Around The Vineyard

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