Cover crops are plants that are grown specifically to cover the soil, rather than to provide food for humans. However, they do provide food for all the organisms that live below the soil surface.
Why are cover crops useful?
Cover crops are usually grown when the soil needs a break, to build fertility or to address specific issues in the soil. Cover crops can help reduce weeds, slow runoff and support the water-holding capacity of the soil. They can also accumulate and recycle nutrients and add in organic matter for those plants that will, in turn, feed you.
Getting the best out of cover crops is a lifelong learning, but the good news is that they can reduce a gardener’s reliance on synthetic fertilisers or animal manures.
What is a green manure? While exploring the world of cover crops, you’ll come across the term ‘green manure’. A green manure is simply a cover crop that has been turned in while still green.
Cover crops can be grouped as legumes, non-legume broadleaf crops, and cereals and grasses.
Legumes
Legumes are plants from the Leguminosae (Fabaceae) family, examples include clover, lupins, beans and peas.
The process of nitrogen fixation
Legumes are known for their nitrogen-fixing properties because of their relationship with nitrogen-fixing soil bacteria called rhizobia. As a legume seedling grows, these bacteria move into the roots of the plant and reside in the root nodules and here they exchange the nitrogen for organic compounds produced by the legumes. Nitrogen fixation varies between plant species, the timing of harvest, and whether the right nitrogen-fixing bacteria are actually present in the soil.
Building fertility with nitrogen-fixing plants
To make the most of using nitrogen-fixing plants as green manures, we need to look for legume varieties that we will also be happy to cut down when they are just starting to flower. As the plants break down in the soil some nitrogen will then be available future crops.
Other nitrogen-fixing plants
Legumes are not the only nitrogen-fixing plants and there are species of perennials, trees and shrubs which fix nitrogen. Many of these make a useful addition to the garden as they can support fruit trees and other plants. Some have the added benefit of adding beauty to our garden, for example, the kōwhai and california lilac.
Non-legume broadleaf plants
Broadleaf plants that do not fix nitrogen are also beneficial in a growing system. Phacelia is one of my favourites, and if I leave some of the crop to flower it looks beautiful and provides lots of bee food. Buckwheat is another quick-growing crop and will also grow in less fertile spots, adding a bit more life to your soil. Chicory is a very deep-rooted perennial broadleaf, planted to tap nutrients deep in the ground with long roots.
Cereal and grasses
There are various types of cereal crops, such as oats and wheat, that can be used to cover the soil. In autumn, a pre-mixed cereal crop can go in to cover the soil during autumn and winter to prevent nutrients from leaching out. This can then be turned in or cut and dropped in spring while it is green before planting vegetables. You may also wish to let some go to seed and dry out before harvest to use as a carbon source for a compost heap.
What are cover crop mixes?
Many garden stores now have specific cover crop/green manure mixes which contain a mix of seeds for planting at different times of the year and for different situations.
What is the best cover crop mix?
The best green manure will depend on the time of year, your climate and what your goals are for your garden. In our garden, I use lupin, phacelia and buckwheat because they are generally easy to grow and a fast cover for a garden bed. I also make use of red clover which I grow over two years along with other broadleaf cover crops such as chicory, as part of a long-term strategy to raise fertility when a bed is resting and before I plant potatoes.
Some cover crops are reputed to have additional functions in your garden. Mustard is said to reduce wireworm or marigolds to reduce nematodes.
A couple of other things
- When planting a cover crop, it may be necessary to initially cover the ground with netting to prevent birds or mice from eating the seed.
- If you decide to chop and drop plants rather than turning in, it’s important to keep in mind that the organic matter will take longer to break down. However, some root matter will still break down beneath the surface.
- If you are turning your cover crop in, you’ll need to wait for two to four weeks for it to break down before planting.
flowering bean organic gardens 