Green Manure/Cover Crops
Cool Season Green Manures
by SCMG Jason Robinson
Green manures, also known as cover crops, are plants that are primarily grown to benefit the soil rather than for consumption or display. There are both warm-season and cool-season green manures. Now that fall weather is upon us, it is time to consider using cool-season green manures in your garden.
Why use cool-season green manures? Particularly in the winter, soil loses nutrients if it is left bare for the season. Green manures help to counteract this and maintain an even soil temperature and moisture content. Some green manures, such as drought-tolerant alfalfa (Medicago sativa), grow deep roots to tap resources unavailable to some crops. Others, such as alsike clover (Trifolium hybridum), produce shallow fibrous root systems to help build structure in the soil. Many, such as phacelia (Phacelia tanacetifolia), have flowers that attract pollinating insects. Green manures replace and hold nutrients in the soil, improve the structure of soil and increase soil’s organic material content. Green manures also smother the soil and prevent weed-growth. They are easy to grow and can be used when standard manures are not available.
A very important function that most green manures serve is to “fix” atmospheric nitrogen which is one of the main reasons I use them. After the cool-season green manures are dug in – usually in the early spring – they enrich the soil with nitrogen which will be available for uptake by your spring and summer crops. Green manures fix nitrogen specifically by using bacteria that colonize their roots nodules. These microbes take nitrogen out of the air and convert it into a form that plants can use.
Leguminous varieties of green manures are the main nitrogen fixers, although they tend to be less winter hardy. Winter rye (Secale cereale), also known as grazing rye, is the most commonly grown non-leguminous cover crop for cool-season planting. Although it does not add nitrogen to the soil, it helps maintain nitrogen levels and grows faster through the autumn than legumes, thereby giving better weed suppression. Non-legumes tend to break down more slowly and add more organic matter to the soil – but use some nitrogen in decomposition. You may wish to use a combination of green manures to balance the benefits of both types.
So how do you plant green manures? Sow the seeds evenly across the area where you plan to grow the green manures: either broadcast the seed or sow larger seed thinly in shallow drills and close rows. Then lightly rake in the seed so that it is covered and will germinate quickly. Water the area thoroughly if there is no rain due or if it does not rain for the following 48 hours. Once the seed germinates, allow it to grow to a healthy state before digging it into the soil. Never let green manure crops set seed or they will become a weed in later crops on the site.
If you practice no-till gardening, an option is to add the cover crop biomass to your compost pile. Once the green manure is well-composted, you can dress your vegetable garden with it. In addition to the benefits of compost, your garden will realize the myriad of cover cropping advantages which include increased nutrient retention, beneficial insects, water permeability, soil pore spaces and weed and erosion control as well as reduced soil-borne diseases.
I am confident that you will have more vigorous spring and summer crops due to the extra soil benefits imparted by the cover crops that you plant now! Good luck with your cool-season green-manure projects!