Cover crops have many benefits in vineyard floor management, but in Washington State, especially east of the Cascade mountain range, they can be challenging to establish. Researchers are studying cover crops in an organic and conventional vineyard to see if the benefits in terms of pest control outweigh the efforts needed to establish the crop.

Dr. Mercy Olmstead, viticulture extension educator for Washington State University, has a special interest in cover crops and vineyard floor management in Washington State. She did her postdoctorate thesis on the subject and recently wrote a WSU bulletin titled Floor Management in Washington State Vineyards to be published later this year. She is also co-project leader in a new study comparing insect populations in vineyard cover crops, funded by the Northwest Center for Small Fruits.

She notes that growers in the Pacific Northwest use a variety of vineyard floor management systems—resident vegetation, clean cultivation, cover cropping, and cultivation. The type of vineyard floor management should tie in with the overall vineyard goals. Is vigor control needed? Protection from soil, wind, or rain erosion? Is the goal improved water infiltration?

Resident vegetation

Resident vegetation requires little input and involves minimal cost, she explains, as it consists of all plant species, both native plants and invasive weeds that are growing within the vine row and alleyways.

Vegetation is usually mowed to control plant height for vineyard traffic, with mowing frequency determined by the type of plants. The area under the vine may be kept weed free by a herbicide application in the spring.

In conventionally managed systems, growers using clean cultivation keep the rows disked or sprayed with herbicides to reduce vegetation, while organic growers use mechanical means to keep the rows clean and weed free.

Olmstead adds that the bare ground can reduce water and nutrient competition for the grapevine, but it can increase the risk of soil erosion by wind or water, especially on slopes. Additionally, the tillage and equipment needed to keep the rows clean can create other problems, such as root zone compaction and poor water infiltration.

Cover cropping

Cover crops are typically seeded in every row, although she said that some growers plant them in alternate rows, each with a solid stand of a different cover, such as a grass and legume, or they rotate the rows between clean cultivation and a cover crop.

Organic growers must use mechanical means to keep the area under the vine free of weeds, but conventional growers can choose to use a herbicide or cultivation to keep the vine area free of plants and reduce water and nutrient competition.

Cultivation

Depending upon the cover crop choice, tillage or mowing may be required when cultivation is the choice, Olmstead stated. “Annual cover crops, such as a number of grass and grain cover crops, may need to be mowed a couple of times per year in order to facilitate access throughout the vineyard or for frost protection in the spring.”

Growers can also till the crop into the soil for nitrogen release and uptake by the vine, she said, adding that it is important to know the decomposition rate to ensure that nitrogen release coincides with the appropriate vine growth stage. Mowing is sometimes needed with perennial cover crops because they can grow three feet high.

As perennial cover stands diminish and need reseeding, she advises growers to consider tillage as an option for re-establishing the stand.

Vineyard floor management has many benefits, depending on the system chosen, she said. “Vineyard age should be considered, as vigorous cover crops can compete with young vines for water and nutrients. But once a vineyard is established and vines are mature, cover crops may be used in areas of excessive vigor to reduce canopy size and maintain an optimal balance between the vegetative and fruiting sections of the vine.”

One of the biggest benefits of cover crops is reducing soil erosion from wind and water. Wind and water erosion can strip the upper soil layers, removing up to 2.5 inches of soil in a growing season, according to Olmstead. Cover crops also help protect the soil surface from high traffic during the season and can increase traction for equipment, an important consideration in the Northwest where harvest extends into the fall when rainfall typically takes place.

Other benefits include reducing surface crusting, improving rainfall penetration, and increasing soil organic matter and nutrient availability. However, in the arid and semi-arid regions of Washington, she points out that the impact of cover crops on increasing organic matter is dependent on soil type, temperature, and soil moisture.

“Many of the microorganisms involved in the decomposition of organic matter need some type of moisture to maintain their activity,” she said, adding that in dry regions, it is difficult to greatly affect the percentage of organic matter in the root zone without supplemental irrigation in the vineyard alleys.

Caution

She also advises growers to be cautious when using mycorrhizal supplements because research under field conditions has not definitively shown consistent benefits. Mycorrhizal fungi are symbiotic fungi that colonize grapevine roots.

On the negative side, cover crops can increase vertebrate (rodent) populations, may increase the risk of frost in early spring or late fall, and can be more expensive to incorporate into the soil than resident vegetation.

“The major challenge to growing cover crops in Washington is establishment,” she said. “It’s getting the water needed to establish the cover crop to the seed.”

Most cover crops in eastern Washington State are planted in the fall to take advantage of winter precipitation. In the spring, cover crops can be irrigated with overhead sprinklers in those vineyards that have them, but in vineyards with drip or micro-irrigation, supplemental water is likely needed.

Some growers with drip irrigation systems bring hand lines or a mobile irrigation system into the vineyard specifically to help establish the cover crop.

Supplemental irrigation adds another expense to farming and should be considered carefully to make sure the economics pencil out, as “the costs may outweigh the benefits,” she added.

A cover crop trial was initiated last fall with four different cover crop treatments planted in an organic, drip-irrigated vineyard and a conventional, sprinkler-irrigated vineyard. Olmstead and Dr. David James, WSU entomologist, are cooperating on the project that will study the treatments to determine which type of cover crop—native vegetation, flower-grass mix, cereal rye, or a drought-tolerant medic mix—performs the best and harbors the most beneficial insects and arthropods.

The trial will also examine which cover species attracts beneficials and serves as a host site for them to overwinter.

“We want to see if we can keep them there in the cover crops during the winter,” she said.

The sites will be managed as conventional or organic, and mowed when the growers would normally mow. In the organic site, the cover crops will be allowed to go dormant in the summer because the drip lines will not reach the cover crop with moisture. Olmstead noted that some organic growers prefer to leave the vineyard row soil untouched because disturbing the soil allows carbon to be released into the atmosphere.

“We want to learn if there may be added benefits from cover crops in terms of pest management,” she said. “It may be that unless you can reduce your chemical sprays for leafhoppers and mites, that it really doesn’t pay for cover crops in eastern Washington due to the challenges of getting water to establish the cover crop.”