Cover crops for the desert
Research shows the best cover crops for eastern Washington are perennial grasses.
This photo of a five-year planting of the Canada mix cover crop shows how it goes dormant in the summer.
It can be tough to get cover crops to grow in the arid climate of eastern Washington, but Washington State University scientists have identified several drought-tolerant cover crop species for vineyardists who want to protect their topsoil and enjoy other benefits of a cover crop.
The benefits are plenty, from reducing spider mite problems because of the reduction of dust, to preventing soil erosion, suppressing weeds, improving soil aeration, reducing vine vigor, and enhancing populations of beneficial insects. Preventing soil erosion is one of the prime reasons for a cover crop. Research shows that growers can lose one-half to an inch of topsoil per year just from tillage. Erosion can also occur from wind and rain.
Although there can be a few negative impacts from cover crops, such as attracting rodents and meadow voles in certain cover crop species and competition for water during drought, those downsides can be mitigated by cover crop choice and spraying cover crops dead in a short water year to conserve soil moisture.
But the challenge for most Washington State grape growers has been to find a cover crop mix that will grow in a drip irrigated vineyard that receives less than seven inches of precipitation annually, said Dr. Mercy Olmstead, WSU viticulture extension specialist. The focus has been to find drought-tolerant species that can help prevent soil erosion instead of trying to add nutrients back to the soil because, in a drip-irrigated vineyard, there is little soil moisture to stimulate microbial activity between vine rows where the cover crop is growing.
She notes that because western Washington receives a wide range of rainfall, with an average of about 35 inches per year, vineyardists there have a wide range of cover crop choices and can grow a lusher cover crop that can suck up water, reduce vigor, and add nutrients back to the soil.
Olmstead began searching in 1998 for cover crops that would grow in vineyard floor management programs in eastern Washington's arid climate. She first planted about 175 native and nonnative species in small plots, planting 45 seeds per plot. Her selection criteria were to find those that would hold down the soil in the vineyard rows, provide traction in the fall months, and go dormant in the summer to minimize negative effects on vine growth.
After narrowing down the list of potential species, she then planted several cover crops in commercial vineyards to compare with resident vegetation. Sites were visited years later to evaluate the cover crop's longevity. Her research measured the influence of cover crops on ease of establishment, percentage of vegetative cover, stand duration, plant height, vine water potential, and soil moisture.
Olmstead found the most economical cover crops were perennial, bunch-type grasses that didn't require the time, expense, and fuel needed for soil preparation and seeding that annual species require. Resident vegetation can work as a cover crop and it lacks the planting costs that seeded cover crops incur, but invasive weeds can be a problem and can be hard to eradicate once the vegetation is established.
From the trial, a clear winner was a perennial grass mix called Canada mix (40 percent crested wheatgrass, 40 percent pubescent wheatgrass, and 20 percent perennial ryegrass). "It was a favorite because it provided staggered germination, goes dormant in the summer but still comes back, and did better on a high ET [evapotranspiration] day when a lot of water was used by the vine," she said.
Olmstead noted that if growers had difficulty locating a seed source for the pubescent wheatgrass, they could substitute and turn the mix into a 50-50 mix of crested wheatgrass and perennial rye.
She also had good results with a fescue grass mix and cereal ryegrass, but trials of medics, including Santiago burr medic, were not as successful as hoped.
Most of the cover crops depressed early season weeds, but for the long term, the Canada mix and crested wheatgrass were best.
Cover crop choices
For Pacific Northwest vineyards, Washington State University recommends the following cover crop species:
Canada mix (crested wheatgrass, pubescent wheatgrass, and perennial ryegrass)
Fescue grass (tall, sheep, or red)
Red, crimson, and subterranean clovers
Burr, barrel, and black medics
Mustards and brassica species
For more information on growing cover crops, download for free the Extension Bulletin 2010 Cover Crops as a Floor Management Strategy for Pacific Northwest Vineyards at http://cru.cahe.wsu.edu/CEPublications/eb2010/eb2010.pdf.