Everything You Need to Know About Green Manure

Jo.Baker
Published on September 12th 2020
5
A pile of green manure being dug into the soil
As autumn draws in and we harvest our summer ripened crops, the question of what to do with our empty beds is high on the agenda.
Green manures are being mentioned more frequently in articles, radio shows and advice columns, but what are they, and why should we use them?

What is green manure?

Green manures are a range of plants that can be grown quickly in between main crops. When they are dug back into the topsoil to decompose, they improve soil structure and fertility.
They also reduce the need for weeding, and some even help to control pest problems.
A close up of lupine  seed pods underneath fresh green velvety leaves.
Lupins being grown at Babylonstoren as a green manure.

What are the benefits?

  • Weather protection
Most plants get the nutrients they need to grow from the top few centimetres of soil. Winter wind, rain and frost can all have negative effects on this topsoil.
On dry days, the wind can blow the top layer of soil away, and rain can wash the nutrients deeper or away altogether. Frost can harm the microorganisms that convert organic matter into nutrient-rich humus and prey on harmful parasites living in the soil.
A green manure cover crop will slow the passage of wind and air over the soil surface, reducing the negative effects of the winter.
  • Weed reduction
Bare soil that has been worked to a fine tilth (small crumbly consistency) is perfect for sowing our vegetable and salad seeds in. However, it's also ideal for weeds.
By filling empty beds with cover crops, weed seeds won't be able to get established and use up valuable resources we'd prefer our crops to have.
If you know your planting area is going to be empty for over four weeks, it's worth planting green manure at any time of the year.
A close up veg. growing in a raised bed.
  • Breaking up the soil
For those of us with heavy clay soils, pick green manures that have long taproots. These will penetrate deep into the soil and help break it up. Opening up the structure of the soil will enable water, air and beneficial organisms to move through the ground faster, improving growing conditions.
  • Fertilisation
The most significant benefit green manures provide is maintaining and increasing the available nutrients in our growing spaces.
Initially, they will absorb these nutrients into their roots, shoots and leaves, preventing them from being used up by weeds or lost to the weather.
Two weeks before you need the space for your next crop, the whole plant can be dug back into the topsoil, where it will breakdown and release those nutrients back into the soil.
Green manures with long taproots will also bring up minerals from subsoil levels, making them accessible (once broken down) to shallow-rooted plants.
Green manures from the legume family are the best at providing additional soil fertilisation. These plants form a symbiotic relationship with bacteria that live on their roots. The bacteria fix gaseous nitrogen into nodules they form on the roots so the plant can absorb it in return for carbohydrates.
This additional nitrogen fixed in the roots will give your next crop a vital boost.
A close up of yellow mustard flower with a white and black butterfly landing on it.

Which green manure should you sow?

With so many varieties on the market, picking the right seed for your space can seem a little daunting. As a general rule, these five are the best to grow at this time of year and in most locations.

White Mustard

White mustard is the fastest-growing green manure, needing only four weeks before being ready to turn back in. It will grow in almost any soil from March until September. Plants sown in autumn will be slower growing and can be hit by the first frost.
A field of Sinapis alba, White Mustard
Ideally, plants are dug back in before they flower. However, time and weather can be against us and leaving a small patch to ripen for the pollinating insects is not a bad thing.
Flowering stems can be cut off around 15 cm (6") above ground level and placed on the compost heap.
This plant is also a bio-fumigant and is especially useful in helping to suppress potato cyst eelworms.The gas emitted from the roots hardens the cases of the cysts, reducing the number of eggs that hatch.
As a member of the brassica family it shouldn't be planted straight after other Brassicas as it can increase the possibility of Club Root.

Winter Tares

As the name suggests, these plants will protect from winter weather. A member of the legume family, they need two to three months to fix nitrogen into the soil and can be sown from July until September.
They grow best on moderately moist soils and dislike acidic conditions, so it's worth testing to make sure your soil pH level is above 6.5.
There is a spring variety that can be sown between March and May that is as equally as good at suppressing weeds.
A close up of some winter green manure plants, rye and vetch. Image taken by Candide user dominic_driscoll
A mixture of winter green manures Rye and Vetch. Image by Candide user dominic_driscoll

Rye

Rye, also known as Hungarian grazing, is a hardy annual that can be grown throughout the winter and tolerates heavy clay soil.
It can be planted as early as March and as late as November, making it a useful plant to go in after the squashes and pumpkins have been cleared.
This green manure can also be planted for long term cover up to two years. It will need to be cut during the second year to prevent it from flowering and setting seed. Cutting will also encourage it to produce more foliage to be dug back in.
Rye grass
Its extensive root system also helps to prevent soil erosion and suppresses weed germination.
Once it's been dug into the soil, rye breaks down slowly. So, to provide fertilisation for longer, it's often planted with quick decomposing plants such as vetch and clover.

Broad Beans

Sown between September and November, these overwintering members of the legume family will grow in most soils but do best in heavy clay. Its deep roots will help break up the soil and fix nitrogen ready for the following crop.
To get the best results, you will need to dig the plants in before the beans develop. However, if you are growing broad beans in an allotment or veg bed, you can cut the spent plant at ground level after harvesting your beans and leave the roots to decompose.
Large farms will sow alternate rows of beans and rye to get the best weed-suppressing and nitrogen-fixing results.

Phacelia

Phacelia is grown for its stunning lavender-blue flowers, which are very attractive to beneficial insects as well as its spreading habit and weed suppressant qualities. It also produces lots of foliage to dig back into the soil.
A purple flower on a plant
This green manure can be sown from March until September and can grow for one to three months before needing to be cut back. Autumn sowings can overwinter on free-draining soils provided temperatures don't get too low.
Many other plants are used as green manures. Check out our knowledge collection for more inspiration and marketplace for local suppliers of seed.
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