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 It's all about the roots.
Did you know that when you plant cover crops, the roots make more compost than the tops?  Winter Rye makes 380 miles of roots per plant! When the tops are cut, those roots make compost right in the soil, so you don't have to haul it, turn it, or shovel it.
When you plant makes all the difference.
With winter cover crops, the size of the root system is largely determined by when you plant. After the soil gets cold, growth slows or stops. We recommend sowing cover crops 2-4 weeks before your  first fall frost. *  If you live where you get overnight frost but the ground generally doesn't freeze, 2 to 4 weeks will work for you. If you live where the ground freezes, make sure you plant 4 weeks ahead of frost. 

Don't be surprised if the plants don't get tall--they will be busy underground. In actual tests, crimson clover that was was only 2 inches tall by November nevertheless had roots 12 inches deep, with many nitrogen nodules already fertilizing the soil.

But how can you plant cover crops in September when so many of your garden beds are still producing vegetables? Here are some options:
  • You can broadcast the seed of your cover crop into the bed while the existing crop is still in place. This is easiest if the crop is fairly upright, but possible for many more. The technique is called undersowing. You could undersow vetch or radish into a tomato bed, for example. Clover is a great crop for undersowing; it doesnt't need much sun. Don't worry if it isn't totally even. If there are going to be places where you can't reach, sow a vining, sprawling cover crop like peas or vetch. They will spread to fill the gaps.
  • Consider carefully whether a bed is still actually producing. It is a reflex for most of us to leave summer crops until frost, but beans, squash, corn, etc are often done by mid-September. If they are no longer producing good harvests, move on. You will gain a lot by building fertility for next year.
  •  You can sow winter wheat or rye into unused corners of the bed, paths, and any empty space, even if it is not directly in the bed. They will make grass you can walk on and even mow if desired. It will still benefit the garden, and hold the soil during the mud season. You will have more compost material in spring.
  • If you have undersown every bed that you can, and set out your fall/winter crops, consider a second sowing later for beds that cannot be sown in September. Rye is the most cold-hardy, and will still sprout late, even after the first frost. In fact, if sown on frozen ground, and covered with straw or other light mulch to keep birds form eating it, rye will sprout during thaws and prevent soil loss while keeping weeds from taking over. Bell bens are another good choice for late planting.
What to Plant?
See the complete version of this article, with detailed guidance on choosing crops and making your own mixes, here.

See all the cover crops we recommend for fall here.

See our cover crop chart here.

You can find your average first and last frost dates by typing your zip code into a search engine online and adding "first and last frost dates."
*Weather tip: For practical purposes, your frost date is when you can expect a forecast of 36 degrees or lower. Why? Weather station equipment is installed at standing height or even at roof level. The temperature below, on the ground, can be colder than the instruments show. So when the forecast expects 36 degrees at the weather station, you can expect 32 degrees at the soil surface. In low spots and areas where air drainage is blocked, it can be even colder.

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