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The key to a good garden is good soil – and there are a number of ways you can improve it no matter where you live. One way is to grow green manures. They’re a short-term crop specifically grown to be dug back into the soil, adding much needed organic matter and nutrients.

There are six main advantages from growing green manures:

• They add organic matter to the soil which helps improve its fertility
• They help make nutrients available to any plants that are grown in the soil afterwards
• They prevent nutrients being washed through the soil in winter (known as leaching)
• They suppress the growth of weeds
• They help break-up heavy soils, improving drainage
• They protect the soil from pounding rain in winter which can cause a hard layer to form on the surface.

The only drawback with green manures is that you have to let the ground fallow (leave alone) while the green manure is growing. In the vegetable garden, this is easy as it’s usually separated into clear sections, so an area can be put aside to grow green manures.

In a more ornamental part of the garden it can be a bit more impractical or undesirable to leave unproductive areas of soil for a length of time. However, Lupins can be used as green manures, so you could have a beautiful display of them.

Adding fertility to your soil

Apart from adding organic matter, some green manures offer other benefits. For example, deep-rooting green manures such as Alfalfa, Red Clover and Lupins will use nutrients from deeper in the soil (beyond the reach of many crops). Then when the green manure decomposes in the soil, the nutrients are deposited in the upper layers where the follow-on crops and plants can use them.

Similarly, leguminous plants like beans, Lupins, Clover and Fenugreek, have special nodules on their roots filled with 'nitrogen-fixing' bacteria that can take nitrogen from the air and make it available to the green manure. Again, when the green manure decomposes, the nitrogen then becomes available to follow-on crops and plants.

Choosing a green manure

There’s a good range of green manure crops, which one you choose depends on when you intend to plant it and how long you’re prepared to leave it growing before incorporating it into your soil. For most people, a green manure that can be sown in late summer or early autumn and left to grow over the winter would be best. Clovers or rye grasses are perfect for this and will have time to grow before digging in during early spring.

In heavy soils (like clay or silt), digging during early spring is not a good idea because it will damage the soil structure. So try a tender quick-growing crop like Buckwheat or Fenugreek instead.

If you have a new garden or a barren plot that you don't intend to cultivate or plant for at least a year, then one of the longer-term green manure crops, such as Alfalfa or Alsike clover are worth sowing. If you find that you’re ready to tackle the area earlier than expected, these green manures can be dug in at any time. Just leave a few weeks for them to rot down before planting.

Which green manure?

Common name	(Latin name)	When to sow	Growing time	When to dig in
Alfalfa	(Medicago sativa)	Late spring	3-24 months Any time while fresh
Buckwheat	(Fagopyrum esculentum)	Spring to late summer	2-3 months	Any time before flowering
Alsike clover	(Trifolium hybridum)	Spring to summer	2-24 months	Any time while fresh
Crimson clover	(Trifolium incarnatum)	Spring to summer	-6 months	Just before flowering
Essex red clover	(Trifolium pratense)	Spring to summer	2-24 months Any time while fresh
Fenugreek	(Trigonella foenum graecum)	Spring to summer	2-24 months	After flowering, before pods develop
Fava beans	(Vicia faba)	Autumn	4 months	Any time before flowering
Italian ryegrass	(Lolium multiflorum)	Early spring	2-3 months	Anytime before flowering
Lupin (Lupinus angustifolius)	Spring to early summer	2-3 months	Just before flowering
Mustard	(Sinapis alba)	Spring to early autumn	Up to 2 months	Any time before flowering
Phacelia	(Phacelia tanacetifolia)	Spring to early autumn	2-6 months	Just before flowering
Rye	(Secale cereale)	Late summer to autumn	4-6 months	Just before flowering
Trefoil	(Medicago lupulina)	Spring to summer	12 months	Any time while fresh
Winter tare	(Vicia sativa)	Spring or late summer	2-6 months	Any time before flowering

Using green manures

Green manures can be either broadcast sown (thrown at random) or sown in rows. Prepare the ground thoroughly as you would for any other crop. If you broadcast the seed, lightly rake it into the surface. If sowing in rows, make drills (small trenches) about 30cm apart with the corner of a hoe and sow in these. Broadcast sowing is quicker and easier, but wasteful, so you’ll use more seed. So, it's best to sow larger seeded Lupins and beans in rows.

Digging in green manures

As soon as a green manure has grown a bit it’s worth digging in. However, the benefit will be better if you let it grow a while. Most green manures should be dug in before they flower and set seed, or before the stems go woody. If allowed to set seed you may find a second generation of green manure becoming a weed problem as they grow up through subsequent crops. If allowed to get woody, the green manure will take a lot longer to break down in the soil and the micro-organisms responsible will remove valuable nitrogen from the soil as they breakdown the woody parts.

Top tip:

It’s worth cutting back long-term Alfalfa crops every few months to keep the growth lush and green.

Three easy steps to digging in:

1. Cut the green manure a few hours before digging in so that the top growth wilts (you can use a sharp spade, shears, or mower to chop it down).
2. Then grab everything (including the roots) and dig into the top layer of soil. It’s best to go about 15cm deep as this is where it will decompose the quickest.
3. After digging in, leave for three weeks before planting or sowing the ground with crops or plants.