Cover Crop Selection for Vineyard Conditions Excerpt from Mark Greenspan's Wine Business Monthly article

Use of sown cover crops is not universal, but in the North Coast, it is quite common. In wet winter climates, they are sown in the tractor rows usually following harvest (and occasionally before harvest if the harvest drags on long enough). Lack of cover crops in wet climates can lead to serious issues of erosion and compaction, not to mention the absence of their benefits, which will be discussed below. On the other hand, in dry winter climates cover crop establishment can be challenging, or borderline impossible, because it requires rainfall—specifically, rainfall when temperatures are moderate. Of course, overhead sprinklers could supplement rainfall to establish a cover crop, but using additional water to grow a cover crop that sucks up more water from the ground in a dry climate does not seem desirable or logical. So, I restrict my thoughts to wetter growing regions.

And by wetter regions, I am thinking primarily of the West Coast winegrowing region, which receives the majority of precipitation during the winter months and has a relatively dry summer season. This is quite different from much of the rest of the country, which receives substantial and sometimes tremendous amounts of summer rainfall. Yes, they need cover crops in those regions and for many of the same reasons we need them here on the West Coast. But, the specific species of cover crops are necessarily different in most regions than out west.

Why Use Cover Crops? 
I feel a little silly going over this because it seem so ingrained in our practices already, but there was a time, even early in my career, when cover crops were seemingly not mainstream, as they are now. So, writing about their benefits seems like preaching to the choir. You already know that cover crops help firm up the soil under wet conditions, thereby reducing compaction by tractors and other equipment, reducing erosion, improving water infiltration, improving soil organic matter, increasing soil aeration, providing substrate for soil microbiota, moderating vineyard microclimate and reducing dust and, thereby, insects, such as spider mites. Didn’t you? And you also knew that cover crops can (albeit moderately) compete with vines for water and nutrients to control vine vigor, fix atmospheric nitrogen by association with specific soil microbes (leguminous plants only) and support insects, both beneficial and potentially harmful—but mostly beneficial.

On the other hand, cover crops can cause excessive competition with vines, potentially over-stimulate vines with nitrogen fixation, deter soil microbiota and harbor vertebrate and insect pests. Hence, while cover crops are largely beneficial, they require management, and that begins with their selection. It’s rather commonplace to select the generic all-purpose blend from the local seed company; and while that is usually better than nothing at all, it’s not suitable for all situations. The most successful growers will determine one or two (or three) blends for their vineyards and apply them where appropriate. Blends can be ordered through most seed companies, so they need some lead time; and if you are choosing blends while you are reading this article, you may possibly already be too late.

Grasses
Grasses form thick, fibrous root systems and are therefore great for building organic matter in the soil, adding stability and firming the surface of the soil. They can also be quick-to-establish, which is important because they are needed most when it starts raining. And when rain falls torrentially, you’ll want to have them established by then. Grasses are also very good at water extraction and are relatively wasteful with water. That can be beneficial for wet spring years, as the grasses will help to extract that moisture more quickly than a vineyard without them. But, as I mentioned in a recent column1, the depth of moisture extraction is restricted mainly to the upper foot of soil depth and is short-lived. Nevertheless, it’s important that the upper soil profile be dried out, after spring rainfall events, to allow for tractor/sprayer access.

The competition effect of grass cover crops may be less than is often believed, but it is still a factor. Grasses consume not only water but also nutrients, and those nutrients can be effectively immobilized temporarily in this manner, which can help modulate the nutrient availability to the vines, as the minerals are released as the organic matter breaks down over time. For most situations, we use annual grasses, such as dwarf barley and oats in the cover crop blends, though other grasses may be successfully used.

Some grasses can be very competitive with vines, but those are mainly perennial types. Perennial grasses can be deep-rooted and compete with vines much more deeply into the soil profile than the annual grasses and are, therefore, rarely suitable except in the wettest of climates or highest water-holding capacity soils. The exception to the non-desirability of the perennial grasses is dwarf varieties of fescues, which can be used in many situations, are shallow-rooted but form a nice firm carpet in the vine rows that can last for years before requiring a refresh of new seed.

Grasses may also compete with vines in other ways besides water and nutrient competition. Grasses, like some other plants, can compete chemically—called allelopathy—where root exudates are mildly toxic to vine roots. Truly, vines don’t get along with others very well, and so maintaining an under-vine cover crop is rarely a good idea, especially with grasses. Ryegrass is thought to have some allelopathic effects on vines and so is often discouraged, especially if it is to grow near the vine row itself.

Legumes
Legumes have a niche in many vineyards for their unique ability to fix nitrogen. Actually, it’s the nitrogen-fixing bacteria that form an association with legume roots that do the work of fixing the nitrogen. A field of legumes can fix 50 to 100 pounds per acre of nitrogen, but fortunately most of that is in the form of organic nitrogen. If allowed to break down slowly, this can form a nitrogen source that can virtually replace fertilizer sources. Yet, many or most growers till in their cover crops, including the legumes, so the breakdown may be rapid, and much may be released back into the atmosphere. Yet, with minimal or no tillage becoming more common, legumes can play an important role in a cover crop blend. There are many to choose from, including various peas that are usually quite successful in a blend. Fava beans, also called bell beans, are often also successful in a blend, and can produce quite a lot of biomass and organic matter for the soil.

Clovers can be quite successful as cover crops, but they emerge late and can often become buried within the taller grasses when they begin their development. I have had limited success with clovers in blends and, as a result, usually don’t include them. However, vineyards that require more legumes than grasses, clovers can be quite successful, producing a thick green carpet with showy flowers when planted mostly on their own.

Most standard cover crop blends for vineyards include vetch, which would be an excellent cover crop were it not for its propensity to creep into the vine rows, where it becomes difficult to manage. Vetch likes to crawl up vine trunks and so can be difficult, especially when herbicides are not used to keep it from doing so. Hence, it is a cover crop component I usually avoid, except for open fields prior to planting, in which case it can be a very good pre-plant cover crop, along with clovers, provided it is mowed down before going to seed.

Flowers
Flowers can provide both a functional role and an aesthetic role. Aesthetics are obvious. Who doesn’t enjoy looking at a row of flowers between rows of vines? Winery customers love them, and, indeed, many rows of under-vine flowers have been established to provide a pleasing entrance to a tasting room or processing facility. But they also serve a functional role, largely as an insectary.

Many growers will sow insectary rows every eight to 10 rows as a means to retain insects in their vineyards. In theory, their purpose is to harbor beneficial insects—those that prey on our vineyard pest insects. In reality, they are not specific enough to favor the beneficial insects: Flowers can harbor and attract both beneficial and pest insects. So, are insectary rows really that beneficial? Well, consider the vineyard ecosystem without showy flowers. Vineyards are usually monocultures, and grapevines themselves are not insect-pollinated. So, vineyards do not attract many insects compared to a more natural ecosystem. We’re not striving for an insect-free vineyard, so attracting a population of insects into our vineyards should help strike a balance between pests and predators. All in all, it just feels right even if it has questionable real benefit to a commercial vineyard.

Brassicas
We often get asked to plant brassica plants, primarily mustards, wild radish and canola. Many people love the look of a vineyard in the spring with a nice stand of showy mustard flowers. Again, an aesthetic function. Is there also a functional reason to plant brassicas?

There is some evidence that brassicas deter nematodes. I spoke with UC Davis professor Andy Walker years ago, and he told me that the nematode suppressive effects of brassicas were modest and that a field infested with nematodes could not be practically relieved by planting a field of mustard. That said, a long-term fallow, such as six or seven years, may include a brassica cover crop because, why not? It couldn’t hurt. But who fallows ground for that long anymore? For short-term fallow, brassicas will offer no real benefit against nematodes.

A friend recently pointed me to a news article about UC Davis assistant professor Rachel Vannette, who has been conducting research on Arbuscular Mycorrhizal Fungi (AMF). AMF are among the most important of the soil microbiome, and grapevines benefit greatly from them. AMF form a symbiosis with many species of plants, including grapevine, obtaining carbohydrates from their hosts while serving as a functional extension of the host plant’s root system. In so doing, they help the plants access immobile nutrients, such as phosphorus and zinc.

Interestingly, Vannette found that AMF do not associate with brassicas. A little digging revealed other sources of this same knowledge. About 5 percent of plant species are known not to form associations with AMF or other mycorrhizae. Of those used as vineyard cover crops, brassicas are the
important ones.

So, are we reducing soil health by sowing brassica cover crops? Perhaps in some ways we are, though we rarely plant brassicas on their own. And it is questionable whether the AMF produced in the row middles even associate with the vines. Considering many growers till their vineyards and break up those associations, this could be a non-issue. However, I tend to think that with reduced tillage practices and reduced use of potentially harmful herbicides, we want to build up our soils in any way, both under-vine and in the row middles. So perhaps we shouldn’t be so eager to see those mustard flowers every year.

Final Thoughts
Cover crop selection should not be made casually, but it does not have to be excruciating either. Don’t stress over a few percentage points of specific species but do focus on the relative amounts of grasses (soil stability and organic matter, but also competitive) and legumes (nitrogen-fixing but vigor-inducing) in the blend. Toss in some showy flowers if you must but I prefer to sow flowers separately from the workhorses (the grasses and legumes). Or alternate rows of grasses and legumes, perhaps including showy clovers. Ultimately, your site limitations and sense of aesthetics should also be allowed to enter into the decisions. Outside of the practicality of cover crops, they can also be a bit of a creative release for us after a long growing season.

Check out Mark's full article in the October issue of Wine Business Monthly. 

Contact Mark and Advanced Viticulture at: 707-838-3805, https://advancedvit.com/, or info@advancedvit.com