Cover Crops 101: Why you need them, and how to choose the right ones

The first frosts of the season have come—and just as quickly gone: dawn’s crystallized world softening back to green, orange and brown when sunlight reaches over the hillside and spreads out morning.  

If you’re in a northern climate like we are, you’re preparing the garden for winter rest.  But don’t put the beds to rest without a blanket!  Before you say “goodnight” for a few months, there’s one more round of seeds to sow.

Cover crops are densely sown crops that do any number of beneficial acts for the soil:

• Build organic matter

• Fix nitrogen

• Smother weeds

• Limit erosion

• Loosen clay or hard-packed soil

They can be used throughout the year from Spring to Autumn.  In the fall, our main purpose for sowing cover crops is to stabilize the soil to limit erosion come snowmelt and spring rains.  Autumn cover crops also add organic matter to the soil when they decompose over winter and spring.  

In September and October, there are two types of cover crops to make note of: winter-kill and not-winter-kill.  

Winter-kill crops are annuals and will die come winter, leaving you with organic matter come spring.  Because the roots of the crops remain in the ground and the vegetation lays on the garden beds, they’ll also limit erosion.  

Common annual cover crops include: buckwheat, oats, field peas, annual rye, and crimson clover.

Cover crops that don’t winter-kill are perennials.  They’ll go dormant come winter, but begin to grow again in the spring.  These are great for areas you won’t plant in immediately come spring, as their growth will stabilize the soil, outcompete weed-seeds, and keep the soil covered to limit erosion.  

Common perennial cover crops are: winter rye, hairy vetch, and medium red clover.   

Timing is everything when it comes to sowing autumn cover crops.

Annual crops need to be seeded earlier than perennial crops.  This gives them enough time to grow before killing-frosts set in.  Ideally, we like to have our annual cover crops grow to 2-3’, leaving us with plenty of organic matter. For annual cover crops, this means sowing in late August to mid-September.

Perennial cover crops, specifically winter rye, can be planted later.  Winter rye can grow much later into cold weather, and begins its spring regrowth earlier than other crops.  Sow up until mid-October.

Types of Cover Crops:

• Oats : creates straw to add organic matter; thick roots stabilize soil and limits erosion; great as a nurse crop for peas or clover.
• Peas : fixes nitrogen in soil; adds organic matter; suppresses weeds.
• Buckwheat : fast-grower; suppresses weeds; adds phosphorus to soil; loosens clay.  *buckwheat prefers warm temps, grow in summer for best results.
• Clover : fixes nitrogen in soil; long taproots bring up nutrients from deep in the soil; adds organic matter.
• Annual Rye : fast-grower; limits erosion; adds organic matter.
• Winter Rye : deep roots add organic matter and limit erosion; grows longer into cold weather and returns early in spring.

If you need help figuring out what cover crop to use, try Cornell University’s nifty cover crop decision tool.