A green manure crop is really just anything you grow for a short while and then slash or dig into your soil before it gets a chance to set seed or fruit.
Why would you do that? Lots of reasons! It helps by:
- increasing organic matter in your soil (increasing nutrient content, aeration and water retention)
- adding nitrogen in the soil (leguminous green manures)
- increasing activity of beneficial microorganisms and soil life
- stabilising soil to prevent erosion or drying out
- bringing up minerals from deeper down in the soil
- providing bee and insect forage (flowers)
- breaking up lumpy or clay soil
- adding humus to sandy soil
- smothering or excluding weeds
- keeping soil moist (living mulch)
It’s a good idea to grow green manures over winter in garden beds that aren’t being used for winter crops. It will improve your soil, suppress weeds, and keep your garden going until you’re ready to plant again in the spring.
You don’t have to restrict green manures to winter, though. They’re also handy as a first crop whenever you set up a new garden bed to break up the soil, increase its biomass, and get the soil life going.
Green manures are usually trampled down and dug in once they reach about knee-high. You want to do this while the plants are still soft and sappy, before they go woody. You definitely want to dig them in before they set seed, or you’ll have a whole new crop of weeds to contend with!
So what should you use as a green manure, and where do you buy the seeds?
Lots of websites give very specific recommendations and ratios, but it’s not rocket science. Generally you want legumes (bean, peas, lucerne, etc), annual grasses (wheat, barley, rye, oats), clover, mustard, and even corn and sunflowers. It’s a bit hard to find a good mix of those things in nurseries and garden centers, though.
I’ve found the simplest, cheapest and best thing is to just buy a bag of organic bird seed. Read the back of the packet and find one with the mix you want. The last lot I planted contained millet, sorghum, wheat, oats, barley, rye, corn and sunflowers. Bird seed will be chemical-free and fresh (since they don’t want to kill your pets!), and very cheap. It’s available at any supermarket.
You can add any out-of-date vegetable seeds you have left over from last season as well. Legumes like beans and peas are especially good, since they’ll fix nitrogen in the soil, but anything else you have will help.
Just scatter the seed around your garden bed, about two handfuls per square meter. Then lightly rake it over to get the seeds into the dirt, and water it in well. You may need to cover the bed with a net if the birds discover the free feast you’ve laid out for them.
When the time comes to dig in the green manure, make sure you use a sharp spade. Trample the plants so they’re lying flat, and slice down through them with the spade. I work in spadefuls of about 10 cm (4 inches) thickness, digging to one spadeful deep. Turn the soil as you dig, so that the greenery ends up buried and the roots are near the surface, to make sure the plants won’t keep growing.
I spread horse manure and sowed a green manure crop into one of my new garden beds in January (Australian summer), and in just over four weeks it was ready to dig in. It really doesn’t take long, and it gets your soil off to a great start.