	-	-	-	-	-	-	-	-	-	-
Jackson	2007	0.12	0.73	−0.12 3.04	0.98	0.20	0.34	−0.03	5.98	0.98	1.53
Springs	2008	0.83	1.26	−0.10 6.27	0.95	0.63	0.81	−0.05	7.19	0.96	1.60

^aThe value of VFSW at which the response plateau is represented by v.

^bThe value of the irrigated VFSW for which pruning are equivalent to the 3.0 m VFSW, nonirrigated treatment.

^cClayton 2008 was not different by irrigation (P = 0.7980), and both levels followed liner the trend as Y = 0.73 + 0.76 VFSW; R² = 0.55.

At Clayton in 2007, the irrigated orchard VFSW to achieve equivalent winter pruning biomass to the nonirrigated grower standard was 2.17 m. At Jackson Springs for the consecutive years of 2007 and 2008, the equivalent irrigated VFSW was 1.53 and 1.60 m for winter prunings, respectively.

Leaf Area

At Clayton, leaf area of individual leaf increased linearly with VFSW for irrigated (Y = 12.23 + 0.96w; R² = 0.70) and nonirrigated (Y = 15.18 + 1.24w; R² = 0.84) trees. Nonirrigated trees had larger (17.45 cm² leaf⁻¹) leaves than irrigated (13.98 cm² leaf⁻¹) trees (P < 0.0001). At Jackson Springs, leaf area of individual leaf displayed a quadratic positive relationship (Y = 11.95–2.91w+ 0.74w²; R² = 0.89) with VFSW for irrigated trees and no effect for nonirrigated trees. The irrigated trees had smaller leaves (10.39 cm⁻² leaf⁻¹) than nonirrigated trees (14.42 cm² leaf⁻¹) (P < 0.0001).

SPAD Measurement

At Clayton, a positive quadratic-plateau relationship was reported for SPAD value with VFSW for irrigated and nonirrigated trees (Figure 3). SPAD values for irrigated trees (36.7) were lower than nonirrigated (39.6) trees (P < 0.0001). The nonirrigated trees had achieved maximal greenness at narrow VFSWs compared to irrigated trees and, therefore had reached maximal SPAD at 1.16 m VFSW compared to the irrigated at 3.07 m.

Figure 3. Effect of irrigation and VFSW on SPAD value of peach, averaged across four sampling dates, Clayton, NC in 2008. The value of VFSW at which the response plateau is represented by v.

At Jackson Springs, SPAD values were lower in irrigated trees compared to nonirrigated trees for early July of 2007 and 2008, but by August 2008 values were same as nonirrigated trees regardless of VFSW (Figure 4 a, b, c). Lower SPAD values represented the light color leaves in irrigated trees compared to nonirrigated trees for early July 2007 and 2008. The normal pattern of darkening leaves from July to August 2008 was seen for both irrigation levels to the point where they became the same color.

Figure 4. Effect of irrigation on SPAD value (a) 0 m VFSW, (b) averaged over 0.6 and 1.2 m VFSW, and (c) averaged over 2.4, 3.0, and 3.6 m VFSW at various sampling dates, Jackson Springs, NC in 2008.

Nitrogen Concentration

The main effects of VFSW and irrigation type were significant (P ≤ 0.05) on nitrogen concentration, with the exception of Clayton where effect of VFSW was not significant during all three years. At Jackson Springs in both irrigation types, N concentration displayed a positive quadratic-plateau response to VFSW, except in 2008 for nonirrigated where the relationship was linear (data not shown). Nonirrigated trees had higher N concentration (4.01, 3.46, and 3.94%) than irrigated (3.69, 2.78, and 3.44%), (P = 0.0021, < 0.0001, < 0.0001), for 2006, 2007, and 2008, respectively. At Clayton, N concentration was not different between irrigation types in 2006, but it was higher in nonirrigated trees (3.34%) than irrigated trees (3.01%) in 2007 and 2008 (P < 0.0001).

Peach Fruit Yield and Quality

At Clayton, peach yield (Figure 5A) and fruit biomass displayed a positive linear response to VFSW for both irrigated and nonirrigated (Table 4). Nonirrigated trees produced heavier fruit and greater yield than irrigated trees. Averaged over VFSW, individual fruit biomass was lower for irrigated plots (43.5 g) than for nonirrigated plots (49.5 g) (P < 0.0001). The fruit yield for irrigated trees was 8.5 versus 10.2 × 10³ kg ha⁻¹ for nonirrigated trees, although the yield difference was statistically nonsignificant (P = 0.0573). The main effect of VFSW, irrigation, and their interaction were not significant for fruit SSC (ranging from 11.25 to 12.25 °Brix) and fruit firmness (ranging from 0.35 to 0.45 kgf) (data not shown).

Table 4. Parameter estimates for peach harvest as influenced by vegetation-free strip width (VFSW) and irrigation, Jackson Springs, NC, 2008.

		Irrigated				Nonirrigated				Equivalent irrigated
Parameter	Unit	a	b	c v (m)^a	R^2	a	b	c v (m)^a	R^2	VFSW (m)^b
Clayton
Yield	10^3 kg ha^−1	4.79	2.03	−	0.87	6.14	2.22	−	0.90	3.95
Fruit biomass	g fruit^−1	38.61	2.69	−	0.82	44.71	2.60	−	0.87	5.17
Jackson Springs
Yield	10^3 kg ha^−1	2.40	1.49	−0.15 5.12	0.83	1.44	1.07	−0.08 7.07	0.86	1.16
Fruit biomass	g fruit^−1	42.4	7.07	−1.25 2.83	0.96	35.6	8.12	−2.07 1.96	0.82	0.07

^aThe value of VFSW at which the response plateau is represented by v.

^bThe value of the irrigated VFSW for which prunings are equivalent to the 3.0 m VFSW, nonirrigated treatment.

Figure 5. Effect of irrigation and VFSW on total fruit yield (A) Clayton and (B) Jackson Springs, NC in 2008. The value of VFSW at which the response plateau is represented by v.

At Jackson Springs, peach yield (Figure 5B) and fruit biomass displayed a positive quadratic-plateau response to VFSW for both irrigated and nonirrigated (Table 4). The yield and individual fruit biomass were higher for irrigated (4.39 × 10³ kg ha⁻¹ and 49.05 g, respectively) than nonirrigated plots (3.03 × 10³ kg ha⁻¹ and 41.43 g, respectively) (P < 0.0001). The fruit SSC was lower SSC in irrigated plots (11.8 °Brix) compared to nonirrigated plots (12.6 °Brix). However, VFSW did not influence SSC (ranging from 11.25 to 12.80 °Brix) and neither irrigation type nor VFSW influenced fruit firmness (ranging from 0.50 to 0.61 kgf) (data not shown). Tworkoski et al. (1997) found similar results in a nonirrigated situation in that size of VFSW within a cover of grasses had no effect on fruit SSC in peach.

Discussion

Similar to Tebeau et al. (2017), seasonal variation in the orchard floor vegetation was observed in terms of different weed species in summer and winter. However, this difference was not affected by VFSW and irrigation. The amount of orchard floor vegetation decreased with increase of VFSW with no significant differences by irrigation. The common trend at both locations was that orchard floor vegetation from irrigated plots contained less N than nonirrigated vegetation. This effect is probably due to NO₃ leaching in irrigated plots. However, reduced N in irrigated plots was not translated into reduced weed growth because irrigated vegetation did not have decreased biomass production compared to the nonirrigated vegetation.

This study focused on peach trees from planting to year three, with VFSW and irrigation treatments that were initiated at planting. Results from these studies indicated that increasing the VFSW in peach results in greater tree growth and greater yield, and these results are in agreement with other studies on young trees (Belding et al., 2004; Glenn et al., 1996; Welker and Glenn, 1989). Glenn et al. (1996) reported an increase in peach TCSA, canopy diameter, fruit yield, number of large fruit, and pruning biomass with increased VFSW.

These results showed that at Jackson Springs, the irrigated VFSW of 1.16 m would produce the same yield as the grower standard of a 3.0 m VFSW of nonirrigated. The irrigated VFSWs which would produce the same TCSA and winter pruning as the grower standard were 1.5, 1.3, and 0.8 m for trees one, two, and three years-old, respectively. However, these recommendations are primarily based on only one location (Jackson Springs) due to the lack of differences in tree growth and yield between irrigation treatments at Clayton. The different response at Clayton versus Jackson Springs is probably due to several factors. The first year after planting, trees are very susceptible to weed competition as they are in the establishment phase. At Clayton, all trees were treated the same for weed management and irrigation during summer of the first year, with no treatments applied. In addition, since Clayton trees were at least nine months older, they were probably better competitors due to increased tree size than those trees at Jackson Springs. The difference in a sand (Jackson Springs) versus a loamy sand (Clayton) would suggest greater benefit of irrigation at Jackson Springs. All of these factors could have contributed to greater treatment differences at Jackson Springs compared to Clayton.

The increase of peach leaf nitrogen concentration with VFSW was probably related to the competition for nitrogen with the orchard floor vegetation. Similarly, Haynes and Goh (1980) and Welker and Glenn (1985), 1996) reported that foliar N concentration increased when trees were grown in bareground compared to grass sod. Tworskoski et al. (1997) demonstrated that competition with grass will reduce fruit yield in young peach trees, largely by interfering with N availability and uptake.

Similarly across locations, peach leaf nitrogen concentration was lower but not deficient in irrigated trees than nonirrigated trees; presumably due to leaching of NO₃ by irrigation. Nitrogen is deficient under 1.7% (Lockwood et al., 2005), so in this study no deficiencies were present at the time of sampling. Reduced leaf size in irrigated trees versus nonirrigated, and the lower SPAD readings in irrigated tree leaves were evidence that nitrogen was reduced in irrigated trees. The role of nitrogen on leaf area has been documented in a number of species including, peach, nectarine, and apples (DeJong et al., 1989; Xia et al., 2009). Xia et al. (2009) reported that total leaf area per apple tree was reduced with reduction of nitrogen rate. Nitrogen dilution effect could be postulated as another reason for the reduced leaf size and lower SPAD values in irrigated trees. A greater growth measured as higher TCSA and winter pruning in the irrigated trees at Jackson Springs compared to the nonirrigated trees may have diluted the nitrogen in the irrigated tree leaves so that leaf N concentration was lower. Unfortunately, leaf number, an indicator of dilution effect, was not recorded in this study. It seems less likely that the N dilution was a factor at Clayton, where similar growth and yields were found for irrigated and nonirrigated trees, but still lower leaf N concentration, reduced leaf size, and SPAD measurements were recorded in irrigated trees. Therefore, at Clayton N differences are probably due to leaching rather than dilution effect.

Data from the present study suggest that a 1.5-m VFSW combined with proper irrigation will produce tree growth and yield in volunteer weedy vegetation similar to the current grower standard. Reducing the VFSW may also help to increase organic matter and maintain soil structure by reducing erosion, and these factors would have a positive influence on the agricultural productivity of the orchard. Reducing the width of the vegetation-free strip will also reduce the amount of herbicide that growers need to apply each year which would reduce input costs. However, the long-term impact of reducing VFSW to 1.5-m on peach tree growth and yield needs to be studied before making any recommendation for farmers as current results are only based on initial three year of tree growth and one year of yield data.

Although our study focused on treatments that were compared to the current grower standard 3.0 m VFSW in nonirrigated peach, results across locations indicated that increase in VFSW had positive effect on peach tree TCSA and yield parameters in both irrigated and nonirrigated conditions. However, goals of individual growers must balance management decisions they face with their yield goals, risk for erosion, need to facilitate movement of equipment and other management decisions. Another important thing to consider that these study conclusions could be differed under high-density peach production systems.

Nomenclature

Flumioxazin, paraquat Prunus persica (L.) Batsch. ‘Contender’.

Acknowledgments

The authors express appreciation to the USDA RAMP project AG for providing funding, Drs. Cavell Brownie and Consuello Arellano for their statistical assistance, and fellow graduate students and the staff at the NC State University Central Crops Research Station (Clayton) and Sandhills Research Station (Jackson Springs) for their assistance in the field work.