Green Manure and Cover Crops
Grow a crop you never intend to eat.
A green manure is a crop you grow with no intention of eating. You sow it into ground that would otherwise sit empty, let it grow, then cut it down and return it to the soil. It is the cheapest soil improvement there is, and the one most home gardeners never try.
What it actually does
- Adds organic matter without you carting compost in. A knee-high stand of mustard or oats turned in is a serious quantity of material.
- Holds nutrients that would otherwise wash away. Bare winter soil loses nitrogen with every rain. A cover crop takes it up and hands it back when it breaks down.
- Fixes nitrogen, if you choose a legume. Peas, beans, clover, lupins and vetch all pull nitrogen out of the air into root nodules.
- Smothers weeds by occupying the ground first.
- Breaks up compaction. Deep-rooted crops like tillage radish and lupin drive down through hard layers that a fork will not reach.
- Stops erosion on any bed that would otherwise be bare through the wet season.
Choosing one
Pick by what the ground needs, not by what is on the shelf.
- Soil short of nitrogen: a legume. Broad beans, field peas, lupins, clover or vetch.
- Soil short of bulk: a cereal. Oats, ryecorn or barley, which make enormous quantities of material fast.
- Compacted soil: tillage radish or lupin, both of which send a taproot straight down.
- A quick gap of six weeks: mustard or buckwheat, both fast and easy to dig in.
- Bees and beneficials: buckwheat or phacelia, which flower quickly and pull in hoverflies and bees.
Many packets sell a mix, which is usually the sensible choice for a home garden because you get several of these benefits at once.
Sowing and turning it in
- Sow thickly. This is not a crop you thin. Broadcast the seed, rake it in lightly and water.
- Let it grow to knee height, or until it is just about to flower.
- Cut it down before it sets seed. This is the one rule that matters. A green manure allowed to seed becomes next season's weed problem, and mustard in particular will haunt you.
- Dig it in shallowly, or cut it and leave it on the surface as a mulch if you garden no-dig. Either works.
- Wait two to three weeks before planting into that ground. Fresh green material ties up nitrogen while it breaks down, so planting straight into it starves the crop you actually want.
Fitting it into a rotation
The obvious slot is the gap between a summer crop coming out and a spring one going in, which is exactly when ground would otherwise sit bare and leach.
Mind the family. Mustard and tillage radish are brassicas, so they belong in the brassica slot and should not be followed by cabbages or broccoli. Peas, beans and clover are legumes and belong in the legume slot. Oats and ryecorn are grasses and fit almost anywhere, which is one reason they are the easy default.
See also: Crop rotation, since a green manure has to fit a slot, and Composting for the other half of the soil story.