The 2012 ASHS Annual Conference

In order to get the benefits of a cover crop, the cover crop must grow vigorously. One of the main determinants of growth rate is planting date. We determined the appropriate planting dates for late-summer cover crops for use on organic vegetable farms in the Great Lakes region. We measured how quickly soil was covered and the ability to compete with weed seedlings, as well as biomass production during the available growing period. Mustard could besown until late August, requiring 1500  to 2000 GDD₃₂. All the crucifer cover crops examined produced minimal biomass by frost if they experineced fewer than 1000 GDD₃₂. Biomass increased linearly with GDD thereafter. The rate of increase will depend on nitrogen fertility and other factors, but relative rates were clear. Mustard, turnips and rapeseed had the same rate of increase, producing about 2 tons per acre after 2000 GDD₃₂. Forage radish (including the whole swollen hypocotyl) grew about twice as fast, and canola about half as fast. Thus the cold-hardiness of canola did not allow it to be planted later than the other cover crop types. The current enthusiasm for forage radish as a cover crop is justified when they are planted early,that is when there are 1700 to 2300 GDD₃₂ remaining in the season. Late-planted crucifers not only provide minimal biomass, but they survive the winter better resulting in undesirable early-blooming volunteers. These values allow growers to adjust planting dates based on climate data for their farm. The optimum planting dates in western New York to southern Michigan are approximately August 10 to August 20.