By: Becky Garrison
The United States Department of Agriculture defines a cover crop as “a crop generally recognized by agricultural experts as agronomically sound for the area for erosion control or other purposes related to conservation or soil improvement.” The USDA National Resources Conservation Service website offers this concise explanation of the multiple benefits of implementing a cropping system. “They can prevent soil and wind erosion, improve soil’s physical and biological properties, supply nutrients, suppress weeds, improve the availability of soil water and break pest cycles along with various other benefits. The species of cover crop selected along with its management determines the benefits and returns.”
While many vineyards have been utilizing cover crops for years, are they maximizing the poten-tial of this agricultural practice? During the 2020 Oregon Wine Symposium, held virtually from February 16 to 19, 2020, two panels delved into the nuances of what defines high quality soil and the role cover crops play in generating organic soil, focusing particularly on how to best utilize cover crops in the vineyard setting.
At the first panel, “Soils and Cover Crops: Back to Basics,” James Cassidy, Senior Instructor in Soil Science & Sustainable/Organic Agriculture at Oregon State University, opened with a short presentation on Soil 101. He described soil as “rotted” rock and decomposed organic matter. Next, he broke down soil into soil solids (mineral 45%, organic 5%) and pore space (air 20-30%, water 20-30%). Carbon from the atmosphere and energy from the sun transform these materials into soil courtesy of the billions of organisms present in a single pinch of soil.
“Internalize the idea that soil is habitat for everything that is alive on this planet,” Cassidy said.
During irrigation, rain or another precipitation event, gravity pulls the water into the soil’s macropores. As water flows through these macropores, it sticks in the micropores inside the soil and eventually fills with water. Then the soil drains, and there’s air in the soil.
“When the soil is draining, it’s actually breathing,” Cassidy said.
The rate at which water moves into the soil is impacted by various factors, including soil type (texture structure, aggregate stability, hydrodynamic characteristics), topography and morphology of slopes, flow supply (rain intensity, irrigation flow), and the initial condition of the soil’s humidity.
Currently, on Earth, a hectare of productive soil is lost every six seconds. While farmers can’t change their land’s soil type or topography, they can manage for stable aggregates, a wide pore size distribution structure and minimized compaction. Well aggregated, stable soil pulls in water. This means the water goes deeper into the soil instead of producing runoff, which carries away the small particles and organic matter that hold nutrients in the soil.
For every 1% increase in organic matter, soil can store up to 25,000 more gallons of water per acre. In Cassidy’s estimation, cover crops represent the easiest way to add organic matter to the soil. “Cover crops improve the soil structure by punching big holes into it with their roots. When those roots decompose, they stabilize the aggregates so those macro pores stay there a long time and can actually reverse compaction.” In comparison, tillage breaks up the macros, thus shredding all the organic matter that’s connecting and holding the soil together.
How to Choose Cover Crops
At the second panel on cover crops, “Digging Deeper into Cover Crops: What’s Happening in Oregon?” Gordon Jones, Assistant Professor, OSU, Southern Oregon Research & Extension Center, delved into the myriad ways to utilize cover crops in the vineyard. Before planting, Jones recommends that growers clearly define their goals. Is their intention to increase production or profitability, improve soil health or appeal to consumers looking for wines made using sustainable means? When viewed in conjunction with a given vineyard’s particular climate and soil, these goals will inform the type of cover crops that would lead to optimal results.
Perennial crops such as ryegrass and tall fescue have been bred for forage and high yield, and they do require frequent mowing. Legumes like fava beans, white clover and strawberry clover can add nitrogen to the soil. Be mindful when sharing the land with livestock, as Jones addressed a fungus – endophyte – that can be present in certain turf-type cover crop grasses and can be harmful to livestock if grazed.
From an environmental perspective, planting deep-rooted perennial crops can improve soil health and decrease runoff. These crops will sequester carbon that helps to address some climate change concerns and build soil organic matter.
The planting schedule for cover crops varies according to a given region. For example, in a state like Oregon, known for hot, dry summers, many cover crops are planted in the fall to establish their root system before the summer.
The way a field is mowed, irrigated or fertilized will result in different mixtures of species of cover crops dominating. While complex mixtures of cover crops are often planted, Jones said that generally, only a few species within a mixture contribute significantly to the cover crop’s biomass. Over seeding in alleyways can result in an improved cover crop, though one must be mindful that any existing cover crop or weeds can be quite competitive with the newly seeded plants.
When debating to till or not to till, Jones said that tillage could be hard on soil aggregation and the pores in the soil. “If you’re interested in maximizing infiltration and soil health, consider a notill system.”
He added that tillage and herbicide can be used to kill all existing plants, and they are important tools in many growers’ toolboxes. Once the soil is devoid of existing vegetation, one can more successfully establish a cover crop free from unwanted plants. Those opting to farm using organic and low-input means by not using pesticides may need to contend with other plants sprouting alongside their cover crops. Also, self-seeding or self-regenerating winter annuals planted in the fall that go to seed by May and sprout again in the following fall can afford minimal soil disturbance.
In conclusion, Jones pointed to preliminary research in Southern Oregon, Ithaca, New York, and South Australia that suggests actively growing cover crops directly underneath the vine can de-crease the need for under-vine herbicide or tillage. Which species of under-vine cover crop to plant in Oregon is still an open question. The ideal species would be low growing, competitive against weeds, but not too competitive with the vines. Further work needs to be done in evaluating undervine cover crop options before growers add this to their toolbox.
Those looking for professional guidance can hire a consultant such as Rebecca Sweet-Smith of Buzz Cover Crop Seeds and Corridor AgLand Consulting, who provides diverse and organic seed mixes for ecologically minded vineyards and farms in the Pacific Northwest. While not in-volved directly in growing grapevines, Steve Groff, founder of Holtwood, Pennsylvania’s Cover Crop Coaching and author of The Future Proof Farm: Changing Mindsets In A Changing World, can offer assistance to those looking to explore organic ways to mimic nature when growing cover crops.
Also, Amy Bartow with the USDA NRCS Corvallis Plant Materials Center presented an overview of the costshare programs available for cover cropping in vineyards along with examples of their projects to date. She recommended that interested parties get in touch with their local Soil and Water Conservation District to get information about specific programs available in their particular area.