COVER CROPS IN THE VINEYARD
February 7th, 2010
Many of our wine club members have asked questions related to cover crops, so I would like to elaborate on the issue based on my personal experience…
A cover crop is a crop grown to cover the soil – serving as a protective layer. Sometimes, it is tilled into the soil as a “green manure.” Other times, it is a living or dead mulch located on the surface of the soil. Basically, a cover crop can be any type of plant. The most common types are grasses, legumes, or grass/legume mixtures.
Cover crops play an essential role for vineyards, especially here in Eastern Washington. Without them, the topsoil would blow away & the weeds would take over. Depending on the type of cover crop used, they can also be beneficial by decreasing water runoff, enhancing infiltration, conserving soil moisture, supplying nitrogen, increasing nutrients & organic matter, improving the structure & overall quality of the soil, suppressing soil born diseases, attracting beneficial insects, & encouraging a diverse ecosystem.
Finding the right cover crop to grow can be quite challenging in the Yakima Valley because of the dry, arid climate & sandy soil conditions. Over the years, we‘ve learned a lot about specific cover crops & their interactions with vineyard sites.
In the past, we’ve tried many permutations. We’ve used Sudan grass, Vetch, & Austrian winter peas. All of these crops are annuals, meaning they grow from seed & do not grow over winter. They all increase the tilth of the soil through the introduction of organic material at the end of the season. (Good tilth refers to soil that has the proper structure & nutrients to grow healthy crops.)
The downfall with these cover crops is that they also increase the total available level of nitrogen in the soil, and wine grapes tend to be low users of nitrogen. Increasing the amount of nitrogen above certain levels can actually cause the vine to produce excess vegetative growth, which in turn, can adversely affect wine quality.
After discovering this issue, we decided to use a cover crop that incorporates perennial grasses. The product that we had the most success with was a blend of several perennial grasses named Companion. Because it was a blended product, it could adapt to a multitude of vineyard conditions, resulting in a highly adaptable cover crop that performed well in a multitude of vineyard environments.
Nevertheless, we found one major obstacle with Companion – rodent control. Eventually the floor of the vineyard became so undulating due to the incessant borrowing of the rodent and coyote populations, that it was difficult to transport equipment, such as mechanical harvesters or sprayers, at the optimal speed.
In 1997, we installed our first drip irrigation system. With this method of irrigation, a drip line was suspended beneath the cordon wire, and it delivered water to each vine via individual emitters. Before conversion to drip, we were using furrow irrigation, which delivered water with ditches. We also used sprinklers, which delivered water over the top of the vine, wetting the entire surface of the vineyard. With the conversion to drip, we lost the ability to irrigate the area between the rows, where we plant cover crops. With the change, we needed to rethink our strategies.
Our new strategy is to use rye, which is an annual grass similar to wheat. Rye grass can often be grown under conditions where other cover crops fail because it establishes quickly and grows throughout the winter. Rye is an excellent choice for soil protection and weed suppression.
At Airfield Estates, we plant the cover immediately after harvest. We utilize winter precipitation to germinate and nourish the cover crop. Once we have established a healthy stand between the rows and before the plants become overly dry, we kill the cover crop. The mulch left by the dead cover crop can be used to help prevent weed germination in that area for the rest of the grape growing season. Control is not always stellar, so from time to time it may be necessary to run a mower through the middles to keep weeds at bay.
This dialog basically exhausts Airfield’s experience with cover crops. In other regions, with other climatic and topographical influences at play, the discussion could be much different. In general, the cover crop must match the site and vine vigor. Growers need to be aware of cover crop and vine interactions, and they must realize how these interactions may change as the vine matures.
Written By Mike Miller
Airfield Estates Grape Grower