TYPES OF COVER CROPS FOR VINEYARDS
A cover crop is grown in the vineyard to prevent soil erosion, suppress weeds, maintain soil structure, provide easier access during wet weather, and contribute to soil fertility through organic matter. Cover crops may also contribute to insect and mite pest control, depending on the pests present and the conditions. Cover crops are increasingly being recognized as an important component of sustainable agriculture production in most wine producing regions of the world.
Growers should strongly consider maintaining and enhancing existing vegetative cover on areas of high erosion potential, such as erodible soils, steep slopes, bench terrace slopes, drainage ways, stream banks, buffer strips, and existing or potential slips.
- Annual Cover Crops Green Manures: Green manure cover crops are typicallyplanted in vineyards from September to early December. They consist of winter annual grasses, legumes, or forbs. If the green manure is used to add nitrogen, legumes are used alone or in combination with nonlegumes, usually cereals.
The cover crop germinates within several weeks with adequate soil moisture and proper soil temperatures. Most species do not develop substantial biomass until day length and temperatures increase in late February or early March. The cover crop is usually incorporated into the soil while still green and succulent, usually from late March to early May. At this time, the carbon to nitrogen (C/N) ratio is low so there is rapid decomposition and a net release of nitrogen. The cover crop is often disked or mowed in late March if there is a danger of frost. If re-seeding is desired, incorporation may be delayed until the cover crop sets mature seed.
- Perennial Cover Crops: Permanent perennial cover crops that grow during the summer usually compete strongly with vines for soil nutrients and water. Research indicates that perennial ryegrass and other sod type cover crops may significantly reduce vine nutrient status. Sod type cover crops are unsuitable in nitrogen limited vineyards. Although these cover crops may be useful in reducing excessive vine vigor, their impact may take several years.
Most perennial grasses do not develop as rapidly as annuals but, when established, have deeper roots and provide excellent erosion control. Perennial clovers, although tap rooted, also reduce erosion because of their invasive growth and their tendency to produce roots at each node. Due to the longer period needed during establishment, perennial cover crops may need to be mixed with a small amount of annual grass seed to provide adequate cover during the first year. Growers should exercise caution when using perennials because they may devigorate vines excessively.
Perennial cover crops, which live year round, offer the advantage of not having to be reseeded each year. However, the stand must be maintained throughout the season, and at least small amounts of water are required to maintain the stand from late spring through fall. Resident vegetation, usually consisting of winter annuals followed by summer annuals, also fits in this category since it can grow and use water for the entire season. Perennial grasses that are less active during the summer offer some hope to less competitive water use.
Plant a perennial cover crop such as straw-berry clover, white clover, birds foot trefoil, ‘Berber’ orchard grass, or perennial ryegrass and plan to mow frequently, which reduces both winter and summer annual weed populations. Additionally, these cover crop plants are generally very competitive and will reduce vine growth in shallow or weaker soils, especially if the vines are young and the cover is allowed to grow into the vine row.
- Fertilization of Legumes: Generally, supplemental phosphorus fertilizer is the only nutrient required for legumes in vineyard cover crop systems. Phosphorous can be applied by broadcasting single superphosphate on the soil surface during seedbed preparation or banding at planting with a seed drill. Soil tests for phosphate phosphorous are a good indicator for fertilizer need. Other fertilization is not necessary. Nitrogen fertilizer should be avoided since it reduces nitrogen fixation. Sulfur is also necessary for good legume growth, but it is usually not limiting in vineyard soils.
- Fertilization of Grasses: Nitrogen fertilizer is usually required for grass cover crops. A common practice is to apply 25 to 50 pounds per acre of nitrogen at planting. Many growers will also apply 25 pounds at the time of incorporation to hasten breakdown and mineralization. If cover crops are used to reduce vine vigor and nitrogen status, or if soil or water nitrogen content is excessive, nitrogen fertilizer use should be reduced or eliminated.
- Cover Crop Selection for Erosion Control: Among the best cover crops for erosion control are perennial sods, which have dense foliage and root systems. Reseeding winter annual grasses, such as ‘Blando’ brome, also greatly enhance erosion control, but do so less than perennial sod because annual grasses begin each fall with small root systems. Reseeding winter annual legumes are often not as effective as grasses because the root systems of grasses are usually more dense.
Although winter annuals that are not self reseeding can reduce erosion once established, the disking required to plant them can cause serious erosion if heavy fall rains occur. It is also important to consider other factors when selecting a cover crop for erosion control, such as seed cost, seedling vigor, reseeding ability, water needs, and the cover crop’s effects on vines, fruit yield, and quality.
For maximum erosion control, the best cover would be a vigorous permanent sod that forms a dense root system, such as perennial ryegrass, tall fescue, or strawberry clover. Winter annual grasses effectively reduce erosion after they become established, and they reseed and die as vine growth begins in early spring. But, the soil may be prone to erosion during heavy fall rains before young seedlings develop sufficient root structure.
- Winter Annual Cover Crops Grasses: Reseeding winter annual grasses, particularly ‘Zorro’ fescue and ‘Blando’ brome, are frequently used on hillside vineyards. They are generally well suited for erosion control because of their dense growth habit and fibrous root system. They are seeded in the fall and germinate and establish rapidly after fall rains, protecting the soil from erosion and anchoring soil particles in place. When properly managed, these species provide a self sustaining cover crop that requires no tillage beyond the first year, although some years if weeds reinvade.
Other winter annual grasses, such as cereal rye, barley, and oat, provide a protective cover crop for one year but generally do not produce sufficient viable seed for rees tablishment in subsequent years. These species are sometimes used in new vineyards where a cover crop is desired for erosion control in the fall and winter, but where further soil disturbance is required for planting, trellising, irrigation, or other activities that will occur the following spring. Disking to smooth the surface will be warranted, after which a perennial or reseeding winter annual cover crop is planted.
- Legumes: Legumes are often included in cover crop mixes to add nitrogen, provide color, or increase diversity. Established legume plants shield the soil from rain-drop impact in late winter and spring. However, young legumes generally provide less protection against soil movement than young grasses because legumes have less fibrous roots. As with grasses, some legumes reseed themselves, such as rose clover and subterranean clover.
Subclover forms a dense network of low growing stems by late winter that helps reduce soil erosion. These legumes may be added to a seed mixture with reseeding grasses. Other legumes such as bell beans and field pea grow only for one season and should be planted when specific benefits are desired, such as adding more nitro-gen to the soil.
- Mixtures of Cereals and Legumes: Plant a mixture of tall winter cereal and a vetch or peas available to fill in the open areas to further reduce the amount of light available to the soil and inhibit growth of winter annuals. Maximum effects are achieved by planting early in the fall or in the late summer into a prepared seedbed followed byan irrigation to increase germination, early growth, and soil cover.
