Hemp as a cover crop in New Zealand vineyards: A feasibility study

Mark Krasnow1 and Kirsty Harkness2

1Thoughtful Viticulture Ltd., Blenheim, NZ

2Hark & Zander Ltd., Auckland, NZ

Executive summary

The results of this study show that hemp is a viable cover crop for New Zealand Sauvignon blanc vineyards. The presence of hemp in the vineyards provides a means of alleviating soil compaction and adding of organic matter to the soil without negatively affecting the vines in any way. Hemp also offers a potential second income stream for the grower, as hemp is ready to harvest before grapes. The hemp did not have a negative effect on the wines, and actually improved quality compared with a wine from grapes not grown alongside hemp when separate wines were made in 2019

Abstract

Industrial Hemp seeds (Cannabis sativa cv Kompolti) were sown in the midrow of a Sauvignon blanc vineyard in Marlborough, New Zealand to assess the effects of hemp as a cover crop/intercrop on the vines and the vineyard soil. The hemp became established without supplemental irrigation, even in an exceptionally dry season when other cover crops failed to thrive. However, quality seed should be sourced to ensure good strike. The presence of hemp did not negatively affect vine nutrition, water relations, growth, or yield. Soils from the hemp treatment had higher soil carbon at 40- 80 cm. Hemp plants grew large roots to at least 30 cm, and were able to grow in compacted tractor wheel tracks in the row, where the root system can alleviate compaction caused by vineyard operations. Juice/must samples from the 2019 harvest showed a higher diversity of yeast species from the hemp area than the control, and produced a perceptibly better wine.

Figure 1: Successful industrial hemp cover c rop establishment between Sauvignon Blanc vines.

Introduction

Many growers opt to plant cover crops in their vineyards in the interrow. Cover crops offer many advantages, such as increasing biodiversity, harbouring beneficial insects, fixing nitrogen, and alleviating soil compaction (Danne et al., 2010; Fagaria et al., 2005; Williams and Weil, 2004). These cover crops are then incorporated as organic matter back into the vineyard by mowing, mulching, rolling, or cultivating into the soil, where they serve as the basis of the soil food web. Rarely are cover crops harvested for any economic gain for the grower, as their benefit to the soil justifies the expense of buying and sowing seed. A cover crop that could have the same benefit in terms of adding soil organic matter, harbouring beneficials, and supporting healthy soils that also yielded a saleable product would be a benefit to growers, offering a second income stream. Hemp offers this sort of possibility for grape growers, as hemp seed or fibre can be grown for sale and harvested before grapes, so as not to interfere with the logistics of winery harvests.

Hemp roots have been shown to grow to depths of over one meter (Amaducci, et al., 2008), and thus have the potential to add more organic matter to the soil than shallower rooted crops. After the plant is harvested and mowed/rolled/mulched the extensive root system will break down slowly and nourish soil microbes and alleviate compaction (Williams and Weil, 2004). However, the fact that they are d eep rooted may mean that they compete with vines more than shallower rooted cover crops which are the norm. The effects of the competition with vines therefore needs to be assessed.

Soil samples from a small scale trial in the 2018-19 season showed that the presence of hemp did not greatly reduce mineral content of the soil, and led to higher organic matter content than soils from the interrow area or a grassed-down area of the vineyard away from vines. The hemp plants were also able to survive in the interrow area of the vineyard despite a very dry late summer/early autumn. Green hemp plants were growing in the midrow when most other plant cover had dried off, indicating it is a resilient cover crop that can establish even in dry years when other cover crops do not thrive. Winery samples from the vineyard with hemp showed no apparent negative effect on the wine. In fact, the feedback from the winemaking team was overwhelmingly positive about the results in terms of wine quality from a trial conducted in 2019.

Bees from hives near the hemp collected much more pollen than hives located elsewhere on the property (personal communication with apiarist). Bees use pollen for food over winter and to feed larvae, and thus the presence of hemp is of potential benefit to bee colonies near vineyards. The large quantity of pollen provided by hemp can not only help them survive, but also to fight off parasites that might cause colony collapse (Di Pasquale et al., 2013). Presumably hemp would serve as a ready food source for other pollen consuming beneficial insects as well, and so might indirectly help protect the vines from predators, parasites, and pathogens.

Considering its rapid growth, its ability to thrive with little care on the part of the manager, provision of a second income stream for the vineyard, and the potential benefits hemp as a cover crop offers to grapevines themselves, it is anticipated that many growers might choose to grow hemp in their vineyards. Before this new cover crop can become widespread, the effects of growing hemp in vineyards on the vines themselves and the wines they produce needs to be researched. This trial was set up to investigate the effects

of using hemp as a cover crop in vineyards and to ensure there was no negative effect on the vines, the soil, or the wines.

Materials and Methods

Trial layout: For all seasons, the trial was set up as a split plot design with four sampling replicates in each plot. Row spacing in the trial block was 2.2 m, with 1.8 m between vines. All vines were pruned to three canes, trained onto on a VSP canopy, and irrigated every day for 90 mins. In both plots, midrow vegetation in every other row was sprayed with glyphosate approximately 10-14 days prior to sowing. In one plot, hemp (Cannabis sativa cv. Kompolti) was planted, and resident vegetation was allowed to establish in the control. Sowing dates are shown in Table 1. The drill was set up to seed the hemp plants 30 cm apart in the centre 1.2 m of the row, to a depth of 2 cm. No irrigation for the hemp plants was provided in the midrow. In each sampling replicate, 4 bays of vines (16 vines total) were flagged off for petiole collection and berry sampling for maturity assessment. 2 data vines were chosen in each replicate (8 vines per treatment) for measurements of vine performance, water status, yield and fruit health.

In 2019-20 plots were chosen in areas of the vineyard with similar vigour as determined by visual examination of midrow and canopy growth. Unfortunately, due to an error in the vineyard maps, the control treatment vines were on 3309 rootstock and the hemp treatment vines on Riparia gloire rootstock.

For the 2020-21 and 2021 -22 seasons, new trial replicates were established so that all vines were on 3309 rootstock. NDVI maps of the vineyard constructed from satellite data were used to better and less subjectively delineate vigour zones within the vineyard. It was ensured that replicates from each treatment were in comparable vigour zones so that a more robust comparison of the effects of the hemp could be seen without inherent soil vigour or rootstock differences potentially obscuring or confounding the results.

Figure 2 : Left: cv. Kompolti hemp seed sown on 13th December, 2019. Right: seed drill sowing an interrow.

Table 1: Sowing dates for the hemp trial 2019 -2021.

the sprayed-out rows. The drill was set up to seed the hemp plants approximately 30 cm apart in the centre 1.2 m of the row. No irrigation for the hemp plants was provided in the midrow. In each sampling replicate, 4 bays of vines (16 vines total) were flagged off for petiole collection and berry sampling for maturity assessment. 2 data vines were chosen in each replicate (8 vines per treatment) for measurements of water potential, bunch number, rot severity, and total yield per vine.

Petiole collection: 100 petioles were collected from leaves opposite bunches in each sampling replicate (4 samples per treatment) in each season at flowering. Petiole samples were sent to Hills lab (Hamilton, NZ) for nutritional analysis.

Canopy gap quantification: In 2020, at veraison in February, digital photos were taken of each data vine’s canopy with a blue background. Image analysis, similar to that of Tardaguila et al., (2010), was used to quantify canopy gaps to assess differences in canopy cover or architecture

Water potential measurement: At midday (between 11:00 AM and 3:00 PM), reflective bags were placed on leaves of each data vine (two leaves per replicate), allowed to equilibrate for at least 20 minutes, and measured using a Scholander pressure chamber (PMS model 615). Measurements were made periodically between veraison and harvest veraison.

Maturity sampling: 50 berry samples were collected from each replicate at various times during ripening. These berries were weighed to determine berry weight, and soluble solids of the juice was measured using a digital refractometer (SPER scientific model 300058).

Harvest analyses: Total bunch number and yield were recorded for each data vine (8 vines per treatment). On each vine, 6 random bunches were assessed for rot using the method of Hill et al. (2010). Before commercial harvest of the vineyard, the total biomass of hemp plants in the four 2 bay sampling replicates (17.3 m2 planted area/replicate) of the hemp plot was determined.

In 2020, a sample of grapes from the area of the vineyard with the most hemp was collected, washed with soap and water to remove any pollen adhered to the surface, and pressed. The juice from this sample was analysed by the Cawthron Institute (Nelson, New Zealand) for THC, THCA, CBD and CBDA to determine if cannabinoids entered the berries and might theoretically be present in wines produced from the vineyard.

Soil analyses: After harvest soil pits were dug to 1 m depth for soil sample collection and profile examination. Two pits were dug in each treatment in 2020. In 2022, a pit was dug in

Figure 3: Left: A germinating hemp seed on the 19 th December, six days after sowing. Right: Kirsty Harkness with plants on 21st January, 2019.

Figure 3: Left: A germinating hemp seed in mid December 2019 Right: Kirsty Harkness with plants in late January 2020

each replicate so a more robust, statistically relevant, sample could be collected (4 pits per treatment).

Visual examination of the soil texture/structure and midrow vegetation roots was carried out, and samples from 0 -40 and 40- 80 cm were collected for subsequent analysis from each soil pit. Samples were analysed by Hills labs (Hamilton, New Zealand) for quantification of organic matter, basic soil nutritional parameters, and volume weight.

2019 wine making and analysis: One truckload of fruit (approximately 20 tons) from each treatment was fermented at the winery using identical tanks. Juices were plated on Bromocresol-containing plates, which differentiate between native and commercial yeasts. Wine quality was assessed by the winemaking team at the winery.

Figure 4: Insect life around plants.

Results

Petiole nutrition. The only consistent significant difference seen was higher sodium in the hemp treatment petioles (Table 2). There were other, less consistent, differences seen in individual years. In general, when differences were seen, the hemp treatment had higher levels of the nutrient in question that the control.

Table 2: Petiole nutrients at flowering for the 2019-20, 2020 -21, and 2021 -223 seasons. Data are averages of four 100 petiole samples per treatment. Values in bold with different lower case letters denote differences at the 95% confidence interval.

Canopy gaps. There was no significant difference in canopy gaps at veraison in 2020 between treatments.

Stem water potential. Midday stem water potential was measured three times after veraison in 2020, and 2021, and once after veraison in 2022. At no measurement date was there any significant difference between the hemp and control vines in terms of water status (data not shown).

Table 3: Percent canopy gaps at veraison. Veraison Control 3.21% Hemp 1.80% T value 0.15388

Berry weight and soluble solids. The only difference in berry weight seen was for the harvest sample in 2020, when berries from the hemp treatment were heavier than the control (Table 3). No differences in soluble solids were seen in any sample in any year (data not shown).

Table 4: Berry weight from the three seasons of the trial. Values in bold with different lower case letters denote differences at the 95% confidence interval.

Harvest parameters. There were no significant difference in bunch number, yield per vine, bunch weight, or rot severity between treatments in any of the years of the trial.

Table 5: Bunch number per vine, yield per vine, bunch weight, and percent rot severity for the three seasons of the trial. No differences were seen in any parameter in any year.

Hemp biomass. The hemp did not grow equally well in every replicate, with stonier replicates growing fewer and smaller plants than siltier replicates. Hemp was able to establish to some degree in every replicate, regardless of soil. Average hemp per 17.3 m2 sampling area is shown in Table 6

Table 6: Average fresh hemp biomass per replicate (17.3 m2).

Soil physical and chemical data. The hemp treatment tended to have lower pH, higher P, K, organic matter, and total carbon. Differences in organic carbon and total carbon were more pronounced at 40-80 cm than at 0 -40 cm.

Table 7: Soil chemical and physical data from after harvest in 2020. Data is from two samples per treatment.

Table 8: Soil chemical and physical data from after harvest in 2022. Data is from four samples per treatment. Values in bold with different lower case letters denote significant differences at the p=0.1 level. Values in bold with different lower case letters and asterisks denote significant differences at the p=0.05 level.

Juice microbial communities from 2019 harvest. In 2019 a winery made wines from vines adjacent to hemp, and a control wine from vines far away from the hemp. There were few differences in basic juice composition, indicating that the hemp did not greatly affect ripening of the fruit. However, the juice from the hemp had very high diversity of yeast species based on microscopic examination and plating (Figure 5). The winery, seeing these results, conducted a native ferment on the hemp wine. Compared with other native ferments, the sample from the hemp showed a greater diversity and presence of nonsaccharomyces yeasts at least 72 hours into the ferment (Figure 6). The winery was very pleased with the quality of the wine produced from the grapes around the hemp. It would have gone into their top tier product, but that wine is only made from estate fruit, not grower fruit.

5:

samples from the fermenting hemp juice 24 (left), 48 (centre), and 72 hours (right). The varying colours of the colonies indicate non- commercial yeast strains. Native non-saccharomyces yeast colonies tend to have a green/blue/yellow colour whereas commercial yeasts have a white colour on these plates. Non-saccharomyces yeasts were present in the ferment to at least 72 hours.

Figure

Plated

Figure 6: The hemp ferment compared with two other native ferments and an inoculated ferment over the first 3 days of fermentation. Notice the persistence of coloured colonies in the hemp ferment, indicating a greater diversity of yeast species for longer in this wine.

Discussion

All cover crops growing alongside grapevines can potentially compete with them for nutrients and water. In some cases, the vine devigouration from this competition is desirable, however in this vineyard the grower did not want to reduce vine growth or yield when using hemp as a cover crop. Based on the measurements made in this study, hemp was not strongly competitive with the vines The only consistent difference in mineral nutrition showed the hemp treatment being higher in Sodium than the control treatment (Table 1). Canopy development was not negatively affected by the presence of hemp when measured in 2019, again suggesting little or no competition that would impede growth

The stark difference in hemp growth between the 2020-21 season and the other two seasons show the importance of using quality seed. The seed supplier inadvertently sent old seed to the vineyard for sowing in 2020, and the strike and overall growth were substantially reduced in that season (Table 6). Seeds were sown in optimal conditions, when the soils had plentiful moisture and were warm enough to support germination. However, the lower quality seed largely failed despite this. Obtaining quality seed is absolutely imperative for all cover crops, and hemp is no exception, as strongly evidenced in this study.

The vines alongside the hemp were not significantly drier than those without, indicating negligible competition for water. This finding has been consistent for the three years of the trial The vines in the trial vineyard were irrigated daily for the duration of the experiment, as is typical in this vineyard, and in the wider Marlborough region. This amount of water was able to supply enough for the vines so that they did not need to complete with the hemp for midrow soil moisture. Were this trial carried out in a vineyard that irrigated less, or not at all, evidence of competition between vines and hemp might more readily be seen. However, unirrigated Sauvignon blanc vineyards in New Zealand are very rare.

Soil pits from the midrows showed the soil profile in both treatments to be a 40 -50 cm deep silt layer overlaying a 10-20 cm layer with large stones, with a very sandy layer beneath that that extended to the bottom of the pits No significant differences in soil parameters were seen in the first year of the study (2019 -20) with duplicate samples, though they suggested that hemp was not greatly impoverishing the soil for any particular element. Soils from the hemp treatment showed trends for having lower pH and higher organic matter than the control in 2020 (Table 7). After the final year of the trial in 2022, samples were taken from all four replicates, doubling the number of samples, and allowing a more robust comparison of the effects of hemp on the soil. There were significant differences in the soils between treatments, but only at 40-80 cm depth. Hemp plots had

Figure 7: Hemp biomass samples being taken, 21st March 2020.

significantly higher total carbon, and lower pH, Magnesium, and cation exchange capacity at this depth (Table 8). Despite some of these differences in the midrow soils, vine petiole nutrition was not greatly affected by hemp cover crops (Table 2).

Fruit development and yield were not affected by the presence of hemp. In fact, berry size and yield was often higher in the hemp treatment at harvest, though usually not statistically significantly so (Table 4). Juice soluble solids, and harvest parameters of bunch number, yield per vine, and bunch weight were not significantly affected by the presence of hemp (Table 5 ). There were no cannabinoids detected in Sauvignon blanc juice from grapes grown adjacent to large hemp plants in 2020 (data not shown), suggesting that there is no uptake of cannabinoids into the fruit. However red wines, which have the skins extract in the fermenting wine for a period of time, could theoretically pick up cannabinoids or undesirable aromatics deposited on the grape’s surface via hemp pollen, hemp plant particles, insect transfer, or direct condensation from the air. This possibility needs to be investigated before recommending hemp as a cover crop in red wine vineyards. Also, if hemp were seeded at higher rates, or if the vines received less water and fertiliser, competition from the hemp might have been more evident. This possibility should be investigated before the use of hemp in vineyards can be recommended more widely, as management practices, such as irrigation an fertilisation, differ greatly between regions, and even between different varieties in Marlborough.

In 2019, the year before this trial began, wines made from vines grown adjacent to hemp plants had higher yeast diversity than vines far away from the hemp plants in the vineyard. High numbers of non -saccharomyces yeasts were present in the ferment for at least the first 72 hours (Figure 5). The hemp ferment had more numerous and longer lasting populations of non-saccharomyces yeasts than other native ferments conducted in the winery that season (Figure 6). Considering some non-saccharomyces yeasts are known to add complexity to wines (Jolly et al., 2014), it is possible that planting hemp in the vineyard actually improves wine quality. The 2019 wines support this possibility. Unfortunately, due to COVID and logistical issues at harvest, no more wines from the hemp trial were able to be made to further investigate this.

Figure 8: Left: Soil pit in Hemp trial area from after harvest in 2022. Center and right: Large hemp roots could be seen down to 30 cm

The lack of any evidence of competition between the hemp and the vines indicates that it is a potentially useful cover crop for irrigated and fertilised New Zealand vineyards. Hemp plants were able to establish with no supplementary nutrition or irrigation. The root system of the hemp plants were well developed and grew visibly 25-30 cm into the soil profile for larger plants (Figure 8). The root system was very vertical in the soil profile, with the hemp plants exploring the midrow soil rather than growing horizontally into the undervine soil. Presumably this growth habit would keep the hemp plants from directly growing into the undervine area and competing with vine roots beneath the dripline, which have direct access to applied irrigation and fertigation. This trait could help explain the lack of competition seen between the hemp and vines in this study. This robust root system explains why the hemp was able to survive the long dry period in 2019 when most other midrow weed species had long since dried off. Hemp plants were seen growing, albeit stunted, in the compacted wheel tracks (Figure 9), a place where most cover crops, or even weeds, cannot become well established. Using hemp as a cover crop could allow for the decompaction of wheel tracks, especially if hemp was intentionally seeded at a high rate specifically in this area. Used this way, hemp could undo some of the damage done to vineyard soils by normal operations. Hemp adds organic matter at depth in the soil profile, even if the aboveground potions are harvested and removed from the vineyard for other uses as a second income stream

Given the possibility hemp offers as a cover crop , in terms of improving vineyard soils, potentially enhancing wine quality, and offering a second income stream from the property, it is expected more and more grape growers will experiment with hemp either as an intercrop or as part of a more diverse cover crop mix. Further work needs to be done on the effects on wines, both in terms of quality, but also the potential for cannabinoid and/or off flavour pickup in red wines grown near hemp plants.

Figure 9: Left: Hemp growing, albeit stunted, in the compacted wheel tracks, a place even most weeds cannot become well established. Right: A bent hemp plant. Run over by tractors but still determined to grow.

Conclusion

From the three year study, hemp appears to be a viable cover crop for use in vineyards. The hemp that was able to become established grew to fruition even in an exceptionally dry season without any supplemental irrigation, unlike most shallower rooted cover crops This allows hemp to continue to grow and sequester carbon in conditions where other cover crops senesce. The hemp plants did not noticeably affect vine growth, nutrition, water relations, fruit development/quality, or vine productivity. These indicate no potential loss in income from using hemp as a cover crop or intercrop. Based on the poor strike in the 202021 season, it is clear sourcing quality seed from reputable vendors is absolutely critical to have the crop grow well.

Soils from the hemp area were higher in organic matter and total carbon, which are beneficial for the health and fertility on the soils in the long term. Differences were especially pronounced at 40 -80 cm, suggesting hemp allows sequestration of more carbon deeper into the soil profile than other cover crops or resident vegetation Results from the 2019 season indicate that the presence of hemp in the vineyard led to a higher native yeast population on the grapes, allowing for a healthy uninoculated ferment to proceed. The quality of the wine from the grapes co-planted with hemp was superior to the control. The findings that hemp plants in the vineyard did not negatively affect the vines, but beneficially affected soils and wines, is very exciting and warrants further study

Acknowledgements

The authors would like to thank Callahagn Innovation for co -funding the project.

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