Green Manure – What, Why and How!
Green manure provides much needed ground cover and protection from flash floods during the winter months. It is often thought to be an essential to organic gardening as you can grow the plants to help the soil with nutrient retention.
- rake through the area you want to grow over
- scatter the seeds as per individual plants preferences (see below) usually in August time
- leave overwinter and then cut back before flowering (unless seed saving)
- slice the wilted foliage into the soil one spade deep
- two weeks later plant out as desired.
With different plants you will find there is significant variation in how the get the most from overwintering. So let’s look more closely at what each plant can do for your soil.
What Is The Purpose Of Green Manure?
This is an historically important routine where fields were left fallow for a year and crops like broad beans were grown to be ploughed into the fields. There is evidence from across the globe in the ancient world including ancient Indian treatise, Ancient Greece and China.
The idea of crop rotation allowed for the field to recover its health and renew its ability to grow fresh crops. Green manure is the culmination of this to allow for smaller scale growers to benefit. We do not use grasses like rye and buckwheat but move more towards legumes and herbs!
So what are the benefits of green manure?
Green Manure As A Weed Suppressant
The plant you grow will form a ground cover. meaning that more stubborn plants cannot grow in it’s pace. You know the ones that a simple half an hour in the garden isn’t going to remove! So green manure can help to reduce weeds by kind of smothering their chances of surviving – therefore potentially being the better weed. So you will have to remove it before it becomes an issue.
Improve Soil Structure During Heavy Rainfall
By covering the ground these plants will also help to bind the soil, meaning that all of those flash floods we suffer from will have a far less drastic effect. You will find that the rain washes so many of the good quality nutrients from the soil. Your green manure crop can help to reduce this impact. This is why farmers use this technique so frequently.
Green Manure As A Nitrogen Fixer
This property of green manure sounds a bit strange but bare with me on this one. Not all plants have the ability to get nutrients directly from the soil. Like the soil has them available but not in the form that the plant is receptive to. So by growing green manure herbs and plants you can zap all of the nutrients out and into the plant, then dig them back in. They are then in the form that most crops want to receive them in.
A sort of redistribution of wealth. This can range between 40-60% increase in nitrogen available to your next crop. Green manure is the Robin Hood of herbs.
Nitrogen is the ultimate in plant feed and basically what we are all striving to get into our plants.
As An Acid / Alkaline Regulator
The type of plant we grow as green manure means that the PH of your soil can be brought back to a more neutral one. Basically you will find that several types of helpful acid are released by the green manure and it can level off heavily alkaline soils.
You can buy a basic Soil pH Tester 
Deep Roots Mean A Deeper Reach For Nutrients
When you grow certain deep rooted legumes like clover and vetch their roots will go deep to bring the plant those nutrients it needs to grow. Other crops you may want to grow as your main crop, would perhaps only have shallow roots, so those nutrients would not otherwise be available to them. Think of them as the gold diggers of plants.
Digging in Green Manure Increases Soil Drainage
When you think about all of those root systems working through the soil it means that your bed will be aerated and ready to go for the next proper growing season. Digging it in also further mixes up the soil, creating a lighter consistency. Sort of like thinking about your soil like cake mixture but you do want it to be easy to dig. If you overwinter your soil in this manner every year it does keep your soil lighter and easier to manage.
It Lifts The Winter Gloom In Your Garden
I have not seen this one listed anywhere before, but think about the difference when washing up and looking out of your window. On the one hand you have a lovely verdant patch that the birds can still feed from, on the other a barren wasteland given over to the near Dickensian gloom of a wet and windy Autumn.
Okay that is a bit dramatic, but overwintering with green manure gives you that colour and will provide a year long biodiversity for any garden or allotment.
Early Season Food For Pollinators
Some green manure plants will become woody once they have flowered so this is all about timing. You do want flowers as early food for bees and butterflies, but you do not want self-seeding or woodiness.
Flowering means seeds and if you can catch them before the wind takes them away you will not have to buy seeds again and your green manure will become an integral part in your passive gardening technique!
What Plants Are Ideal For Green Manure
We have put together a list below of the crops we like to grow but if you want to have a full list of cover crops check out an article from the Agricultural Sustainability Institute.
- Borage is our ultimate herb. Tasty leaves and flowers that actually go rather well in an early Spring cocktail in the garden! Besides all of that borage forms excellent ground cover and can completely fill any available space. Borage is quite architectural and tall in stature, so as you can imagine has long taproots to suit. Meaning it is an excellent nitrogen fixer and one of those gold digger herbs we spoke about earlier. Borage seeds can be scattered thinly over soil, but bare in mind they look like little missiles so will not need much soil to cover them. In theory borage will react to sunlight to aid germination, so as little soil on top as possible. It is a lovely green manure and will reward you with beautiful star shaped flowers that will attract all of the bees and butterflies you could ever hope for. Once the flowers have ended cut it back. Leave on the soil to wilt and then dig in, after around two weeks you will have a viable flower bed and can grow your main crop.
- Clover a wonderful legume that can grow easily overwinter and fill any space that you have recently emptied. What is really cool about clover is that it will grow to around 30cm, but this takes as little as 3-4 months. So you could cut it back for mulch before you then use it as green manure where it grows. It is a known nitrogen fixer and will grow from any time in April to August as sown. This means that you could cover an area in your allotment or garden that you have no intention of growing on for a few years. Clover will keep going for as long as two years, sometimes longer. So it will look after your beds for as long as you need it to.
- Fenugreek in the UK is not a known nitrogen fixer. But it will grow to cover in around two months and is the perfect speedy solution to getting improved soil condition. Also great as a weed suppressant in a short space of time! All parts of fenugreek are edible as well so a total bonus there, salads will never be the same again! Sow seeds from March through to end of August and two months later you can gather your first harvest. If you harvest the seeds they make a great inclusion in curries as well as being viable for the year ahead.
- Mustard will take around 1-2 months to cover an area and protect it from weeds. A great suppressant as well as tasty if eating the leaves.
- Vetch is a wonderful nitrogen fixer and can be sown later in the season to be overwintered. Scatter thinly and you will be rewarded in a few months time with total weed annihilation.
With all of your ground cover plants you will need to cut back before they can get woody and too settled in. Once you have cut them back and chopped them through, leave on the surface of the soil for a day or two. This will give them the chance to wilt. Then dig them to around a spades depth and chop them further. The more chopped they are the easier it is to mix through.
Use your spade for this as it is easy with a sharp spade!
Collecting Seeds To Save For Each Year
This is an absolute must for those of us looking to save money in out organic gardening. So often people say they don’t go organic as it is more expensive, however this is one of many ways you can save money. Your green manure now becomes the ultimate free fix!
AS you will know by now we are lovers of propagation and dividing plants to increase our yield, so why not collect seeds as well?
Potential Problems
Self- seeders! Your average green manure crop is sort of self sufficient and keen to keep on growing. As they are ground coverage we know that their tactics include smothering all other plants and blocking out their daylight. So if you are letting some of the crop flower – and I think you should, then make sure you capture the seeds before the wind carries them to another patch.
Remember – letting them flower means that there will be food early in the season for the pollinators and free seeds for that winters crop.
Slugs and snails love ground cover. So make sure to keep on top of that, I can recommend rescue chickens as our will keep on top of the slugs and snails as well as converting them into organic free range eggs. Friends of ours highly recommend their small flock of geese as well. However slugs and snails can form an essential part of your gardens biodiversity.
By encouraging natural predators like toads, hedgehogs etc you are working with nature to turn a problem into an opportunity – a key to any organic gardeners strategy. This holistic approach forms the backbone of many permaculture ideals. It is how we work with nature.
Now, from a lazy gardeners point of view this is fantastic and forms part of my hands off approach. If I can get plants to grow over winter and cover my garden from weeds, as well as form the basis of next years crops nutritional content and all for a very low cost, then why would I spend money on solutions that are not guaranteed to work?
On a side note we find that having a good range of mini beasts means that the kids are more likely to entertain themselves whilst we get on with gardening, so a bit of a win win situation. Although be warned to remove all slugs from pockets before returning inside!
Why Use Green Manure In Conclusion
- If using last years seeds it is free
- nitrogen fixing
- soil aeration
- organic and part of a wider permaculture movement.
- works with nature
- uses ancient techniques
- increased tolerance to flash floods and heavy rainfall
- provides a vital food source for pollinators early in the season
- overwintering provides a green and pleasant focal point in the garden
- it is not very labour intensive at all and requires almost no skill
- works with nature not against her!
Alternatives To Green Manure
You could use the foliage as mulch for other plant beds. You could also make up your own fertiliser by steeping the leaves and foliage in water for several weeks. This tea is great for potted plants and can be a great early feed for bay trees that live in the same pot.
Both Mulching and liquid feed take a bit of time and certain level of skill and experience. So why not start with a simple overwintering green manure and then look at other organic ways to improve your garden?
Good luck with it and please do let us know how you get on!
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