Leaving soil bare over the winter period can have detrimental effects on its structure and fertility. Rain leaches nitrates from the soil and weeds can gain a foothold creating extra work for the gardener. Green manure plants can help in a number of ways. Members of the legume family can fix nitrogen in the soil improving fertility for the following crop, all of them help to suppress weeds and when dug in add organic matter. Some, like the broad bean, Vicia faba can also provide food during the growing period. Here are a few of the most beneficial plants for sowing now (late summer/autumn) and overwintering.
The Nitrogen Fixers
Alfalfa (Lucerne) Medicago Sativa
This crop is a tall perennial with deep roots which can occupy a plot for a whole season. It provides plenty of organic matter when dug in, is a good weed suppressant and is a nitrogen fixer coming from the legume family. It can be sown in late summer for digging in come spring.
Broad (or fava) Bean Vicia faba
This crop is highly versatile withstanding the coldest winters, providing organic matter when dug in and again is a nitrogen fixer. Additionally, if sown in rows 30cm apart with 10cm between seeds then you will get a crop of beans too. Seeds can be harvested for the following year’s crop of green manure or beans if desired. Sow in autumn or early summer.
Red Clover Trifolium pratense
Red clover is low growing with a wide spreading root system. It fixes nitrogen and provides plenty of organic matter when dug in. It needs to be sown before autumn, preferably in spring or late summer. Sow in rows 15cm apart scattering the seed at 30g per sq metre. When the land is needed just dig it in.
Winter Tare Vicia villosa
This plant is extremely useful as an overwintering crop where land is unoccupied. It has an extensive root system, fixes nitrogen and supplies a good return of organic matter to the soil. Seeds are sown in late summer, in rows 15cm apart and about 7cm between each seed. It can also be sown in spring and summer if needed.
Non Nitrogen Fixers
Rye Secale cereale
This agricultural crop is a perennial with an extensive root system. It doesn’t fix nitrogen but adds a good amount of organic matter to the soil when incorporated in spring. The perennial variety can be sown in late summer or autumn, scattered in rows 23cm apart. It can also be broadcast sown at 30g per sq metre. If you want seeds for the following year, allow some plants to set seed and save.