Once the onions have been lifted, the potatoes are out and another growing season begins to wind down, it can be tempting to clear the beds and forget about them until spring.
But an empty bed does not mean an inactive bed.
Through autumn and winter, bare soil is exposed to heavy rain, weeds, nutrient losses and months without living roots growing through it.
One option is to sow a green manure.
Green manures are plants grown primarily to protect and support the soil rather than to harvest. They can be particularly useful on allotments and vegetable plots during the gaps between one crop finishing and another being planted.
Instead of asking only:
“What am I going to grow here next?”
it can be worth asking:
“What can I do for this soil before the next crop goes in?”
What is a green manure?
A green manure is a crop grown for the benefit of the soil.
It is allowed to grow for a period and is then cut down, dug in or left on the surface to decompose, depending on your growing method.
Different green manures perform different jobs.
Some provide dense ground cover.
Others develop substantial root systems.
Legumes such as vetch can fix nitrogen through their relationship with bacteria living around their roots.
Many also compete with weeds while occupying a bed that might otherwise remain bare.
The Royal Horticultural Society recommends green manures for covering soil between vegetable crops and particularly during autumn and winter.
Why protect bare soil during winter?
Across Ireland and the UK, autumn and winter can mean months of persistent rainfall.
Bare ground takes the full force of that weather.
Growing plants help provide a protective layer above the soil while their roots help hold the soil together below.
Green cover can also take up some of the nutrients remaining after the previous crop rather than leaving them vulnerable to being washed further through the soil.
Irish research and guidance from Teagasc highlights winter green cover as a way of reducing nutrient losses, protecting soil from rainfall and supporting soil structure.
For an allotment grower, the principle is fairly simple:
if a bed is going to be empty for several months, it does not necessarily need to be bare.
Green manures can help suppress weeds
Anyone who has left an allotment bed untouched for a few months knows that nature rarely leaves bare soil alone.
If you do not grow something deliberately, weeds will often fill the space instead.
A well-established green manure competes for light and space, helping to suppress some weed growth while the bed is out of production.
It will not eliminate every weed, but it can make unused ground much easier to manage.
Living roots are useful below the surface too
The part of a green manure you can see is only half the story.
Roots are growing through the soil underneath.
Different species produce different types of root systems, which is one reason the right green manure depends on what you want to achieve.
Some produce a dense network of fine roots while others reach deeper into the soil.
These roots interact with soil organisms, help bind soil particles and leave channels behind when they eventually decompose.
Green manures are not a miracle cure for serious soil compaction, drainage problems or poor soil structure, but they can form part of good long-term soil management.
What happens to the nutrients?
Green manures are sometimes described as if they simply create fertiliser.
It is a little more complicated than that.
Non-legume green manures can take up nutrients already present in the soil. Those nutrients become held within the growing plant rather than remaining unused in bare ground.
When the plant is later cut down and decomposes, some of those nutrients can cycle back into the soil.
Legumes such as vetch and clover work differently because they form relationships with nitrogen-fixing bacteria.
The benefit you get will vary depending on the plant, soil, weather, sowing date and how the green manure is eventually managed.
So think of green manure as part of nutrient management and soil protection, rather than free fertiliser appearing from nowhere. Teagasc likewise cautions that benefits vary significantly between species, soils and rotations.
Which green manure should you grow over winter?
There is no single best green manure for every allotment.
Before choosing one, consider:
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when you are sowing
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how long the bed will be free
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your soil type
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what crop was grown there previously
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what you intend to plant next
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your crop rotation
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whether you practise dig or no-dig gardening
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how you plan to remove the green manure
At this time of year, several options are particularly useful.
Grazing rye
Grazing rye (Secale cereale) is a popular option for winter ground cover.
Importantly, this is grazing rye rather than ordinary ryegrass.
It is hardy, develops an extensive root system and can remain growing through winter.
The RHS gives a sowing window of approximately August to November, making it one of the more useful choices when beds become available later in autumn.
It can produce a substantial amount of growth, though, so consider how and when you will remove it before sowing.
Winter tares or vetch
Winter tares, also known as vetch, are another useful overwintering option.
Vetch is a legume and can fix nitrogen through bacteria associated with its roots.
It is hardy, provides good cover and is often grown in combination with grazing rye.
The two plants have different growth habits and root systems, which can make them a useful mixture.
If you follow crop rotation, remember that vetch belongs to the legume family alongside crops such as peas and beans.
Winter field beans
Winter field beans are another legume that can work well as a green manure.
They are particularly useful on heavier soils and produce deeper roots than some other options.
The RHS recommends sowing winter field beans from around September to November, so they are worth considering when beds become free later in the season.
Phacelia
Phacelia is a quick-growing green manure that is particularly useful earlier in the growing season and during shorter gaps between crops.
It is not closely related to most common vegetable crops, which makes it useful when crop rotation is a concern.
If allowed to flower, phacelia is also attractive to pollinating insects.
However, the RHS gives its main sowing period as approximately April to August, so by later September there may be better choices if your main aim is reliable winter cover.
What about mustard?
Mustard grows rapidly and can be useful when you have a relatively short gap to fill.
There is one important issue for vegetable growers, however:
mustard belongs to the brassica family.
That means it needs to be considered alongside cabbage, kale, cauliflower, broccoli and other brassicas when planning crop rotation.
If clubroot or brassica rotation is a concern on your plot, mustard may not be the best choice.
The RHS specifically recommends avoiding following mustard with other brassicas because of the potential to contribute to disease problems such as clubroot.
Can you use a mixture?
Yes.
In fact, mixtures can make a lot of sense because different plants perform different functions.
A common example is grazing rye and winter vetch.
The rye provides vigorous growth and an extensive root system, while the vetch contributes the characteristics of a legume.
The RHS notes that mixtures of green-manure species can sometimes be more useful than relying on one species alone.
But more species does not automatically mean better.
Choose according to your soil, rotation and next crop rather than simply buying the packet with the longest ingredient list.
Think about your next crop before you sow
This is one of the most important parts of using green manure successfully.
Do not decide how you are going to remove it in March after it has already filled the bed.
Decide before sowing.
Ask:
When will I need this bed again?
A bed needed for very early spring planting may require a completely different approach from one that will remain empty until May.
Likewise, a vigorous overwintering crop such as grazing rye may be ideal where you have plenty of time before planting but inconvenient in a bed you need urgently at the beginning of spring.
Green manure should work with your growing plan rather than getting in its way.
When should you cut green manure down?
Most green manures should be managed before they become overly mature or begin setting seed.
For an overwintered crop, that often means cutting it down towards the end of winter or in early spring.
If you plan to dig the green material into the soil, allow time for decomposition before planting your next crop.
The RHS recommends allowing roughly a month after incorporation before sowing or planting, while similar guidance suggests around three to four weeks.
No-dig growers do not necessarily have to bury the material.
Depending on the species and your growing system, it may instead be cut at soil level and left on the surface to break down, potentially with compost or another mulch added above.
Will green manure transform poor soil?
Not by itself.
Green manures are useful, but they should not be treated as a quick fix.
One season of green manure will not suddenly transform severely compacted or depleted soil.
Teagasc notes that while cover crops can contribute to soil characteristics and organic matter, the changes achieved can be modest and depend heavily on species, conditions and how they are managed.
The real value comes from combining good practices over time.
That might include:
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growing green manures
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adding compost
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using well-rotted manure
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mulching
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avoiding unnecessary soil compaction
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protecting beds during wet weather
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rotating crops
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keeping living roots in the ground where practical
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supporting the wider soil environment
Healthy soil is built gradually.
Green manure isn't the only option
Not every empty bed needs to be planted with a green manure.
You could also protect soil with compost, leaf mould, organic mulch or another suitable covering.
Some beds may continue producing winter vegetables.
Others may be needed too soon in spring to make an overwintering green manure practical.
The important idea is not:
“Every bed must have green manure.”
It is:
“What is the best way to protect and support this soil until I need it again?”
Where does Humusoil fit?
Humusoil for Gardens, Allotments & Growers is based on the same soil-first approach.
It contains humic acid, seaweed and beneficial soil microbes and is designed to support soil structure, nutrient availability and a more biologically active growing environment.
Humusoil does not replace compost, crop rotation, green manures or good growing practices.
Instead, it can form another part of the way you look after your soil throughout the growing season.
Whether you are growing in vegetable beds, raised beds, containers, a greenhouse, polytunnel or a full allotment plot, the principle remains the same:
healthy plants start with the conditions you create in the soil.
Your growing season doesn't end with the harvest
When the last crop comes out of a bed, it is easy to see that patch of ground as finished until spring.
But autumn and winter are part of the soil-management calendar too.
Covering bare ground, growing living roots, making compost and planning next year's rotation can all help give you a better starting point when spring arrives.
So when you lift the last of this year's crops, do not just ask what you are going to plant next.
Ask:
What can I do for this soil in the meantime?