Why grow ‘green manure’ crops?
Green manures (also known as cover crops) are plants that provide multiple benefits to the food garden by enhancing soil structure and fertility. In doing so, they can improve yields by up to 47%. As they cover bare soil, green manures also have the added benefit of suppressing weeds.
How are they used?
They can be sown after edible crops have been harvested when the soil is laying bare, or ‘undersown’ between crops. Once mature, green manures are cut down so that the nutrients contained within them are released into the soil. This can be done by digging them into the soil or mowing them. This should be done before they go to seed, or 3-4 weeks before you want to sow another crop – whichever comes first. Avoid using herbicides to kill them as this damages the soil ecosystem and the wider environment.
How to choose green manures
There are two main things to consider when choosing green manures:
- Beneficial properties: Different types of green manure bring different benefits. Some have deep tap roots that draw up minerals from the subsoil, others fix nitrogen into the soil and others attract beneficial insects. For best results, use a mixture of species with different properties.
- Growing time: Another thing to consider is how long you want to leave them in the ground. Are you looking for fast-growing crops that you can sow between summer and winter crops or are you looking for something to cover the soil over winter? Or perhaps you could allocate a part of your garden to slow-growing green manures that will bring benefits to your soil over a number of years.
Suggested growing mixes
Each of the mixes below comprise plants with a range of beneficial properties:
- Summer mix: mustard, crimson clover, phacelia and Persian clover. These can be planted from late spring onwards or immediately after harvesting summer crops. They will be ready to cut in 8-10 weeks, and can be followed by an edible winter crop.
- Winter mix: grazing rye or Westerwolds ryegrass and vetch. These can be sown between mid September and mid October and can be left in the ground until the spring. Wait until the soil warms up if you want them to fix nitrogen in the soil.
- Long term mix: red and white clover with perennial ryegrass. Grow for at least one full year to realise its full potential. If you can leave the plants in the ground for at least two years, use cocksfoot instead of perennial ryegrass.
For a more in-depth introduction to green manures, check out this guide by Garden Organic.
Improving soil quality with compost
Another excellent way to incorporate organic matter into your soil is to add homemade compost. Check out our guide to making your own compost.