If you have a bit of space in your garden, growing green manures or cover crops is a great way to improve soil fertility and add organic matter to your soil. They can also be used as weed suppressants, and assist in the control of pest and diseases. So what are they, and where and when can you use them?
What is green manure?
Green manures or cover crops describe a range of temporary, fast growing leafy plants which are sown from seed generally in autumn or spring, grown through the next few weeks or months (depending on the season), and then, just before they begin to flower and set seed, they are slashed and turned in to the soil, or used as mulch for whatever’s to be planted next.
Why is it grown?
You might consider growing a green manure crop in a new garden area to supress weeds, improve drainage, reduce compaction, stabilise the area to limit erosion, and improve the soil before you put in permanent plantings. Green manure crops are often used in vegetable gardens, in beds, or parts of beds which are given a break from vegetable production for a season to improve the soil and set the bed up for the next crop.
Vegetable gardeners might have some space to spare after the summer harvest, and if you have a crop rotation system, you could think of incorporating a green manure crop into the rotation – check out our guide to crop rotation incorporating green manure crops here.
When to plant
There are different green manure crops that will thrive in different climates and different seasons.
If you want to plant a cool/cold season crop and you live in
• an area with cool/cold winters (Tasmania, Victoria and southern New South Wales, southern South Australia and southern Western Australia), the best sowing time is early autumn. If it’s been very dry, wait for a bit of autumn rain before sowing. You can also sow cool/cold season crops in these areas in early spring;
• a warmer area (northern New South Wales and southern Queensland, warmer parts of South Australia and Western Australia) plant cool/cold season crops a little later – in April or May;
• in a frost free area, cool/cold season crops can be planted from autumn until the end of winter.
If you want to plant a warm season crop, and you live in a colder area, plant in spring, once there’s no risk of frost. In warmer areas, plant in summer whenever you can rely on rainfall, or are able to irrigate. If you live in a tropical area, you can plant warm season crops throughout the year, so long as there’s moisture available.
What plants to grow
Cool/cold season crops:
Fava beans, broad beans, tick beans, fenugreek, lupins, oats, subclover, woolly pod vetch, ryecorn, yellow and black mustard seed, other brassicas, feed oats, wheat or barley.
Warm season crops:
Buckwheat, cowpea, French white millet, Japanese millet, lablab, marigolds, mung bean and soybean
Benefits of Particular Plants
Different crops have different benefits, and can be grown in combination. Seed sellers will often sell individual seed types and green manure mixes. Some examples:
Biofumigants, like marigolds (Tagetes patula) planted in spring, brassicas (Brassica napus and Brassica campestris) and mustard, planted in autumn help to control root knot nematodes and root rot fungal pathogens. These crops must be dug in to release beneficial gases as they decompose.
Legumes, like lucerne, clover, beans and peas, which fix nitrogen and will make it available to whatever follows the green manure crop.
Weed smotherers include lablab, cowpea, lucerne and buckwheat.
Establishing your crop
it’s usually as simple as choosing your seed mix, clearing the bed of weeds, broadcasting the seed and raking to cover, but check the seed packet to see whether your seeds have any special requirements. Most green manures will need moisture to germinate and while they’re growing, so depending on rainfall, you might need to water the seeds in when you sow them, and to give them a hose as they grow.
For maximum benefit, harvest as your crop starts to flower – once the crop has fully flowered (and then set seed) the nitrogen content decreases, and, if the plant goes to seed, you’ll likely see it pop up all over the place next season. You can either dig the crop in, or cut it and use it as mulch on the bed where it grew, or around the garden. If you decide to dig the crop into the soil, it will break down faster if you mow or chop it up a bit first. Allow 4 to 6 weeks after you dig the crop in before planting new seeds in the bed. If you need to plant seeds sooner, use your green manure crop as a surface mulch.
Some garden centres supply mixes of different seeds or sell them individually. However, you can make your own selection quite cheaply by buying seeds of many individual species from bulk food stores which sell nuts, grains and pulses. These normally germinate well, but it might be wise to buy only a small quantity first and test their viability. This can be easily done by placing a few on moist tissue paper in a small container and watching for root formation.