Once the summer crops are harvested and gone, organic gardeners will often consider sowing green manures into their growing areas.
These crops might not always be of use as an edible crop. But they will have a range of benefits for you and your garden.
In my garden, I frequently choose to add green manures to my growing areas in September or October, and chop them down in the spring.
While we also have the manure from our fifteen rescue chickens to incorporate into the growing systems, green manures are a far less pungent option and work well whether you live in the countryside or a city.
What is a Green Manure?
A green manure is a crop that is grown not for primarily for your own direct use (for example as an edible crop). Instead, it is a crop that is grown to benefit and potentially improve the soil in which it is grown in some way.
After a period of growth, green manures are chopped and cropped.
Sometimes, the organic material is dug into the soil.
In a ‘no dig’ garden, however, the green manure is simply chopped down and left to lie on the soil surface.
Organisms in the soil below, thriving in a no dig system, will do the work of incorporating this matter and its nutrients into the soil as it breaks down.
We implement a ‘no dig’ system here on our property. To learn more about creating a no-dig garden, have a read of our article here.
We are fortunate enough to have a nutrient-rich, deep, clay-loam soil but even with this good quality soil, we are conscious of the importance of maintaining (and even improving) its fertility over time.
Our topsoils are precious and fragile and if we do not care for them, they will be gone.
A ‘no dig’ system, which takes care to disturb the soil ecosystem as little as possible, is the best way to preserve growing areas for years to come. When we leave the soil undisturbed as much as possible, we allow the soil web to thrive.
Studies have shown that yields can be improved with a ‘no dig’ or ‘no till’ approach, and my own experiences bear that out.
Green manures are an important part of that ‘no dig’ picture.
Why are Green Manures Used in Winter?
To Prevent Nutrient Loss
Where I live, rainfall in winter is relatively high. The conditions are much drier and sunnier than in most of the country, but in the coldest part of the year things can still get pretty wet.
The problem is that heavy rain (and occasionally snow) can wash nutrients from the topsoil and away into the deeper layers of soil.
Our clay rich soil is better at nutrient retention than other soil types. But nonetheless, can suffer from depletion.
A green manure will cover the soil over the winter months and prevent the nutrients from being washed away.
Rather than losing the nutrients from the growing area, planting a green manure ensures that these nutrients are gathered by the plant roots.
Then, when these are chopped and strewn on the soil surface, they will be returned to the top layer of soil where they can be taken up by the next plants to be grown there.
To Prevent Soil Compaction or Erosion
Another benefit of a green manure is that it will protect the soil from compaction or erosion. When it absorbs water during the winter months, our clay-rich soil can be prone to compaction.
Not only does having a cover crop help to avoid bare soil, which reduces this problem, it also adds organic matter in the spring, which will help considerably to improve the soil texture and keep it aerated.
Lighter soils can be prone to erosion in winter winds and rain – a green manure helps to combat those issues too.
Erosion can be a particular problem on sloping sites.
Avoiding bare soil by planting a green manure will help soil from washing or blowing downhill. The roots of the plants you choose for your green manure can also help to anchor soil in place.
To Fix Nitrogen
Outside the dormant phase, legumes used as green manures will also co-operate with beneficial bacteria on root rhisomes to fix nitrogen from the air and make it available in the soil.
There is some disagreement as to how much nitrogen different plant species will fix in different bioregions, and debate as to how much nitrogen is then available for the uptake of plants following these ones in the soil.
Evidence suggests, however, that nitrogen fixing plants can play a key role in effective organic food producing systems.
Details of some nitrogen fixing green manures can be found below.
To Reduce Weeds
Finally, growing a densely planted green manure over the winter months can also help to keep your growing areas free from weeds.
This means that you will have a clean, clear planting area to sow into or plant up in the spring.
Choosing a Green Manure
When choosing a green manure, the first thing to consider is that what works well in one location will not do as well in another.
As with any plant, green manures should be chosen with reference to the climate and conditions in your area. You should be sure to think about:
- Temperatures and expected weather conditions where you live.
- The soil type in your garden: clay, silt, sand, loam etc..
- Soil pH: is your soil acidic, neutral or alkaline?
If you do not match your green manure to your soil type then the results that you achieve are not likely to be as good.
It is also important to consider your green manure not only as a stand-alone crop but as part of your entire garden.
Think about which other crops might be grown nearby, and about how the green manure you are considering will fit into your crop rotation and planting plan.
Think about which crops will precede, join and follow your green manure, and whether your green manure will help or hinder you in your overall garden plan.
5 Types of Green Manure
To help you decide which green manures you might like to incorporate into your winter planting scheme, here are some of the common green manures for the winter months:
1. Winter Field Bean (Vicia faba)
Winter field beans are the same species as the fava bean or broad bean that is grown primarily for culinary use. However, field beans show better cold tolerance and are more vigorous.
They are a different variety with produces smaller beans.
While these are also edible, if grown as a green manure, they are usually chopped and dropped before flowering.
Personally, I do sometimes leave a few plants to produce beans for our consumption, while using the rest of the crop to feed the soil.
Though the beans of field beans are smaller than broad or fava beans, more pods are generally produced per plant.
(If you intend to harvest field beans for human consumption, pick them young for the best texture and flavour.)
Used as a green manure, field beans are sown between September and November. Recommended sowing density is 20g per m2.
They are particularly good on heavy soils as their deep roots penetrate and break up the soil, helping create a looser, more aerated soil structure.
As a legume, field beans are a nitrogen fixer, and they can replace other legumes, or follow summer legumes, in a crop rotation. I sometimes also sow field beans as a crop between winter brassica such as kale or winter cabbage.
The foliage will provide some organic matter to the soil, but in order to improve weed suppression they are often sown alongside winter rye.
In my garden, a winter sowing of field beans is preceded by summer peas or beans and often followed by a spring brassica crop. These leafy vegetables benefit from the nitrogen that has been fixed by the beans.
Any leafy vegetables will tend to do well when planted following legumes in a crop rotation.
2. Winter Tares/ Vetches (Vicia sativa)
Winter tares, also known as vetches, is another nitrogen fixing green manure that you could consider for the winter months.
This is another hardy crop that can be sown between July and September for overwintering.
This is another good option to combine with winter rye for a good soil cover. Like field beans, it can be included after summer legumes and before brassicas or other leafy vegetables in crop rotation.
Good coverage is achieved by sowing 5.5g per m2.
Winter tares is another crop that works well on heavy soils. However, it will not do well on very dry or acidic soils.
One thing to bear in mind about winter tares is that it is beloved of slugs and snails, and (as we found out here, where we have a large flock of the birds nesting in a nearby barn) pigeons have a taste for it too.
Another thing to remember is that after it has been growing for 2-3 months, and has been chopped and dropped, it is best to wait a while before sowing seeds in the area.
Winter tares releases a chemical that inhibits the growth of small seeds (in particular carrots, parsnips and spinach).
While it will not pose a problem for transplants or young seedlings, leave at least a month before sowing seeds in the area.
3. Red Clover (Trifolium)
Trifolium is the best form of clover for fixing nitrogen from the air.
It is also one of the green manures that is excellent both for weed suppression and for improving the structure of the soil.
The roots are deep and so are particularly beneficial for binding light soils together and preventing erosion over winter. It is also good for preventing nutrient leaching.
Red clover is generally sown between April and September and can either be chopped and dropped after around 3 months, or left in situ for up to two years.
Trifolium pratense is hardy and a good option to choose for overwintering.
Sowing at a density of around 2-3g per m2 will give good cover and create a bulky soil cover that will reduce weeds. It will work well in areas with a good loam or a sandy soil.
When red clover is growing it can be beneficial for wildlife, especially when left to flower. The flowers attract a range of pollinators and are beloved of bees.
When chopped and dropped, the nitrogen is said to be released quite quickly, to be benefit of nitrogen hungry plants following it in the area.
4. Winter Rye (Secale cereale)
Winter grazing rye is a great green manure for a range of purposes. It is fantastic for our clay-loam soil but also works well for sites with a wide range of soil types.
Its deep roots mean that it is very good at breaking up soil or anchoring soil layers.
It is fantastic at suppressing weeds, grows quickly and, crucially, continues to grow well even in cold weather.
I have found that germination can be patchy if we have a wet fall. But most years, sowing at a density of around 17g per m2 in September gives good results.
You can sow any time between August and October.
When chopped and dropped, rye is excellent for adding organic matter and improving soil structure.
Another interesting thing is that rye is very good at taking up nitrogen and can then release up to 90% of the nitrogen it lifted for the use of the next crop.
This can be a very useful feature with this green manure.
Mustard is another interesting green manure option to consider for the winter months.
It is a member of the brassica family and produces a huge quantity of organic matter that can help to improve soil texture and moisture retention.
It can work very well when grown in most soils.
One of the things that I like about this green manure option is that it requires even less work. It is not very hardy and though it can make it through a milder winter, here, it gets frost damaged.
The frost damaged plants can then simply be left in place as a soil covering mulch, so you do not even have to chop it and drop it.
If you have a problem with wireworm, consider sowing a winter crop of mustard before potatoes. It is believed that mustard can reduce wireworm populations by stimulating the pest to complete their life cycle more quickly.
Wireworms are common in areas that were covered by grass, and can decimate a potato crop.
Rotting mustard leaves are also said to suppress nematodes and pathogenic fungi and help prevent a number of common soil diseases. Certain mustard strains are used to cleanse soils that have heavy pathogenic loads.
Never sow mustard, however, before other brassicas, as it could increase the chances of club root or other diseases that attack this plant family.
Pick a variety that is edible and you can even pick the odd leaf in fall and early winter for use in your salads, as well as using it as a green manure.
Try A Green Manure For Yourself
The right green manures for the winter months will of course depend on where you live and the conditions to be found there. Your own requirements with regard to your planting plan will also come into play.
There are, of course, other options to consider. But considering the above options could be a good place to start.