How To Use Green Manure Seeds To Get A Better Yield

By far, the easiest way to improve your soil and add nutrients is through sowing green manure seeds when the garden beds are bare. High in nitrogen, green manure crops are ideal for growing before a hungry crop is planted in the same area later in the year.

In this article we will explain what green manure is and how you can use green manure seeds to improve the health of your soil.

What Is Green Manure?

Green manures are plants that are planted in a bare bed and then dug into the soil when they are still green.

These plants are planted as a cover crop in beds, so not for harvesting. This may sound weird to a vegetable gardener, but it is an organic way to keep weeds down over autumn and winter and reintroduce nutrients into the soil.

It is called green manure, because it is purely plant based rather than coming from animals, such as horse manure.

Green manures are quick growing, so you should see some growth very soon after you have sown your green manure seeds.

This means, you can do this between different crops in summer, to give your vegetables the best possible soil.

Benefits Of Growing Green Manures

green manure mustard

Sowing green manure seeds has a lot of benefits and is very easy. Using green manure has the following benefits:

  • it will add valuable nutrients into the soil, such as nitrogen and thereby improve your soil fertility.
  • the roots of the green manure plants will improve the drainage or water retention, depending on the soil condition. With this, it will improve the soil structure.
  • green manure can cover big areas quickly, so weeds have no chance. This is one of the easiest ways to keep the soil surface of your beds weed free.
  • many green manure plants attract pollinators and predatory insects, so it is a great way to get these beneficial insects into your garden.
  • if you have heavy soils, such as clay soil, green manure plants can help break up the soil to make it easier to cultivate
  • sowing green manure seeds in autumn will also reduce soil erosion that can occur over autumn and summer due to rain.

There are also some things you have to consider though before you start sowing green manure seeds.

  • by growing green manure plants over winter, you give slugs and snails the ideal breeding conditions. The same is true if there were weeds. Therefore, this should not put you off growing green manure crops. Just keep it in mind and protect your news seedlings and young plants accordingly in spring.
  • after you have cut down and dug in your green manure crop, you need to wait at least two weeks, four would be better, before you can plant your vegetables. This is because you have to give the foliage time to compost. First, because otherwise the decomposing leaves could inhibit the growth of the new plants. Second, because some plants release toxins into the soil, which will restrict germination of your vegetable seeds. So make sure you cut down and dig in the green manure in good time.
  • if you are using a mustard variety as green manure crop, keep in mind that it is part of the brassica family. So don’t plant it in the beds you want to plan broccoli, cauliflower or cabbage, as these are also from the same family. Your crop rotation should also include green manure plants if you are growing them.
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Types Of Green Manure Seeds 

There are generally two types of green manure seeds to grow, spring and autumn sown types.

Spring Sown Green Manure

Alfalfa flower

Spring sown varieties add a significant amount of bulk to build the soil structure. Suitable varieties are quick growing and will take 6 – 10 weeks from sowing the green manure seeds to digging in the plants.

If you start sowing as soon as the soil is manageable, you can improve your soil before your vegetable plants are planted.

Just keep in mind that you have to wait for 2 – 4 weeks after digging in, before you can plant new plants.

But sowing some green manure seeds in spring can bring other benefits too:

  • they will suppress weeds that would come up as soon as the sun gets warmer. 
  • they will provide flowers for bees and other insects at the start of spring, and they will also look nice.

Autumn Sown Green Manure

Autumn sown green manures are much more popular. Once the main garden crops have been harvested, the green manure seeds are sown and allowed to grow until early Spring when the plants are dug in the soil.

After the last harvest, in late summer or early autumn, you sow green manure seeds in the beds. They will use up the nutrients left in the soil to prevent them from getting washed away in the rain and being wasted.

When they then get dug in, they will release the nutrients back into the soil, ready for your veggie plants to absorb.

Green manure sown as an overwinter cover crops will also prevent weeds that would normally germinate in cold temperatures and establish themselves over winter. 

And that means, you don’t have to do as much weeding in spring!

Summer Sown Green Manure

While most people sow green manure seeds in autumn, you can also sow them in summer. 

You can use them as catch crop between two main crops. You might have a bed to fill because one crop has finished, but you are not yet ready to plant a new one.

Using green manures in that way will not only give your soil a boost, but will also keep weeds down, so you don’t need to worry about weeding this bed.

You can also plant the catch crop with your main crop and use them as companion plants. Read more about this gardening method in our guide to companion planting for vegetables.

Green Manure Seeds To Try

a field of buckwheat

There is a wide variety of green manure seeds that you can buy, so deciding on which ones can be difficult. Here are some that are worth trying.

Crimson Clover: this is a perennial plant of the legume family. It works best in light soil, such as loam or sandy soil. 60 days after sowing you can cut it down and dig it in. But you can also leave it longer, then it will produce beautiful crimson flowers.

Phacelia: This can be sown in summer or autumn. It’s fast growing and has beautiful flowers that will attract beneficial insects. It’s great at suppressing weeds. It can also be left over winter. It works with most soil types, but thrives in dry soil.

Grazing Rye: this is a great green manure crop to overwinter. It will improve the soil structure, thanks to its large roots. It grows around 60cm (23-24in).

Buckwheat: this summer green manure is fast-growing and will suppress weeds brilliantly. It also attracts hoverflies, which is a very beneficial animal to have you in your garden.

There are lots more, but these are all worth a try.

Sowing Your Green Manure Seeds

Before you start sowing your green manure seeds, you need to prepare the ground. 

Get rid of all the weeds and then dig it over. No need to be too precise with that.

You can sow your green manure seeds either by scattering (also called broadcast sowing) or sowing them in rows.

Scattering is generally seen as the better option, because it will ensure good coverage and therefore good weed suppression.

Once you have scattered or sown your seeds, rake them in lightly and give them a good watering. In dry weather, keep watering them until the plants are well established.

Cutting Down Or Digging In?

You want to cut down and dig in your green manures before the set seed. Because otherwise, they will come up again.

To make sure the plants have not set seed, it is often advised to cut them down before they are flowering.

This has the additional advantage that the stems are still soft, which means they will decompose quicker. They will also still have more nutrients in them.

Each green manure variety can be left for a different amount of time, so refer to the seed packet if you are unsure.

Once grown, the green manures should be cut at ground level and left for a day to wilt. It’s then time to dig this organic matter into the soil. 

You want to dig down about 15 cm (6in) and turn the soil over. 

It is also possible to just leave the wilted foliage as mulch on the surface. As they decompose, worms will drag them down into the soil, thereby getting the nutrients where the need to be.

Adding and digging in such a mass of green organic matter in the soil will seem strange, but this will very quickly compost into the soil.

Ideally, you should allow two weeks to four after digging the green manures into the soil before planting anything else in the bed.

The tender green manure leaves will quickly rot down into the soil.

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How To Use Green Manure Seeds In A Vegetable Garden

vegetable garden

If you are now keen on trying out sowing green manure seeds, then here are some ways you could use green manure to benefit your vegetable garden:

  • if you are only just starting with growing your own vegetables, then you can start off with growing green manures to revitalise your soil, ready for your veggies.
  • Most vegetables finish cropping at different times. This can sometimes result in parts of the vegetable patch not being used. Of course, you can just add a new vegetable plant, but you could also sow some green manure seeds. This will not only help out the soil, but will also help keep weeds down.
  • Sowing green manure seeds in autumn after your vegetables have finished cropping is an obvious thing to do. 
  • If you grow your vegetables in rows, then there will be a lot of weeding to do. To avoid that, you could grow green manures between the rows. This will only work though, if your vegetable plants are big enough not to be overshadowed by the green manure plants. 
  • As we have mentioned before, many of the green manures attract beneficial insects. So you could use them to have these useful helpers in your garden to keep pest numbers down. You will have to let them flower for this, but make sure you dead head the flowers before they start to set seed.

Now that you know the benefits of sowing green manure seeds, why don’t start planning when and where to sow them, so that your vegetables will thrive this summer.

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