Covering the Spaces

By Jessica Cortell

Even though I grew up in Oregon, I still get tired of all the rain in the spring. BUT I love all the green!

A big part of the green in the valley is grass seed fields and cover crops grown in vineyards. In the vineyards, it is time to start bringing out the equipment between rain showers to mow and cultivate the cover crops as needed. While a few growers use sheep to graze the cover crop before bud break, most rely upon tractors.

There are several options for vineyard floor management. These include clean cultivation (no vegetation during the summer months), resident vegetation (includes all plants growing, both native and non-native species) and the use of both annual and perennial cover crops. Annual cover crops typically will be planted in the fall and cultivated into the soil in spring. If a cover crop is mowed and left lying on the soil surface, 5 to 14 percent of the ammonia nitrogen could be volatilized, contributing to greenhouse gases.

The various cover crop management strategies can often be used in combination, depending on the vineyard goals. Cover crops usually consist of a blend containing grasses, grains, legumes and other species planted between the vine rows.

Cover crops can prevent soil erosion, reduce soil compaction, increase soil organic matter, soil nitrogen and nutrient availability, promote beneficial insects, reduce pest populations, and minimize noxious weeds. They can improve soil health determined by the interaction of soil structure, soil chemistry (pH and nutrition) and soil biology.

The strip under the vines is usually managed separately from the vineyard floor (row middles). This strip of about two feet is kept clean through the use of herbicides or in-row cultivators. However, some growers are experimenting with low-growing plants directly beneath the vines.

With the use of legumes, we are able to harvest our own nitrogen fertilizer literally from the air. How does this work?

While atmospheric nitrogen is unusable by most plants, legumes have the ability to fix nitrogen through a symbiotic relationship with Rhizobium bacteria. Legume cover crops can fix from between 50 to 200 pounds  of nitrogen per acre. The amount of fixation depends on the crop species, soil pH, soil temperature, soil moisture and a few other variables. Nitrogen fixation is a very sustainable practice compared to purchasing nitrogen fertilizers, which have high energy costs for production.

In addition to the above benefits, the use of cover crops in vineyards systems can be used to achieve specific goals in the vineyard. As wine quality is influenced by vine vigor, cover crops are used to improve or reduce vigor as needed. They are important tools for growers.

For example, in young blocks, a common practice is to clean cultivate the row middles during the late spring and summer months to reduce the chance of competition. Then, in the fall, plant a soil fertility-building annual cover crop. At this time of the year, weather permitting, the annual cover crops will be mowed and incorporated into the soil. One challenge is sometimes it grows too tall to mow and integrate the huge biomass. I have been in a cover crop one foot taller than me.

In young vines, there are some benefits of mowing and cultivating it under between two to three feet, as it will be easier to work and have a higher nitrogen-to-carbon ratio. When your goal is good growth and rapid establishment, the higher nitrogen ratio is beneficial because there will be a quicker microbial breakdown of the biomass and the nitrogen will become available to the vines more quickly. The cover crop can provide for all the nitrogen requirements of the young vines.

One of my favorite annual cover crop blends is crimson clover with cereal rye. The cereal rye works well in Oregon as a fall-planted cover crop, as it will rapidly establish to prevent erosion and can survive the winter. It is the best cool-season cereal for absorbing unused soil nitrogen and can hold up to 100 pounds per acre. This is important in reducing leaching and loss of nitrogen. It provides a large amount of biomass, up to 10,000 pounds dry matter per acre. The influx of organic matter will stimulate populations of microbes, which provide numerous benefits to the soil and vines.

Another advantage of a cereal rye cover is that it out-competes many broadleaf weeds and reduces weed pressure. Cereal rye was also found to suppress many weeds allelopathically (produces chemicals that act as a natural herbicide), including dandelions and Canada thistle. Both these weeds are common problems.

There are a number of benefits of having crimson clover in the blend with cereal rye. It establishes quickly in the fall to prevent soil erosion, has spectacular red flowers that are an abundant source of nectar for many types of bees and can fix nitrogen (70 to 150 pounds per acre) to provide for vine needs.

One challenge with growing crimson clover in the Willamette Valley is that it does not perform well in soils deficient in phosphorus or potassium, or with a low pH. These conditions often exist in vineyards. One study found that no nitrogen was fixed at a pH of 5.0 and small but inactive nodules formed in phosphorus-deficient soils. In order to have healthy legume cover crops, lime or other nutrients may be needed.

There are other cover crop mixes with similar properties, such as the mix used by Jason Tosch, director of viticulture at Anne Amie Vineyard in Carlton. These mixes often contain a number of grass, grain and legume species. A few other legumes often included are Austrian field peas, vetch, various clovers and fava beans.

Rebecca Sweet, vineyard manager at Van Duzer Vineyards in Dallas, prefers triticale over cereal rye due to the spreading growth habit. While oats generally perform well, some species are not sufficiently cold tolerant to survive a cold snap during the winter.

These annual cover crops can also be used in mature vines to improve vine vigor and nutrient availability. However, the timing of incorporation must be considered in order to prevent a high influx of nitrogen near bloom. This excess nitrogen can cause fruit-set problems. Generally, the nitrogen mineralization/mobilization process will take about three weeks after the cover crop is incorporated.

What if the vines are very vigorous and the goal is to reduce the vigor? In this case, as Tosch mentioned, cereal rye can be grown without the legumes in the mix. The rye will absorb extra water and nitrogen and help reduce vine vigor. In contrast to the young vines, this cover could be allowed to grow later into the spring, resulting in a higher carbon-to-nitrogen ratio. This will cause slower microbial breakdown and release of nutrients.

Another option is to plant perennial grass in all, or alternate, rows. Perennial grasses have a net nitrogen demand, so they will reduce the amount of nitrogen available to the vines by soil microbial processes.

Flowers are often blended into perennial mixes to provide pollen and habitat for beneficial insects and for aesthetics. The use of all — versus alternate — rows with the perennial cover can be used to calibrate the level of vine vigor. However, as a management tool, continual assessment is needed to decide when a change needs to be made in the type of cover crop being used. Dormant pruning weights are one way to measure the influence of the cover crop over several years.

A third option several growers are experimenting with is planting grasses, herbs and flowers directly under the vine in what is generally a clean, weed-free strip. Tosch says they are experimenting with chamomile and wooly thyme under the vines. One question is whether these herbs can positively influence the aroma and flavor of the grapes being grown above on the fruiting wire.

Herbs naturally grow under the vines in Otago, New Zealand, and are believed to contribute positive aromas and flavors to the wine. Winemakers often feel that nearby plants could have a positive or negative effect on flavor and aroma depending on what is being grown.

Sweet is taking her vine vigor reduction a bit further by planting perennial ryegrass blended with Prunella vulgaris (attractive low-growing native flower) in the vine strip to provide additional competition. She is also considering the white flower Alyssum under the vines. ◊

Jessica Cortell received an M.S. degree in Horticulture and a Ph.D in Food Science and Technology from Oregon State University. Currently, she owns her own consulting and vineyard management company, Vitis Terra Vineyard Services, and teaches at Northwest Viticulture Center in Salem.

A Growing Glossary

Allelopathy: A biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival and reproduction of other organisms.

Biomass: Biological material from living, or recently living, organisms.

Cover Crop: Crops planted primarily to manage soil fertility, soil quality, water, weeds, pests, diseases, biodiversity and wildlife in ecological systems managed and largely shaped by humans across a range of intensities to produce food, feed or fiber.

Legume: A seed, pod or other edible part of a leguminous plant. Well-known legumes include alfalfa, clover, peas, beans, lentils, lupins, mesquite, carob, soy and peanuts.

Nitrogen Fixation: A process by which nitrogen is converted from its inert molecular form to a compound more readily available and useful to living organisms.

Triticale: A hybrid of wheat and rye first bred in laboratories during the late 19th century. It was originally developed in Scotland and Sweden.

Vetch: A flowering plant in the legume family. Common vetch, together with broad bean cultivars such as horse bean or field bean, is included among the 11 most important legumes in the world, according to the Food and Agriculture Organization.