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Tips and inspiration

Green manuring – why and how?

22 August 2022

An empty garden plot isn’t just boring to look at – bare soil isn’t very good for your garden or your future crops. Green manuring means planting crops to improve the soil and can be done in several ways.

Why try green manuring?

Green manuring is the easiest way to fertilise the soil, reduce nutrient leakage, improve soil structure and reduce weed growth. At the same time, green manuring creates the very best conditions for bees, butterflies and small micro life to love your garden. There aren’t many other forms of fertiliser that can claim to do all of this.

For the space-conscious, the green manure crop can be grown before or after other crops. Green manuring means planting crops to protect and improve the soil. The plants should keep the soil surface covered, loosen the soil with their roots and return nutrients when they are dug back into the ground. Specific properties can be found in different varieties suitable for green manure crops.

How should I grow green manure? 

Main crop

Instead of leaving the soil bare, the green manure crop is grown as a main crop throughout the growing season. Most green manure crops are cut before flowering and then dug into the ground (follow the instructions on the seed bag carefully), providing additional organic material and nutrients. This makes the soil loose and nutritious. Some green manure plants have deep taproots that improve soil structure while accessing nutrients and minerals from deep soil layers. Other green manure crops, such as crimson clover, have a nitrogen-fixing capacity. These crops fix nitrogen in the air and then release it once they have been dug into the ground. 

Catch crops

A catch crop is grown after another crop. You can sow a catch crop where you have just harvested your onions. Catch crops work by capturing and binding the excess nutrients in the soil. When the crop is then dug into the ground, the nutrients are released back into the soil. In this way, nutrients that would have contributed to acidification are not leaked into the soil, but are captured and eventually used by plants that will be planted in the future.

Living mulch

Living mulch is grown at the same time as other crops. Grow marigolds next to the vegetables in the vegetable patch. When the growing season is over, the marigolds are dug into the ground, adding organic matter and nutrients to the soil. This method of cultivation can also be seen as a form of compact cultivation, which prevents weeds from taking over.

Different varieties with different purposes

To loosen up the soil, it is a good idea to sow varieties with deep roots, such as lacy phacelia and borage. These also produce a lot of leaf mass, which you can then dig into the ground.

Crimson clover, like other types of clover and legumes, are nitrogen-fixing. This means that these plants are able to absorb nitrogen from the air and draw it into the soil so that it can be taken up by the next generation of plants. Nitrogen is the nutrient plants need most.

One advantage of green manuring is that it prevents weeds from growing. Buckwheat is fast-growing and quickly forms a green carpet that prevents unwelcome plants from taking hold.

Since you usually want all the different properties, it’s a good idea to sow different varieties in the same place. There are ready-made mixed seed packets that make it easy to get started with your green manuring. You can of course create your own mix too.

Different methods

Sowing green manure seeds in spring

You can sow green manure seeds in early spring in places where you will then grow crops. The seeds will have time to germinate and grow a little in the spring before you turn them into the soil. About 2–3 weeks before sowing the year’s vegetables is usually just about right. When the time comes to plant your pre-sown seedlings outside in early summer, the soil will already have improved in terms of nutrition and structure.

Sowing green manure seeds in spring or summer

If you have a bed that needs to be completely rested from vegetable cultivation, sowing green manure seeds is an excellent option. In this case, you leave the plants in place for the entire growing season. Turn the green mass into the soil in autumn. This will improve the soil for the next growing season.

Green manure seeds are also suitable for sowing between rows of plants. This keeps out weeds and attracts useful insects. Since you never weed the plants but turn them back into the ground in the autumn, the nutrients are retained at the site.

Sowing green manure seeds in autumn

If you have a plot that you have finished growing plants on for this year, you can sow green manure seeds. They will have time to germinate in the autumn, and will provide excellent protection against autumn rains that otherwise risk washing away soil and nutrients. Leave the plants in place over the winter and turn them back into the soil the following spring. This prevents the soil from leaching and reduces weeds.

Allow to flower or not?

Lots of the plants we use as green manures produce beautiful flowers – for example, lacy phacelia or borage. When you allow them to flower, they also attract useful insects to your garden. You can of course also dig flowering plants into your garden. However, make sure you do it before your plants go to seed – unless you want them to reappear in your crop, of course.

What shouldn’t I dig in?

Digging mature plant matter back into the soil is a way of returning nutrients to the soil, regardless of the type of plant. Therefore, you can even dig in your vegetable plants after harvesting them. Avoid digging in plants affected by diseases, such as tomatoes with potato blight.