Growing green manures is an age-old gardening technique that draws on a large range of fast growing crops to build, regenerate and maintain soil health. I like to think of it as growing soil instead of food – which is all the same thing really. Among the many benefits is this impressive list adapted from Green Harvest’s website: www.greenharvest.com.au/SeedOrganic/CoverCrops/GreenManureIntroduction.html
- help prevent and treat soil disease
- increase organic matter, earthworms and beneficial microorganisms
- increase the soil’s available nitrogen and moisture retention
- stabilise the soil to prevent erosion – we’ve used them extensively on our steep banks directly after fresh earthworks, which has worked brilliantly
- bring deep minerals to the surface and break up hardpans – when you choose plants with deep taproots, such as lupins
- provide habitat for beneficial insects and reduce populations of pests
- improve water, root and air penetration in the soil
- can smother persistent weeds – good choices for weed suppression include lablab, cowpea, lucerne and buckwheat.
Gardeners will often grow a mixed selection of green manures, including both legumes (which fix nitrogen into the soil) and grasses (which provide/build organic matter). By ‘grasses’, I mean cereal grasses such as rye or oats, and definitely not weedy types like kikuyu or twitch.
Green manures are usually integrated into a crop rotation cycle so there’s a seasonal rhythm to planting them. In Tasmania we usually grow them over winter to rejuvenate the soil for spring plantings; however, if you’re leaving your garden for an extended period of time, there’s always a type of green manure you can grow any time of the year – it’s best to not leave your soil naked.
How Do We Grow Them?
Make sure your soil has good nutrients available, such as compost or aged manure, and that it’s not compacted, or use a garden fork to puncture and massage (not turn) the soil before planting. If you have sandy soils, add lots of organic matter (compost, aged manure and mulch in warmer seasons). To sow the seeds we simply broadcast them, which involves throwing handfuls of seed over the bed, and then using a rake and/or garden fork to shuffle the soil around until there’s a layer of soil covering all the seeds. Add water, and then you can pretty much forget about them.
Unlike growing food, don’t let green manures flower or set their crop. Otherwise the plants will divert most of their energy and goodness into the flower/crop instead of into the soil, which is where you want it.
Once you see the plants are about to flower you can either: chop them up and dig them into the soil; or ‘prune’ them. We’ve been pruning for the past two seasons, which means we get an extended life out of the plants and, in the case of broad beans, have lots of salad greens for us or our chickens to eat, as the young leaves are super tasty.
What Types Should I Grow?
Cool season/climate green manures include broad beans, fenugreek, lupins (have deep taproots), oats, rye, mustard, peas, sub clover (not recommended for your vegie patch as it’ll get weedy) and vetch. Warm season/climate green manures include buckwheat, cowpea, French white millet, Japanese millet, lablab, mung bean and soybean.
Are green manures only for small gardens? No, they’re used on both backyard and broadacre scales. How they’re applied varies greatly; often tractors are used on a large scale to plant seeds evenly and efficiently.
Can You Have Perennial Green Manures?
I’m increasingly interested in all things perennial, as long-term crops lead to improved soil and crop health, and ongoing harvests. Some green manure plants, like clovers, will grow continuously, although die back in winter. However, you could consider some tree crops as perennial green manures, for example certain acacia varieties, tree lucerne/tagasaste (a declared weed in Tasmania), cowpea and Siberian pea tree. Such trees are often referred to as ‘nurse trees’, and are planted in orchards to regenerate the soil, for example to fix nitrogen and provide organic matter. They may also provide animal fodder, shelter, protection from the elements (i.e. as windbreaks) and firewood. Once the fruit trees are mature and require more space, the nurse trees are usually removed, but you can keep them there as long as they are functional.
Overall, green manures provide an affordable, easy and effective solution to restoring or maintaining soil health. And when we have healthy soil, we have healthy food, which of course means we have healthy people.
Hannah Moloney runs Good Life Permaculture with her husband Anton Vikstrom in Hobart, Tasmania. They provide hands-on courses in everything from food production to fermentation, composting, permaculture design and more. For further information see www.goodlifepermaculture.com.au.