February 2007 Issue of Wines & Vines

Your Vineyard's Other Crop

A guide to floor management in Northwest vineyards

by Mercy Olmstead, Ph.D.
Wildflowers mixed with native grasses can provide an attractive cover crop, and can persist from year to year. They may help to attract beneficial insects.

  • Cover crops provide benefits including less soil erosion and compaction, more water infiltration, fewer pests and increased beneficial invertebrates, less chemicals, fewer weeds, lower vine vigor and greater soil organic matter.
  • They can also affect microclimates, increasing frost risk during early spring and late autumn, encourage vertebrate pests and raise costs compared to resident vegetation
  • It's important to select the right cover crops and plant and manage them properly for greatest benefit.
As the growing season begins, the first plants in the vineyards to show growth are usually not the vines, but what grows between them. Cover crops have become more and more important to vineyard managers, and should be included when considering the entire ecosystem in sustainable production of quality grapes. Vineyard floor management can have a large impact on the microclimate and both beneficial and unwanted life in a vineyard, especially soil health. Here is a guide to cover crops, developed specifically for Pacific Northwest vineyards.

Floor Management Systems

Resident vegetation. This consists of all plants growing in the vineyard, including both native plants and invasive weeds. Weeds may be difficult to eradicate once established. To manage vegetation, mowing is usually required, but herbicides can be used in vineyards not certified organic.

Clean cultivation. In conventionally managed systems, vineyard alleys are disked or sprayed with herbicides to reduce vegetation, while in organically farmed systems, mechanical means or approved products keep alleys weed-free. This reduces competition for water and nutrients, but the bare soil can increase erosion. Travel through bare alleys can also cause compaction and reduce water infiltration.

Cover cropping. Vineyard cover crops are usually seeded in every alley to provide cover throughout the vineyard. They can be planted in alternate alleys, with a different cover or clean alleys. The area under the vine is often kept clear with herbicide or cultivation to reduce competition for nutrients and water, especially with drip-irrigated systems.

Choices in Floor Management

Vineyards like this, with sloping, clean-cultivated alley rows, can be particularly vulnerable to soil erosion.
Protection of the soil surface is one of the biggest benefits of cover crops. Wind and water erosion can strip up to 2.5 inches of the upper soil in a season, but soil accumulates in areas with vegetative cover. Cover crops, especially grasses, protect the soil, reducing dislodging by rain and water runoff.

Cover crops can also reduce surface crusting and improve rainfall penetration, important for saline soils. Species with large root systems like oilseed radish penetrate hard pans to improve water infusion.

Cover crops can increase organic matter and nutrient availability, but their impact is highly dependent on soil, temperature and rainfall. Moisture is needed, and may require supplemental irrigation.

Cover cropping may also enhance the beneficial mycorrhizal fungi that colonize grapevine roots. Colonization of grapevine roots requires contact between them and cover crop roots. It can be expensive to inoculate a vineyard, so inoculating vines when establishing a vineyard is more practical. To gain the most benefit from mycorrhizal inoculations and cover crop partnerships, use proven vendors and do not fumigate prior to planting if possible. Reduce use of phosphorus fertilizers and tillage.

Cover crop choice can also affect insect populations. For example, some vineyard pests such as cutworms prefer to feed on broadleaf covers rather than grasses, so eliminating cover can drive cutworms into the grapevine to feed on grapevine buds. Sprays can prevent this, but a more sustainable approach is to maintain the broadleaf cover until this threat has passed. Cover crops can also reduce dust and spider mite populations in vineyards with dusty, dry conditions, but they can also encourage voles, moles and gophers.

Vineyard age should be considered before planting cover crops, as vigorous cover crops can compete with young vines for water and nutrients. Once vines are more than 4 years old, cover crops can help reduce excessive vigor.

Certain thick cover crops can reduce temperature near the ground by reducing soil warming during the day. Tall cover crops can reduce temperatures closer to the fruit zone, damaging flowers or fruit.

Choices and Management:
Seed-Bed Preparation

Seed beds should be moist, well-mixed and free of plant material. Shallow ripping or rotovation (10--15 inches) of the soil may be needed, but should be done when the soil is fairly dry. Water is required before and after seeding for good germination. Most cover crops in the Northwest are planted in the autumn to take advantage of winter precipitation. The vineyard alleys are disked and leveled, then seeded to a depth depending upon the size of the seed. Cover crops can be seeded by drilling or broadcasting, but broadcasting usually requires a higher seeding rate, and some crops establish better when drilled.

Cultivation of Cover Crops

Tillage or mowing may be required. Annual cover crops such as grasses may be mowed a couple of times per year to facilitate access or for frost protection in the spring. Tillage can be useful for legumes and forbs that release nitrogen in the soil, but timing is important to ensure that nitrogen release coincides with the appropriate vine growth stage.

Overview of Cover Crop Types

Grasses. Many grasses form large, fibrous root systems, which prevent soil erosion in windy areas. In vigorous vineyards, grass can take up nitrogen and tie it up over time. Unlike legumes, grass does not contribute nitrogen, but can provide biomass. Annual grasses like cereal rye, oats, barley and triticale are often planted in the autumn and tilled under or mowed in the spring for frost protection. Tilling and mowing will increase exposure of the soil surface for greater heat absorption during the day and release at night. Mowing grasses can leave stubble that reduces dust, provides traction and competes with weeds. Grasses usually compete with existing weed populations better than legumes or forbs.

Perennial grass mixes planted in this young vineyard contribute biomass, but in vigorous vineyards, can tie up nitrogen over time.
Legumes. Legumes are broad-leaved, annual or perennial species that contain nodules on their roots that house nitrogen-fixing bacteria (Rhizobium spp.), but legume seeds must be inoculated with the bacteria to fix nitrogen. Nitrogen is released after the cover crop begins to decompose or in response to mowing. Legumes include a tap root that can increase water infiltration, but can also attract rodents to the vineyard, so are best used in established vineyards.

Insectary mixes. These broad-leaved, flowering annual and perennial plants can attract beneficial insects, but can be difficult to establish if not targeted for the region.
Some mixes contain weeds, however. Check the Washington state noxious weed list (nwcb.wa.gov). A number of introduced species such as yellow starthistle and purple loosestrife have become noxious weeds, and are very difficult to control.

Annual Cover Crops

Annual ryegrass (Lolium multiflorum). Annual ryegrass or Italian ryegrass is a cool-season bunchgrass with an extensive fibrous root system, useful in areas with excess water or nitrogen and better on finer-textured silt or clay soils than perennial ryegrass. Annual ryegrass can compete with vines in marginal areas. It is typically seeded in the autumn, tilled in late spring or early summer and matures between June and September.

Cereal cover crops. Barley (Hordeum vulgare), oats (Avena sativa), triticale (Triticosecale hexaploide) and wheat (Triticum aestivum) can be interplanted with vetches, because they support the vine-like growth. Choose varieties resistant to diseases that could infect nearby crops. Barley and wheat are more drought-tolerant than oats or triticale. Cereal crops are often tilled into vineyards in early summer, but can be mowed for extra frost protection in the spring. They form a fibrous root system, adequate for reducing soil erosion and removing excess nitrogen. Cereal crops are planted in the autumn to take advantage of winter moisture.

Cereal rye (Secale cereale L.). Cereal rye or winter rye can be used to increase organic matter and can produce about 3,000--10,000 lbs./acre in dry biomass. Its extensive fibrous root system can take up excess water and nitrogen in a vineyard. It is normally planted in the autumn or early spring and mowed before it begins to senesce. Cereal rye is tolerant to minus 30°F.

Clovers (Trifolium spp.). Crimson clover (T. incarnatum), rose clover (T. hirtum) and subterranean clover (T. subterraneum) are annuals often used in vineyards managed without tillage. Crimson and rose clovers can reach 6--8 feet, but subterranean clovers exhibit low, prostrate growth. Clovers may be difficult to establish in soils with poor nutrition.

Field pea (Pisum sativum). Field pea is a popular cover crop in the eastern Northwest as a winter/spring annual, tilled or mowed in early summer to supply organic matter and release nitrogen. Its succulent stems break down rapidly, providing a burst of nitrogen in the soil.

A mustard cover crop can be planted before the vines when establishing a vineyard. It can act as a soil fumigant and weed suppressant, and is successful in dry climates
Medics (Medicago spp.). Bur medic (M. polymorpha), barrel medic (M. truncatula) and black medic (M. lupulina) were once used for pastures. All are reseeding annuals or short-lived perennials. Bur medic performs well on soils with a pH above 6.5, establishes with relatively little seed and produces large quantities of seed for future growth, but seed pods are often spiny. Bur medic performs well in vineyards with minimal irrigation, provided winter rains are adequate. Barrel medic prefers soil pHs ranging from neutral to alkaline, and requires about 11 inches of precipitation. It produces a large number of hard seeds, maturing in mid-spring.

Mustards (Brassica and Sinapsis spp.). Mustards are annual forbs that produce glucosinolates that break down to isothiocyanide, which can act as soil fumigants and weed suppressants. Mustard cover crops can be used before a vineyard is established to eliminate chemical fumigants. They reseed and should not be mowed until seeds have set. Both white mustard (Sinapsis alba) and oriental mustard (Brassica juncea) have shown success in dry environments.
Vetches (Vicia spp.). Vetches such as hairy vetch (V. villosa) and common vetch (V. sativa) are seeded or volunteer in vineyards. Hairy vetch is more prone than common vetch to climb beyond the vine alley into trellises, especially on deep soils. Both types of vetches are shallow-rooted plants, unlike many of the tap-rooted legumes, and have flowers that can attract beneficial insects. Hairy vetch is more cold-tolerant than common vetch, with most of the biomass production in the early spring and summer. Allow vetches to reseed if a continuous stand is desired.

Perennial Cover Crops

Fescues (Festuca spp.). This group includes tall fescue (F. arundinacea), sheep or hard fescue (F. ovina) and red fescue (F. rubra). Sheep fescues are bunchgrasses, while red fescues have a slow, spreading growth habit. Many turf-type fescues form a dense mat, providing excellent weed suppression and traction, and reduced summer precipitation will induce dormancy and a layer of mulch. Fescues are well-suited for dryland vineyards and should be autumn-seeded to take advantage of precipitation. Some fescues produce large roots that are difficult to eradicate.

Indian ricegrass (Orzyopsis hymenoides or Achnatherum hymenoides). This native western North American grass is often used for erosion control on sand dunes and sandy soils. Although green in the spring, it goes dormant in summer, but retains its hold on soil, making it useful for sandy, windy sites. It is a cool-season cover crop and very drought-tolerant.

Meadow barley (Hordeum brachyantherum). Meadow barley is a perennial grass that performs best in wetland or riparian areas. It is also known as California barley, and can tolerate clay with low calcium and low water-holding capacity or high magnesium serpentine most commonly found in California. It competes poorly with weeds.

Perennial ryegrass (Lolium perenne). Perennial rye is a cool-season, moderately drought-tolerant bunchgrass that can be competitive when overseeded. It establishes quickly and germinates early in the growing season. It is ideal for grass mixes with other species, and these mixes remain green longer than pure perennial rye. Perennial rye fights soil erosion because of its extensive fibrous roots. It can be seeded in the late autumn in areas with mild winters; it should be seeded during the late summer in areas with potentially cold winters. Seeds should not be planted deeper than one-half inch.

Wheatgrasses. Wheatgrasses includes crested wheatgrass (Agropyron cristatum) standard crested wheatgrass (Agropyron desertorum), pubescent wheatgrass (Agropyron trichophorum) and other species. Wheatgrasses are known for their drought tolerance and persistent stands (five to seven years). With good early establishment, they can significantly reduce weeds in vineyard alleys. They can be seeded with forbs to attract beneficial insect populations.

White clover (Trifolium repens). White clover is a perennial that performs better on heavy soils than on lighter, sandy soils because it is not as drought-tolerant as other clovers. It is considered a short-lived perennial that may require replanting every three to four years. White clover seeds are small and a well-prepared seed will aid in establishment.

Wildflower and forb mixes. These mixes often consist of various varieties of native flower and grasses to enhance ecological diversity and persistence from year to year.


Vineyard floor management should consider overall goals for vineyard management and the surrounding area. Vineyards benefit from cover crops that help in vigor control, compete with the existing weed population and control soil erosion by wind and water. Remember that sustainable grape production takes into consideration labor concerns, economics and ecological impacts.

This article was adapted from "Cover Crops as a Floor Management Strategy for Pacific Northwest Vineyards," which can be found
at cru.cahe.wsu.edu/cepublications/ eb2010/eb2010.pdf.

(Mercy Olmstead is an extension viticulture specialist and assistant professor for Washington State University. She interned in the Napa Valley as an assistant enologist before earning a master's degree in viticulture at WSU, and earned her Ph.D. at Michigan State University. Contact her through edit@winesandvines.com.)
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