By: Annie Klodd, University of Minnesota Extension Educator
Under-vine cover crops are managed plants grown underneath the rows of vines. Vineyard managers grow them on purpose to help achieve certain goals in the vineyard. I find them fascinating to talk about because their benefits and challenges are complex and never guaranteed.
Grape growers working with overgrown vines might bring in competitive cover crops to slow their vigor. Organic-leaning vineyards often use cover crops to suppress weeds in place of herbicides. Those on steep hillsides should think about installing cover crops to stop erosion. If the cover crop is selected and managed well, vineyard managers should find success with these goals for at least several years.
Other benefits are less straightforward. In some cases, cover crops might help increase soil organic matter and retain nitrogen, but the effects are often less that one might expect. Grasses tend to re-capture the nitrogen they put back into the soil after decomposition, so it may never reach the grapevines. Flowering cover crops will help attract pollinators like bees to the vineyard. This is wonderful as long as you are avoiding insecticides, which can kill those bees congregating around your flowering cover crop.
Growers considering cover crops should first decide what goals you want to accomplish. Then, learn which types of cover crops to plant based on what will help achieve those goals. For example, a simple perennial fescue mix works wonders for most goals – vigorous fescues can easily outcompete most weeds, slow excess vine growth for several years, and support soil structure while requiring minimal mowing. Flowering species can be incorporated if desired. Vineyards hilling up graft unions each winter should consider annual cover crop species because the soil is frequently disturbed.
Slow Down Overly-ambitious Grapevines
Is excessive vigor a problem in your vineyard? Are you constantly hedging and thinning to keep them under control? A strong, thick stand of under-vine cover crops might help slow the growth of overly-vigorous grapevines. Plants with dense root systems like perennial grasses compete with grapevines for water, nutrients, and space in the soil. When those roots get established, they tend to take up so much space in the soil that the grapevine roots avoid the top several inches of soil. The top few inches of soil are often the richest in nutrients and organic matter. So, when the grapevine roots are forced out of this area of soil, it can cause the vines to grow more slowly as they have limited access to the buffet of top soil resources.
To use cover crops for vine vigor management, select a short, densely-growing perennial grass such as fine fescue. Wait to plant it until the vines are at least 3 years old, so that the grass competition does not stunt their early growth.
There are many fine fescue species to choose from. The species that I have personally worked with is creeping red fescue. Fine fescues are short, only growing about 8 inches tall, and only need to be mowed 1-2 times per season. These grasses are also effective weed suppressors because they form a thick thatch of grass blades along the soil surface, blocking sunlight from hitting the soil surface. During my graduate research on fine fescues in vineyards, creeping red fescue formed a dense tangle of roots in the top 10 cm of soil, forcing the grapevine roots to avoid that area and grow deeper into the soil.
To plant this cover crop, spread the seeds on lightly cultivated soil under the vine rows. Fescues are cool-season grasses that germinate best in cool, rainy weather. So, plant them in the spring after snow-melt for best results.
Say No to Weeds
To suppress weeds, the stand of cover crops should be dense enough that it prevents direct sunlight from hitting the soil. Perennial fescues, described above, achieve this well. Clovers do a relatively poor job of densely covering the soil unless they are planted at very high seeding rates. If you want weed suppression but also insect attraction and nitrogen fixing, mix clover in with a fescue seed mix rather than planting it alone.
Annual cover crops generally germinate and establish much faster than perennial species. They will do a better job of fighting weeds in the three months after planting. The obvious tradeoff is that they must be re-seeded each year. Perennial cover crops are less likely to provide effective weed management in the first year but will perform better long-term if managed well.
Many annual species can be used for weed suppression if planted at high seeding rates. It is wise to include grasses in an annual cover crop mix if weed suppression is a goal, because they will help cover the soil more densely. Hairy vetch has also been shown to have good weed suppression but can become weedy. Whatever you do, just be sure to select low-growing species.
Cover Crops for Soil Health
Cover crops improve healthy soil by adding organic matter, reducing erosion, and improving soil structure. Reducing erosion and improving soil structure are always good things in vineyards. But those considering cover crops for organic matter and nitrogen additions should ask themselves two questions first: 1) Will my cover crop add enough organic matter to make a difference? And 2) Does my vineyard actually need higher organic matter and nitrogen, or is my soil already ideal for grapevines?
Sometimes certain cover crops can help retain or increase soil nitrogen, but this effect does not necessarily benefit the grapevines. If the soil and foliar tests indicate that the vines have sufficient nitrogen already, adding more is not beneficial. Most vineyards I know in the Midwest do not need to increase nitrogen.
Secondly, even though grass cover crops put nitrogen back into the soil when they break down, their roots tend to take this nitrogen back up rather than releasing it into the soil for the grapevine roots to use. A 16-year study at a Cornell University vineyard found that bark mulch under the rows contributed more nitrogen to the soil than grass or herbicide treatments.
This same 16-year study also found that bark mulch contributed more organic matter to the soil than cover crops, along with a 10-fold increase in phosphorous. While all plant matter contributes organic matter to the soil, the amount of matter added via cover crops is generally less than large outside inputs like mulch, compost, or manure.
The ideal soil organic matter content for grapevines is 2-3%. Adding organic matter beyond that may not actually benefit your crop and can lead to excess vine vigor. Therefore, it is OK if a cover crop’s organic matter contribution is small.
If, after reading this, you think that your vineyard’s soil could use more organic matter and nitrogen, consider cover crops, compost, or wood-based mulch accordingly.
Flower Power or Flower Downer?
Flowering cover crops attract beneficial pollinators like bees. I am a full supporter of bees. That said, when deciding to add flowering cover crops to your vineyard, consider whether or not you plan to use insecticides going forward. Many broad-spectrum insecticides, including some organic ones, are highly toxic to bees.
Because flowers attract these beneficial and sensitive insects to the vineyard, pollinator experts at Michigan State University and University of Minnesota have advised against spraying insecticides if flowers are in bloom. If you do use insecticides throughout the season, opt to plant flowering plants outside the vineyard rather than in the rows. Alternatively, you could try growing flowering plants and mow off the flowers before spraying insecticides.
Planting a Vineyard? Hold that Thought
Just like weeds, thick stands of cover crops in the rows will slow the growth of newly planted vines, causing a delay in yield. In general, I advise waiting to plant cover crops under vine rows after Year 3, or until the grapevines have both cordons established (you should still plant grasses in the aisles during this time).
The exception to this 3-year rule would be if the vines are excessively vigorous during establishment – in that case, a cover crop could help manage their rate of growth and suppress bull canes. I have seen this occur in a Minnesota vineyard with over 5% organic matter. Due to the extreme vine vigor during Years 2 and 3, I broke my rule and recommended planting an under-vine cover crop during vineyard establishment.
If cover crops seem like the right choice for you after reading this far, give them a try! Reach out to your local seed representatives and ask for some samples to try in small areas of the vineyard. If it works for you, let your neighbors know!
Sources:
• turf.umn.edu
• Atuche et al. 2011. Long-term effects of four groundcover management systems in an apple orchard.
• HortScience 46(8)
• Klodd et al. 2016 Coping with cover crop competition in mature grapevines. Plant and Soil 400: 391-402
• MSU webinar recording – April 8th, 2021
I have 2 areas of my vineyard, a small hillside that has less fertility and less weed growth beneath the vines than my flatter, more fertile bottom field. Each is about 3 acres in size.
I’m wondering if the creeping red fescue might work in the my bottom field, which, even with herbicides doesn’t seem to control weeds so well.
With the red fescue can I have the 8″ of growth beneath the vines and not have to mow it? Your article mentions mowing twice a year. This would work between the rows, where now have a grass. but I assume you are not speaking about mowing directly beneath the vines.
I also have drip irrigation but with 2 emitters directly on either side of my vine trunks.
I have also ben having “horseweed” issues the past 2 years, which is a lot of work pulling, spot spraying , and in other ways trying to ocntrol it.
Any suggestions you can make would be helpful
I enjoyed your article very much.
Sincerely,
David Mitchell
Mitchell Vineard, Oregon, WI.