Green Manures and Cover Crops for Autumn and Winter
Green manures and cover crops are plants grown primarily to protect or improve the soil rather than to provide an edible harvest. They are particularly useful when vegetable beds become empty during late summer or autumn and will not be planted again for several weeks or months.
Their roots keep the soil active, while their leaves shield the surface from rain, wind and erosion. Depending on the species chosen, they may also suppress weeds, capture nutrients, add organic matter or provide flowers for pollinators and other beneficial insects.
The right green manure depends on the climate, sowing date, soil condition, crop rotation and how soon the bed will be needed again.
Green Manure or Cover Crop?
The terms green manure and cover crop are often used interchangeably, although they can describe slightly different purposes.
A green manure is generally grown to improve the soil before being cut down and returned to it.
A cover crop is grown mainly to keep the soil covered and protected. It may also be cut and left on the surface, composted or removed before the next crop is planted.
In a vegetable garden, the same plant often performs both roles.
Why Avoid Bare Soil?
Soil left bare throughout autumn and winter is exposed to weather and weed growth.
Heavy rain can:
Break down the soil surface
Cause compaction and capping
Wash soil from sloping ground
Leach soluble nutrients
Create waterlogged conditions
Reduce the air available to roots and soil organisms
Wind can remove fine particles from dry, light soil, while uncovered ground provides an opportunity for weeds to establish.
A living cover softens the impact of rain, slows water movement and holds soil together through its roots.
Benefits of Green Manures
A suitable green manure can:
Protect the soil surface
Reduce erosion
Suppress weeds
Capture nutrients that might otherwise be lost
Add organic matter
Support soil organisms
Improve soil structure
Create channels for water and air
Provide habitat for beneficial insects
Produce flowers for pollinators
Reduce the need for imported mulch
No single green manure provides every benefit equally. Choose according to the main purpose of the bed.
Start with the Available Growing Time
Before choosing a green manure, decide when the bed will next be needed.
Some green manures grow quickly and are suitable for short gaps. Others need several months and may remain in the ground throughout winter.
For a short gap, consider a fast-growing species that is easy to clear.
For a bed that will remain unused until spring, choose a crop that can tolerate the expected winter or one that will be killed naturally by frost and form a protective surface covering.
Record the planned start and end of the cover-crop period so it does not interfere with the next vegetable planting. Growing Spaces in Your Growing Companion can be used to mark the bed as resting or fallow until it is needed again.
Phacelia
Phacelia is a fast-growing green manure with fine roots and abundant foliage.
Its advantages include:
Rapid ground coverage
Good weed suppression
Attractive flowers for pollinators
Easy cutting
No close relationship to most common vegetable crops
Because it is not part of the main vegetable crop families, phacelia fits easily into many rotation plans.
It is not reliably winter-hardy in cold climates. Frost may kill it, leaving the collapsed growth as a protective mulch. In mild regions it may survive for longer and need to be cut before flowering or setting seed.
Mustard
Mustard germinates and grows quickly, making it useful where only a short period is available.
It can:
Cover soil rapidly
Suppress some weeds
Produce substantial leafy growth
Be damaged or killed by frost in colder regions
Mustard belongs to the brassica family. This is important where cabbage, kale, broccoli, cauliflower, radish, turnip or other brassicas form part of the rotation.
Avoid mustard where clubroot or another persistent brassica problem has occurred. It may also be unsuitable before or after a brassica crop if a strict rotation is being followed.
Cut it before it produces mature seed.
Buckwheat
Buckwheat is a quick-growing warm-season cover crop that can be useful during a short late-summer gap.
It provides:
Rapid leaf growth
Good surface coverage
Flowers for beneficial insects
Easily managed soft growth
A crop-family option separate from most vegetables
Buckwheat is sensitive to frost and performs best while the soil remains warm. It is therefore more suitable for early late-summer sowing than for cold autumn conditions.
Cut it before seeds mature unless self-seeding is acceptable.
Field Beans
Field beans can be used as a cool-season green manure, especially where autumn and winter conditions are relatively mild.
Their roots support nitrogen-fixing bacteria, and the plants can produce useful organic material.
Field beans may:
Tolerate cool conditions
Grow in heavier soils
Develop strong roots
Remain standing through part of the winter
Provide substantial material for cutting and mulching
They belong to the legume family, together with peas and other beans. Consider this when planning crop rotation.
Field beans used as green manure are normally sown more closely than beans grown for a harvest.
Forage Peas
Forage peas can provide leafy growth and help protect the soil during cool weather.
They are often used:
As a short-term cover
In mixtures with oats or other cereals
To add soft organic material
Where a legume fits the rotation
Their cold tolerance varies by variety and climate. In severe winters they may be killed by frost, while in milder areas they can continue growing.
Forage peas need support from neighbouring plants if tall growth is expected, which is one reason they are often included in mixtures.
Vetch
Vetch is a legume that can produce dense growth and survive relatively cold conditions, depending on the species and variety.
It may be useful for:
Winter soil coverage
Weed suppression
Supporting nitrogen fixation
Producing organic matter
Mixing with cereal cover crops
Vetch can become tangled and difficult to clear if allowed to grow for too long. Cut it before it flowers heavily or forms mature seed.
Its vigorous growth makes it more suitable for beds that will not be needed early than for very short gaps.
Clover
Clover can form a low, living cover and may be useful where ground will remain unused for a longer period.
Different clovers vary in height, lifespan, soil preference and cold tolerance.
Clover may:
Protect the soil
Support nitrogen-fixing bacteria
Provide flowers for pollinators
Cover paths or longer-term spaces
Remain in place for several months
It can be slow to establish and may be difficult to remove completely from a bed required for fine seed sowing. It is generally better suited to longer resting periods than to brief gaps between vegetable crops.
As a legume, it should be considered within the crop rotation.
Oats
Oats produce fibrous roots that help hold the soil together and improve its structure.
They can:
Establish quickly in suitable conditions
Produce substantial root growth
Capture nutrients
Suppress weeds
Combine well with forage peas or vetch
Many oat varieties are damaged or killed by hard frost. In cold climates, this can be useful because the dead growth forms a surface mulch that is relatively easy to manage in spring.
In milder climates, oats may survive and need to be cut before the next crop is planted.
Grazing Rye
Grazing rye is one of the hardiest winter cover crops.
It can:
Continue growing in cold conditions
Protect soil throughout winter
Produce extensive roots
Capture nutrients
Suppress weeds
Grow on relatively poor ground
Its strength can also be a disadvantage. Grazing rye develops a dense root system and may be difficult to clear if allowed to become large.
It can temporarily compete for moisture and nitrogen as its mature material decomposes. Cut it early enough before the next planting and leave time for the residues to begin breaking down.
Grazing rye is generally best for beds that will not be needed early in spring.
Green-Manure Mixtures
A mixture can provide a wider range of benefits than a single species.
Common combinations may include:
Oats and forage peas
Rye and vetch
A cereal with a legume
A quick-growing flowering mixture
A frost-sensitive blend for winter-killed mulch
The cereal provides fibrous roots and upright support, while the legume adds softer growth and supports nitrogen fixation.
Mixtures can be harder to manage if the plants mature at different times. Check that every species suits the climate, sowing date, soil and planned clearing time.
Understanding Nitrogen Fixation
Legumes such as clover, vetch, field beans and forage peas form relationships with bacteria living in nodules on their roots. These bacteria can convert atmospheric nitrogen into forms used within the plant.
This does not mean that planting a legume immediately fills the soil with freely available nitrogen.
Much of the nitrogen remains within the roots, stems and leaves. It becomes available gradually as the plant material decomposes. If the entire crop is removed from the garden, much of this benefit is also removed.
Nitrogen fixation is most effective where the correct bacteria are present and the plants are growing successfully.
Choose According to Crop Rotation
A green manure is still a crop and should be considered within the rotation.
Important families include:
Legumes: clover, vetch, field beans and forage peas
Brassicas: mustard and some radishes used as cover crops
Grasses: oats and grazing rye
Other families: phacelia and buckwheat
Avoid using a green manure that could encourage an existing pest or disease problem.
For example, mustard may be inappropriate where clubroot has occurred, while repeated legumes may be undesirable where a legume-specific disease is present.
Phacelia and buckwheat are often useful because they are not closely related to the most commonly grown vegetables.
Prepare the Bed
Green manures do not usually require elaborate soil preparation.
After removing the previous crop:
Remove perennial weeds.
Clear diseased material.
Level major holes or ridges.
Water the ground if it is extremely dry.
Create enough loose surface soil for seed contact.
Sow at the recommended rate.
Rake or cover the seed to the appropriate depth.
Water gently if rain is not expected.
Very compacted or heavily weed-infested ground may need additional preparation.
Avoid applying large amounts of fertiliser. The purpose is to protect and improve the soil, not to force excessive soft growth.
Sow at the Correct Rate
Cover-crop seed is often sown more densely than a vegetable crop because rapid ground coverage is desirable.
Sowing too thinly can leave gaps for weeds. Sowing far too thickly can create weak plants, poor airflow and unnecessary competition.
Follow the supplier’s recommended rate where available. Broadcast the seed evenly over the bed or sow in closely spaced rows.
Mixing very small seed with dry sand can make even distribution easier.
Protect newly sown seed from birds where necessary.
Water During Establishment
Late summer can still be hot and dry. A green manure cannot protect the soil if the seed fails to germinate.
Water the bed before sowing if necessary and keep the surface moist until seedlings are established.
Once rooted, many cover crops need little additional watering, although prolonged drought can still affect them.
Do not sow a water-demanding cover crop if irrigation is unavailable and reliable rain is unlikely. A protective mulch may be the more practical choice.
Prevent Green Manures from Becoming Weeds
Most green manures should be cut before they produce mature seed.
Self-seeding may create unwanted plants among later vegetable crops, particularly when the cover crop belongs to a family already present in the garden.
Watch for flowering and seed development. Some plants progress quickly during warm autumn weather.
If flowers are being used temporarily by pollinators, allow a limited flowering period but cut the crop before viable seed develops.
When and How to Cut the Crop
Cut the green manure when:
The bed will soon be needed
Plants begin flowering
Growth becomes difficult to manage
Severe weather is expected
The cover threatens to set seed
The next crop requires preparation time
In a no-dig system, cut the plants at or just above soil level. Leave the roots in the ground to decompose naturally.
The top growth can be:
Left as a surface mulch
Chopped into smaller pieces
Added to the compost heap
Used to mulch another suitable area
Removed if it carries disease or mature seed
Soft, young growth decomposes more quickly than mature, woody material.
Allow Time Before Planting
A thick layer of freshly cut green manure can obstruct direct sowing and temporarily create difficult conditions around small seedlings.
After cutting:
Leave soft material on the surface to wilt
Chop bulky stems into smaller pieces
Move heavy residue aside from sowing rows
Allow time for dense roots to begin breaking down
Add compost if the next crop requires it
Large transplants can often be planted through remaining surface material. Small seeds need a clear, fine seedbed.
The time required depends on the species, maturity and weather. Grazing rye generally needs more management time than young phacelia or buckwheat.
Frost-Killed Cover Crops
In cold climates, frost-sensitive green manures may be deliberately chosen so winter weather clears them naturally.
Possible frost-sensitive crops include:
Buckwheat
Phacelia in colder conditions
Mustard, depending on variety and temperature
Some oats
Tender mixtures
After frost, the collapsed foliage covers the surface and the roots begin to break down.
Winter-killed crops are convenient, but results depend on the severity of the climate. In a mild winter, they may survive and require manual cutting.
Living Covers in Mild Climates
In regions with mild winters, cover crops may continue growing for much of the season.
This provides long-term soil protection but means the plants must be monitored.
Check that they are not:
Producing seed
Spreading beyond the bed
Harbouring persistent pests
Becoming too dense to clear
Competing with nearby winter vegetables
Remaining wet and diseased
Cut vigorous crops in stages if necessary and plan their removal well before spring planting.
Green Manures in Raised Beds
Green manures can be used successfully in raised beds, but choose species that fit the scale of the space.
Very vigorous or deep-rooted crops may be difficult to manage in a small bed. Compact, fast-growing choices such as phacelia, buckwheat or a short-term mixture may be more practical.
Do not allow roots or stems to block irrigation outlets or spread into paths.
Where only part of the bed is empty, sow a small patch rather than waiting for the entire area to become available.
When Mulch Is a Better Choice
Green manure is not always the best way to cover an empty bed.
Use mulch instead when:
The bed will be replanted very soon
Water is unavailable for germination
It is too late for reliable establishment
The ground is heavily infested with perennial weeds
A suitable green manure does not fit the rotation
The bed is needed for very early spring sowing
Managing the cover crop would create unnecessary work
Mature compost, leaves, straw or another appropriate covering can protect the soil without introducing a living crop.
Record the Results
Keep a record of:
The species or mixture sown
The sowing date
The bed or growing space
Germination success
Winter survival
Flowering time
The cutting date
How easily it was managed
The condition of the soil afterwards
The Garden Planner and crop records in Your Growing Companion can help connect this information to the correct bed and the crop planned to follow it.
Keep the Soil Living
Green manures and cover crops turn an empty bed into an active part of the garden.
Their purpose is not to produce a perfect stand of plants or the largest possible amount of growth. It is to protect the soil, support its living systems and prepare the space for future crops.
Choose a species that fits the available time, climate and rotation. Sow it promptly, prevent it from setting unwanted seed and clear it before it interferes with the next planting.
Used thoughtfully, a cover crop can protect the ground throughout autumn and winter while reducing the amount of repair and preparation needed in spring.
