Effects of Mulching on Soil Temperature, Scion Rooting, and Soil Moisture of Mounded Grapevines

ABSTRACT

Soil-hilling around grapevines is required in the eastern United States for winter protection of grafted and cold sensitive grape cultivars (Vitis vinifera). The effects of simazine-treated mulches (STM) on soil temperature, scion rooting, and soil water conservation were measured. Neither simazine nor interactions between simazine and mulches affected any variables measured in this study. Wood and straw mulches increased the minimum temperature of media around the graft union as effectively as hilled soil, reduced scion rooting by a factor of 2 to 75 compared with soil-hilling, and increased soil moisture content 45 to 75% at Wooster and 8 to 23% at Kingsville. These results indicate that mulches have positive attributes and should be considered as an alternative to traditional soil-hilling in vineyards for temperature mitigation around graft union.

KEYWORDS:

• Graft union
• herbicide
• Vitis vinifera

Introduction

Grape cultivars from Vitis vinifera are grown for making wines in the northeastern United States. However, V. vinifera grapes are very sensitive to phylloxera, an insect that severely parasitizes the root system. This pest has been effectively controlled by grafting a vinifera scion onto a phylloxera-resistant rootstock (Pouget, 1990). Most vinifera grape cultivars are also cold-sensitive, and the graft union needs protection to survive through the winter. Currently, winter protection is achieved by mounding soil, called “soil-hilling,” to cover the graft union in the fall (Dami et al., 2005). Growers remove the hilled soil, called “dehilling,” at the beginning of the following growing season to prevent the scion from rooting and the potential phylloxera infestation (Dami et al., 2005).

Although soil-hilling effectively prevents winter injury, experience suggests that the practice may not be sustainable over the long term. Soil-hilling requires fuel, labor, and specialized equipment, increasing the cost and the carbon footprint associated with the final wine products. Soil-hilling requires two, and sometimes three, additional tillage operations (Zabadal, 2003) that can oxidize soil organic matter. Because these practices often are implemented when soil is very wet they can be particularly destructive to soil physical quality. The practice exacerbates soil erosion on hilly sites. In a research vineyard at The Ohio State University, soil-hilling for 10 consecutive years resulted in a loss of the top 8 to 12 cm of soil under the trellis of vines planted on a slope of less than 5 degrees (Jiang, 2010). Soil erosion also threatens the grape production in other states (Battany and Grismer, 2000; Leib et al., 2005). Soil-hilling may exacerbate pesticide offsite movement as a result of increased surface runoff on various soils (Fu et al., 2006; Seta et al., 1993), and through vertical leaching in soils with good internal water drainage (Gish et al., 1995). This is a particular concern because grape is a pesticide-intensive crop. According to the U.S. Department of Agriculture (2009) vineyards in the United States annually are treated with 48 kg·ha^–1 a.i./year, on average. Finally, soil-hilling complicates weed control. More weed species at higher density were found in Ohio vineyards that had been hilled compared to those that were not (Jiang et al., 2008). An alternative to soil-hilling that provides equivalent protection against cold injury to graft union but without the associated environmental costs is highly desirable.

Zabadal (2003) reported that a mulch of wheat straw protected the graft union of vinifera grapes from winter injury as effectively as mounded soil. Mulches are known to effectively reduce soil erosion and chemical run-off (Maass et al., 1988; Smolikowski et al., 2001). Moreover, when herbicides were applied to mulches, better weed control was achieved than herbicide or mulch alone (Case and Mathers, 2006). These results provide evidence that herbicide-treated mulches may be a viable alternative to soil-hilling and traditional herbicide-based approaches for weed control. However, organic mulches, such as wood/bark and straw, may decrease N availability in soil because of the high C to N ratio, affecting the mineral nutrition of the grapevine (Eiland et al., 2001). Considering the vital importance of winter protection and the need for more sustainable methods, the effect of herbicide-treated mulches on vinifera grapevines needs to be examined.

In this research, simazine-treated mulches (STM) were applied in two vineyards for 2 years. The hypothesis was that STM are equivalent to soil-hilling for winter protection, while simultaneously providing weed management and moisture conservation in vinifera grape production. The objectives of this research were to determine the effect of STM on: (1) soil temperature, (2) scion rooting, and (3) soil moisture.

Materials and methods

Field trials

Experiments were conducted in research vineyards of the Ohio Agricultural Research and Development Center at Wooster (40° 46’ 43” N, 81° 55’ 51” W) and at the Ashtabula Agricultural Research Station in Kingsville (41° 53’ 5” N, 80° 41’ 52” W) in 2008 and 2009. The soil type was Wooster silt loam (U.S. Department of Agriculture taxonomic class: Fine-loamy, mixed, active, mesic, Oxyaquic Fragiudalfs) at Wooster (Mahboubi et al., 1993) and a Bogart loam (U.S. Department of Agriculture taxonomic class: Fine-loamy, mixed, active, mesic, Aquic Hapludalfs) at Kingsville (Ferree et al., 2000). The cultivars included in this trial were grafted V. vinifera Auxerrois (planted in 1994) for the 2-year experiment at Kingsville, own-rooted hybrid Vitis species Seyval (planted in 2001) for the 2-year experiment at Wooster, and grafted V. vinifera Pinot gris (planted in 1994) for the 1-year experiment at both Wooster and Kingsville. The spacing between rows was 3 m for all cultivars, and the spacing distance within a row was 1.8 m for Auxerrois, 1.0 m for Seyval, and 1.2 m for Pinot gris. ‘Seyval’ had a high cordon training system (1.7 m) and both ‘Auxerrois’ and ‘Pinot gris’ had a vertical shoot position training system. Spur pruning was performed before bud break for all three cultivars. ‘Seyval’, ‘Auxerrois’, and ‘Pinot gris’ at Kingsville were pruned to 25, 30, and 20 nodes per vine, respectively. No cluster thinning was performed to ‘Auxerrois’ or ‘Pinot gris’, while ‘Seyval’ was thinned to one basal cluster per shoot at the bloom stage as recommended by OSU Extension (Dami et al., 2005). Because of severe bud injury (>95%) to ‘Pinot gris’ during the winter of 2009 at Wooster, all canes were hedge-pruned to five-node spurs and all spurs were retained (Dami et al., 2012). Fertilizers and pesticides were applied following recommendations by OSU Extension (Dami et al., 2005).

The field experiment of each cultivar had four replications of nine treatments, which were three types of mulches (wood/bark, wheat straw, and hilled soil) combined with three rates of simazine (Syngenta, Greensboro, NC, USA; 0, 2.7, and 5.4 kg·ha^–1) applied in both autumn and the following late June (2008) or early July (2009). The treated plots were 3.6 m by 1.0 m for ‘Auxerrois’, 2.0 m by 1.0 m for ‘Seyval’, and 2.4 m by 1.0 m for ‘Pinot gris’. Mixed hardwood-species wood/bark (125 t·ha^–1), or wheat straw (20 t·ha^–1) was applied to a depth of 10 cm above the graft union and a depth of 5 cm for the rest of plot area in November each year. The 10-cm-high cone around the grape vine had a diameter of about 1 m. The standard method of winter protection, soil-hilling, was included as the control and covered the graft union up to 20 cm. Prior to the second winter, an additional 0.8 kg of straw or 5 kg of wood/bark was added to cover the graft union of each vine and restore the rest of the plot area covered by the mulch to the depths established the previous autumn. Soil was also hilled at the same time for the winter hilling treatment.

Simazine was applied to the top of the mulches or soil at 0, 2.7, and 5.4 kg·ha^–1 as an aqueous emulsion on the same day mulches were applied in vineyards at Wooster and Kingsville. A CO₂ backpack sprayer with a single 8003EVS nozzle (Spraying System Co., Wheaton, IL, USA) that delivered 230 L·ha^–1 at 240 kPa pressure was used to apply the herbicide. In late June or early July the mounded soil or mulch that covered the graft union were removed, and mulches that covered the rest of the plot were kept. Simazine treatments matching those applied in autumn were then applied after hand weeding on the same day.

Effect of STM on soil temperature

The probes (n = 4) of temperature loggers (StowAway Tidbit XT, Onset Computer Corporation, Cape Cod, MA, USA) were placed on the surface or buried 10 cm deep under straw or wood/bark and 20 cm deep under soil right after mulches were applied. Temperature was recorded once every 5 min from Nov. through late June 2008 and early July 2009. The winter of 2009 in Ohio was unusually cold, with official low temperatures of –28.1 °C in Wooster and –23.6 °C in Kingsville (OARDC Weather System, 2009). In Wooster, ‘Pinot gris’ sustained more than 95% primary bud injury while ‘Seyval’ sustained less than 10%. In Kingsville, ‘Pinot gris’ and ‘Auxerrois’ sustained less than 10% primary bud injury. Therefore, whole plant injury was assessed on ‘Pinot gris’ in Wooster at the pre-bloom (12 June) and post-bloom stages (9 July).

Effect of STM on scion rooting

The number of new roots (longer than 0.5 cm) generated at the base of the scion of grafted ‘Pinot gris’ and ‘Auxerrois’ was recorded at the time of mulch or soil removal around the graft union.

Effect of STM on soil moisture

Two soil cores, 5.3 cm in diameter and 10.5 cm in depth, were taken at random from each plot and combined together into a zip-lock plastic bag to prevent moisture loss during sampling and transportation. For mulched plots, straw or wood/bark were removed before collecting cores, and replaced after sampling. Soil dry weight was determined by drying the cores to a constant weight. Constant weight was achieved at 50 °C after 3 days. Soil moisture content was expressed as the percentage of water relative to soil dry weight.

Experimental design and data analysis

A complete randomized block design was used with four replications of two vines per plot unit. Since the experiment had two factors: simazine and mulch, the data were analyzed by two-way ANOVA using the ANOVA model in SAS 9.3 (SAS Institute Inc., Cary, NC, USA). Means were separated by Tukey’s test at P < 0.05 level.

Results and discussion

Effect of STM on soil temperature

In this study, the F test showed that simazine had no significant effect on all variables measured or any significant interactions with mulch. Therefore, the results are presented showing mulch as the main effect.

Wood/bark and straw increased the minimum temperature around the graft union as effectively as did hilled soil (Table 1). In the winter of 2007, the lowest temperatures recorded under 10 cm of wood/bark and straw were –8.1 and –8.3 °C compared to –19.4 °C on the soil surface. A similar result was also observed at Kingsville in 2008 and 2009. These results agree with those of Zabadal (2003), who reported that wheat straw could protect the graft union as effectively as hilled soil. Neither simazine nor mulch treatment affected winter injury of buds, or injury at the whole plant level (data not shown).

Table 1. Minimum temperatures (°C) recorded in the Kingsville and Wooster research vineyards during winter months in 2007, 2008, and 2009.

Treatment^z	Wooster	Kingsville
	6 Feb. 2007^y	11 Feb. 2008	2 Mar. 2009
Soil surface	–19.4b^x	–11.7b	–11.5b
Wood/bark	–8.1a	–1.5a	–1.5a
Straw	–8.3a	–2.0a	–7.0ab
Soil	–11.2a	–1.7a	–5.7ab
Air temperature^w	–21.3	–16.1	–12.8

^zTemperature was recorded at soil surface, under 10 cm of wood/bark, 10 cm of straw, and 20 cm of soil.

^yDate when minimum temperature was recorded.

^xMeans followed by the same letter in the same column were not significantly different from each other by Tukey’s test at P < 0.05.

^wAccording to OARDC Weather Station, available at www.oardc.ohio-state.edu/newweather.

Effect of STM on scion rooting

Simazine application rate did not affect scion rooting of ‘Pinot gris’ or ‘Auxerrois’ (data not shown). This lack of effect fits well with the photosynthesis-inhibitor mode of action of the herbicide. However, mulches largely inhibited the development of roots from the scion compared to soil-hilling (Table 2). Fifteen roots developed from the scion of ‘Auxerrois’ when the graft union was covered by hilled soil (2008), compared to 3.6 and 0.2 roots per vine when covered with wood/bark and straw, respectively. Similar results were observed with ‘Pinot gris’ in 2009. Straw and wood/bark have different physical characteristics than soil, which are apparently less favorable for scion root development. In fact, the bulk density of wood/bark and straw was 0.25 and 0.04 g·cm^–3, respectively, while the soil bulk density was 1.6 g·cm^–3. Higher bulk density of soil would have resulted in a closer contact between the vinifera scion and the soil that probably facilitated scion rooting. Fewer phylloxera-sensitive roots generated from the scion might be a factor in reducing the risk of phylloxera infestation. Reduced scion rooting by straw and wood/bark mulches could provide grape growers a wider time window to remove winter-protection from the graft union at the beginning of the growing season.

Table 2. Effect of mulching on scion rooting in Auxerrois and Pinot gris grown in Kingsville, Ohio.^z

	Root Number·Vine^–1
Mulch	Auxerrois 2008	Auxerrois 2009	Pinot gris 2009
Wood/bark	3.6b^y	1.6ab	4.5b
Soil	15.1a	3.2a	11.2a
Straw	0.2b	0.1b	2.8b

^zThe graft union was covered by soil and mulch in November, and soil or mulch was removed on 19 June 2008 and 23 June 2009 to investigate scion rooting.

^yMeans followed by the same letter in the same column are not significantly different from each other by Tukey’s test at P < 0.05.

Effect of STM on soil moisture

Simazine application rate did not affect soil moisture; however, mulches increased soil moisture compared with the soil-hilling treatment (Table 3). The highest soil moisture at Kingsville was observed under wood/bark (15.9%), followed by straw (14.0%), and by soil-hilling (12.9%). An identical pattern was observed at Wooster. Similarly, organic mulches have been reported to reduce soil water evaporation (Jones et al., 1968; Unger and Parker, 1976). The higher bulk density of wood/bark (0.25 g·cm^–3) compared with straw (0.04 g·cm^–3), and the closer contact of wood/bark fractions were likely factors contributing to its greater resistance to water evaporation.

Table 3. Effect of mulching on soil moisture content (% dw) at the Kingsville and Wooster research vineyards in 2008.^z

Mulch	Kingsville	Wooster
Soil	12.9c^y	12.8c
Wood/bark	15.9a	22.5a
Straw	14.0b	18.6b

^zAt Kingsville, soil samples were taken at 4 d after the most recent rainfall occurred from 30 Sept. to 3 Oct., totaling 49.3 mm; and at Wooster, soil samples were taken at 15 d after the most recent rainfall, which occurred from 12 to 14 Sept., totaling 41.9 mm.

^yMeans followed by the same letter in the same column were not significantly different from each other by Tukey’s test at P < 0.05.

Conserving soil moisture, which guarantees stronger growth during periods of rainfall deficit, is a likely explanation for the enhancement in vine vigor in response to mulch. This was supported by the recorded precipitation data (OARDC Weather System, 2009): a total of 472 mm of rainfall occurred during the growing season of 2008 (1 May to 1 Nov.) at Wooster with only 32 mm of precipitation for the entire month of August; whereas the long-term average is 598.4 mm for the former and 98.6 mm for the latter (The Weather Channel, 1996). Similarly, in the growing season of 2009 a dry month was also recorded: the total precipitation was only 25 mm from 21 June to 21 July.

In conclusion, mulches conserved soil moisture likely by reducing the evaporation from the soil surface and indirectly through suppression of weed competition for soil water. This effect enabled grapevines to better resist extreme drought stress during the growing season. Mulch provided winter protection that was as effective as the traditional soil-hilling. In contrast to soil-hilling, mulches did not stimulate scion rooting; thereby, leaving a wider time interval for removal of winter-protection from the graft union in spring.

Although the novel mulching methods tested in the study possess many environmental benefits in the long term, the relatively high cost may restrict its adoption. Zabadal (2004) estimated that the cost of equipment and labor to conduct soil-hilling was $320·ha^–1; whereas, the cost of using straw at 6.7 t·ha^–1 was $1410·ha^–1. Accordingly, when straw was applied at 20 t·ha^–1 the total cost would be $4230·ha^–1. Considering straw and shredded wood/bark are similar in price and field application procedure, the total cost of using shredded wood/bark would also be around $4000·ha^–1. It should be noted that the material cost of straw or shredded wood/bark was the predominant contributor to the total cost (Zabadal, 2004). Given that the mulches only cover the area under the trellis, which is about a quarter of the total vineyard, the material cost based on a 1-hectare vineyard field is about $1058 for straw and $1000 for bark. It also needs an additional input of nitrogen fertilizer at $30·ha^–1. Together, the cost of applying mulches is slightly over three times of soil-hilling. However, during the second year only 40% of mulch materials used in the first year were needed to resume the same coverage, thus significantly reducing the annual cost in the long term. If the negative environmental cost by soil-hilling, i.e., soil erosion and associated chemical runoff, and the positive environmental benefits by mulches were taken into the budget, this novel mulching method could readily become economically feasible, especially for high value grapes.

Acknowledgments

We wish to thank Bruce Williams and John Elliott for helping manage vineyards in Wooster and Gregory Johns for taking care of vineyards in Kingsville. We also thank Tim Koch for helping with the mulching and simazine treatment, and Cathy Herms for helping with processing temperature data.

Funding

Funding for this project was provided by the Ohio Grape Industries Committee.

Additional information

Funding

Funding for this project was provided by the Ohio Grape Industries Committee.