by Pierre Helwi (October 2017)
Cover crops are an important component of sustainable viticulture systems as they have a major and direct impact on the health of vines and the surrounding ecosystem. Growing a cover crop minimizes the use of chemicals which may negatively affect the environment and reduce the physical impact of frequently running heavy equipment on vineyard soil. This article discusses the benefits and drawbacks of using cover crops in sustainable viticulture and includes guidelines for sound practices.
A cover crop can be defined as any vegetation grown in vineyard middles and occasionally under vines without being harvested. Cover crops may be planted annually fall and spring or maintained perennially. The implemented technical aspects of this approach are delicate and must be well considered in order to benefit from the positive effects of the planted crop.
Benefits of cover crops
Improve soil structure: vegetation roots bind soil particles together, ameliorating soil structure and water infiltration. In additions, the mechanical action of cover crop roots loosen the soil up to 60 inches of depth, reduce its compaction and improve the penetration of water and air.
Improve mineral fertility: besides increasing soil nitrogen, the decomposition of the cover crop increases soil cation exchange capacity therefore the ability of a soil to hold and exchange nutrients allowing their restitution to the vine in an assimilable form. Cover crops limit also mineral leaching by rain by storing them during the winter time. In addition, legumes contribute to enrich the soil with nitrogen by symbiotic fixation of the atmospheric form.
Improve soil biological activity and organic matter content: cover crops stimulate rapidly and intensely the biological activity of the soil during their growth and especially after decomposition. The quantities of formed humus (organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms) allow to maintain the organic matter content of the soil.
Protect against erosion and run-off: cover crop protects the soil surface from raindrop impact that dislodges soil aggregates, enabling them to move with water run-off.
Limit weed germination and growth.
Provide habitat for beneficial insects and predators: some cover crops attract beneficial insects and arthropods which can contribute to control harmful insects and mites.
Suppress some populations of nematodes: an anti-nematode action is sometimes described for some cover crops. This action, due to compounds released during the decomposition of the plant, concerns only Root-knot and Pratylenchus nematodes responsible for direct damages.
Influence grapevine growth: the presence of vegetative cover influences grapevine growth by competing for water and nutrients or by providing additional nitrogen for vine development.
Provide firm footing for cultural operations and are aesthetically pleasing.
The use of cover crops may also have some drawbacks. The presence of a cover crop may increase water use, frost hazard, and the competition with vines for soil moisture and nutrients. Pest problems may also result from the presence of cover crop mainly when it is not kept in a reasonable height, in addition to a possible increase in costs and management.
Which species can be used as cover crop?
Many types of plants can be used as cover crops. Legumes and grasses including cereals are the most extensively used, but there is increasing interest in brassicas (such as rape, mustard, and forage radish) and continued interest in others, such as buckwheat.
Families of cover crop are classified according to their ability to provide carbon (“slow” or “fast”) and nitrogen (N). “Slow” carbon sources correspond to materials rich in cellulose and lignin such as cereals, “rapid” carbon sources are associated with grasses and brassicas with easily degradable sugars, and legumes are the N providers. In order to ensure that microorganisms can properly degrade the organic matter without depriving N, it is desirable to use a cover crop with a balanced formulation between slow carbon, fast carbon and N sources. Legume-grass mixtures complement each other in their soil improving functions. This blend offers the benefit of both tap and fibrous root systems and supplies the vines with moderate N. Monocultures may be preferred where the species has a history of proven performance. Single-species plantings should usually be rotated to reduce the potential for buildup of insects or pathogens. For each specific crop, ask the seed supplier about seedbed cultivation, as well as moisture and fertilizer requirements.
The table below adapted from “Principles of Cover Cropping for Arid and Semi-arid Farming Systems, NM State University” lists some of the species that can be used:
Winter annual
| Name | Family | Characteristics | Seeding Rate (lb/ac) |
| Annual grasses (wheat, barley, oats, annual ryegrass, cereal rye, triticale) | Grass | Cold-tolerant, high lime tolerance, low drought and generally low salinity tolerance, moderate moisture use | Wheat, barley, oats, triticale: 60-120
Annual ryegrass: 15-30 |
| Austrian winter pea | Legume | Moderately cold and drought tolerant, moisture efficient | 60-80 |
| Brassicas (mustards, turnips, forage radish) | Tap-rooted, moderate to high drought tolerance | Mustard: 5-12
Turnip: 4-7 Radish: 8-12 |
|
| Hairy vetch | Legume | Cold tolerant, moderate tolerance to drought and soil lime; low salinity tolerance | 15-20 |
Winter annual cover crops are most often planted in vineyards because they grow during the dormant season and spring when rainfall is often most abundant, thereby aiding in erosion control and are not in competition with the vines for water and nutrients. They are sown in the fall and are mowed and disked in the spring or killed with an herbicide. Summer annual cover crops are usually planted in the spring and they are ready to mow or till in in about a month assuming adequate rainfall.
Summer annual
| Name | Family | Characteristics | Seeding Rate (lb/ac) |
| Buckweat | Grass | Cold sensitive, moderate drought tolerance | 50-60, drilled |
| Cowpea | Legume | Drought tolerant | 50-100 |
| Foxtail millet | Grass | Cold sensitive, drought tolerant | 15-20 |
| Lablab | Legume | Vining and spreading legume | 50-60 |
| Pearl millet | Grass | Cold sensitive, drought tolerant | 15-20 |
| Sesbania | Legume | Fast and vigorous growth | 30-40 |
| Sorghum-Sundangrass | Grass | Cold sensitive, drought tolerant | 15-40 |
Perennial cover crops are generally sown in the fall, but some can be planted in early spring. They usually do not require replanting for several years. Perennial species are most commonly used in vineyards planted on fertile sites where vines are seriously out of vegetative balance but are also utilized in less fertile sites in order to maintain soil structure in the aisles and provide firm footing for viticulture operations.
| Name | Family | Characteristics | Seeding Rate (lb/ac) |
| Alfalfa | Legume | Cold tolerant, drought tolerant | 15-18 |
| Red clover | Legume | Cold tolerant, moderate tolerance to soil lime, low drought and salinity tolerance | 20-28 |
Legumes
Legumes provide N to the soil with the aid of symbiotic bacteria. A legume plant produces a tap root that does not penetrate well into compacted soil layers, so they are less useful for loosening soils and improving water penetration than cereals. A legume green manure cover crop can provide all of the N required under ideal circumstances after 2 seasons of careful management. The N contribution could be reduced by planting alternate row middles, combining legumes and cereals in the cover crop mixture, or reducing the width of the cover crop band. Legume cover crops should be used with caution in excessively vigorous vineyards and high rainfall areas of the state. Legume seed must be inoculated with the appropriate strain of N-fixing rhizobium bacteria prior to planting.
Grasses
Grasses do not fix N but may be useful as a trap crop to take up soil N and release it more slowly upon decomposition in the soil. Grasses have numerous small, fibrous and fine roots that are more likely to grow into compacted layers.
Cover crop management
Prior to seeding, the soil must be sufficiently cultivated to allow for good germination. Many growers begin by shallow ripping using a shallow tiller. The soil is then moistened and disked twice (about a week apart), leveled, and seeded. Seeding will ideally be from mid-September to mid-October for most cover crops, when soils are warm and rainfall is likely. Seeding after mid-October in many areas of the state becomes risky due to cooling soil temperatures, slow germination, and early frost. No-till drilling method is highly recommended for seeding cover crops because, besides conserving soil texture, it offers a uniform seed placement and an excellent seed-to-soil contact, which leads to a high cover crop establishment rate. After seeding, the seedbed should be firmed to lightly pack the soil. Irrigation after seeding helps ensure successful germination and establishment.
Cover crops should be fertilized and soil amendments should be added on the basis of soil test results. Grasses and brassicas may require the addition of N for adequate growth. When planting legume-grass mixes, avoid or limit N fertilizers, which stimulate grasses to the point that the will shade out the legumes. Many growers use compost, which in most cases will adequately provide what the cover crops need.
The presence of the cover crop increases the risk of damage by spring frost. Often the cover crop is mowed in early spring for frost protection and then allowed to resume growth and go to seed. After the seed matures, the cover crop is mowed and left on the soil surface or incorporated into the soil using a shallow tiller.
The following factors should be considered when choosing whether or not to incorporate the cover crop:
- It may allow for rapid N release and availability for the current season.
- Maximum N release occurs about 3 weeks after incorporation, assuming that the soil remains moist.
- For perennial cover crops, several mowings might be required to keep the foliage from growing excessively tall.
Conclusion
Vineyard floor management is an important component of sustainable winegrowing systems. If a cover crop is to be utilized, choices in cover cropping should be site-specific. Growers must consider their style of farming, yield and quality objectives, and any other criteria that they consider important.