When the main growing season starts to wind down, it's easy to look at an empty bed and think about what needs doing next year. But before you start planning your spring crops, it's worth thinking about what happens to those beds over winter.
Leaving soil bare through the colder months can make it more vulnerable to heavy rainfall, erosion and nutrient loss. With shorter days and less opportunity to work outside, winter is also a good time to think about how you can protect and improve your soil without creating more work for yourself in spring.
This is where cover crops and green manures can be useful. Rather than leaving beds empty, you can use suitable plants to protect the soil, support soil biology and prepare growing areas for the following season.
What Are Cover Crops and Green Manures?
The terms cover crops and green manures are often used interchangeably, although they aren't quite the same thing.
Cover crops are generally grown to protect the soil rather than for harvest. They help provide ground cover, reduce the impact of heavy rainfall and can support soil structure and biological activity.
Green manures are crops grown specifically to improve the soil, often by being cut down and incorporated or left on the surface as organic matter. Some varieties can also help capture nutrients that might otherwise be lost over winter, while legumes can fix atmospheric nitrogen through their relationship with soil bacteria.
In a market garden, the two approaches can overlap. A crop might protect the soil through winter and also provide useful organic matter when it's time to prepare the bed for another crop.
Why Is It Important to Avoid Leaving Soil Bare?
One of the biggest challenges for UK growers over winter is rainfall. Several weeks of wet weather can have a significant effect on exposed growing areas, particularly on lighter soils or sloping ground.
Bare soil is more exposed to the impact of rain, which can lead to surface sealing, erosion and the movement of nutrients. Repeated wetting and drying can also make some beds more difficult to work when spring arrives.
A living cover helps soften the impact of rainfall and provides roots that hold soil particles together. It can also provide food and habitat for soil organisms, helping to maintain biological activity during a period when most vegetable crops have slowed down.
It won't solve every soil problem, but keeping something growing in the ground wherever practical is a useful part of looking after your growing space.
Choosing the Right Cover Crop
There isn't one perfect cover crop for every market garden. Your choice will depend on when you're sowing, how long the bed will be out of production, your soil type and what you want the crop to achieve.
Some commonly used options include:
Winter Rye
Winter rye is a popular choice for providing winter ground cover. It establishes well in suitable conditions, develops an extensive root system and can help protect soil from erosion. It is particularly useful where beds will be out of production for several months, although it needs managing carefully before the next crop is planted.
Field Beans
Field beans can be useful as a winter cover crop and green manure. As a legume, they can fix nitrogen when associated with the appropriate soil bacteria. They can also provide useful ground cover and organic matter. They are often considered where growers want to improve soil fertility as part of a longer rotation.
Winter Vetch
Vetch is another legume that can be included in winter cover crop mixes. It can contribute nitrogen fixation, provide ground cover and produce useful plant material for the following season. It is sometimes combined with cereals to provide a mixture of different root structures and growth habits.
Mustard
Mustard establishes quickly and can provide rapid ground cover when sown at the right time. However, because it belongs to the brassica family, it isn't suitable for every rotation, particularly where clubroot or other brassica-related disease issues are a concern.
Before choosing a variety, always check its suitability for your sowing date, soil conditions and rotation.
When Should You Sow Cover Crops?
Timing makes a big difference to how successful a cover crop will be.
Many cover crops establish best when soil is still relatively warm and there is enough moisture for germination. This means late summer and early autumn are often better opportunities than waiting until November.
That doesn't mean there is no point sowing later, but growth will be slower as temperatures and daylight hours decrease. If you're sowing in November, choose varieties suited to late sowing and check the recommended sowing windows carefully.
It's also worth thinking ahead. If you know a particular bed will be needed for early carrots, salads or brassicas, you need to choose a cover crop that can be terminated in time without delaying your spring planting.
What Happens When Spring Arrives?
One of the most important things to consider when using cover crops is how you will deal with them before planting your next crop.
Leaving a cover crop to grow for too long can create additional work, particularly if it becomes mature and difficult to manage. Some varieties can also compete with the next crop for moisture and nutrients if they aren't terminated properly.
Depending on the crop and your growing system, you might cut it down and leave the material on the surface, use it as mulch or incorporate it into the soil where appropriate.
In lower-disturbance systems, leaving the cut material on the surface can be a useful option. However, you need to allow enough time for the material to break down and for the bed to be ready for the next crop.
Think about this when choosing your cover crop rather than leaving the decision until planting time.
Cover Crops as Part of Your Rotation
Cover crops are most useful when they're considered as part of your wider crop rotation rather than simply something to sow when a bed is empty.
For example, a legume cover crop might be useful before a crop with higher nitrogen requirements, while a cereal cover crop might be chosen for its ground-covering and rooting characteristics.
However, it's important to remember plant families when planning rotations. Mustard, for example, shouldn't automatically be treated as a neutral break crop if you're managing a rotation that includes brassicas.
Keeping a simple record of what was grown in each bed, including cover crops, will help you make better decisions from one season to the next.
A Little Planning Now Can Make Spring Easier
Cover crops and green manures aren't a magic solution, and they do require a little planning. But they can be a useful way of keeping beds productive even when you're not growing vegetables for sale.
Rather than looking at an empty bed as another job waiting for spring, think about whether there's something useful you can grow in it over winter.
Your soil will benefit from the protection, and you'll have another opportunity to improve your growing system before the busy season starts again.
Sometimes the best thing you can grow over winter isn't something you'll harvest at all.