I awoke to very frosty fields this morning, as is expected this time of year in Maine. The basil died long ago, the peppers have been limping along through light frost after light frost until they finally died last night. Now we can really see which crops are the true fall superstars in the field. As the sun finally warms everything, the broccoli looks just as good as it did yesterday. And so do the phacelia, crimson clover, and radish. I want these cover crops to die eventually- just not yet.
Cover crop cold hardiness
Like cash crops, cover crops vary tremendously in their cold hardiness. In terms of frost tolerance, buckwheat is to basil what hairy vetch is to parsnip and forage radish is to daikon (wait! yes, that was a trick because forage radish and daikon are pretty much the same crop).
Sudex is not frost tolerant, whereas forage radish will survive temperatures down to the teens (F). This mixture can work well in Maryland where the first frost isn’t often until late October. Photo credit: Ray Weil.
With cash crops, you might go to extra effort to eek out a couple more weeks of production by covering with heavy row cover.
Although I advocate treating cover crops like cash crops in many regards, I don’t think it’s practical to extend the life of cover crops with row cover. I’m not that crazy. Instead, I think it makes sense to select cover crops and cover crop mixtures with their cold tolerance in mind. Choose cover crops that grow AND die when you want them to.
Winterkilling cover crops
There are advantages to winterkilling cover crops. For spring field work, dead cover crops do not get in the way as much as living cover crops. When and if a cover crop will winterkill in your area depends on the weather. Chart 3A in Managing Cover Crops Profitably is a useful reference for the hardiness of cover crops.
The chart also has valuable information on the minimum germination temperatures for many cover crops. The beginning of life for cover crops (germination and early growth) is a key consideration in cover crop selection, though here I will not focus on that.
We have been especially interested in cover crops that will continue to grow into the fall, beyond the first frosts, but will still winterkill in the northeast. These include, depending on where you are located, forage radish, phacelia, oats, other brassicas, and some clovers.
For the winterkilled (but not frost killed) cover crops, an early seeding date is essential. These cover crops were seeded in early August in Maine (with an Earthway seeder, no less!).
A month and a half after seeding, these cover crops blanket the ground. The intent is for them to die during the winter. Foreground: phacelia, sudex/crimson clover, forage radish… other brassicas.
Two months after seeding and after a few light frosts, most of the cover crops are still alive, but sudex died.
Regional differences matter
What might be a great cover crop mix for one purpose in one area might not work well in another. For instance, sudex, when seeded with forage radish in late August in Maryland, can put on a lot of biomass and be a nice grass companion to radish (see photo above). In Maine, however, the risk of early frost is too high to make sudex a reliable fall cover crop (see below). Crimson clover, which generally winterkills in Maine, will overwinter in Maryland. Get familiar with the hardiness of different cover crops, plan, and plant accordingly. Among brassicas, there are wide differences in cold tolerance, as there are among clovers.
An early August seeding of sudex and crimson clover in Maine wasn’t such a great mix because the early frost in September took out the sudex. Crimson clover should winterkill in Maine, but will overwinter in Maryland.
