The Optimal Time to Establish Late-summer Cover Crops in the Great Lakes Region

For organic growers, planting a cover crop after vegetable harvests is an important tool for weed management and soil building. In the Great Lakes region, there is often not sufficient time for a second vegetable, but there is enough growing season left for weeds to become a serious problem. Cover crops are only effective at producing these results if they are sown at the right time of the season. We identified the optimal planting date range for sudangrass and mustards, developing a degree-day model that allows growers to estimate the best time in their location. In order for the model to be applicable across the region, we did sequential plantings in two states: Michigan and New York. Sudangrass required a minimum of 700 growing degree days with a 50 °F base temperature (DD₅₀) before frost to suppress weeds and produce meaningful biomass and suppress weeds. 'Idagold' and 'Tilney' mustard required 1700 to 2200 DD₃₂before a hard frost to produce sufficient biomass. The biomass increased sharply with DD within that range, so a few days delay in planting can substantially reduce the cover crop value. The crucifer-planting window is approximately two weeks long, occurring in early-mid August in the cooler parts of the region, and late August in the warmer parts. When mustards were sown earlier (> 2200 DD₃₂) they produced no more biomass, but they did produce seeds. Those seeds create a high risk for volunteer mustard, that is a difficult weed problem. Tests of other crucifer cover crops (albeit not on organic ground) show that the response of cover-crop radish, brown mustard, forage rapeseed, forage turnip, and winter canola have exactly the same optimal planting window. All have a tendency to bolt and go to seed in the fall sown later than ideal, and a tendency to overwinter and go to seed in spring if sown too soon.