Protecting the soil in a winter garden primarily involves keeping the soil covered and maintaining a living root in the soil whenever possible, for as long as possible. This involves growing winter crops with appropriate mulches and using cover crops or green manures that will either remain in place over the winter, to be chopped and dropped in spring, or naturally break down once the coldest weather arrives.
Growing Winter Crops
I live in an area of the UK where winter temperatures can dip below freezing between late October and mid-April, but rarely go below 14 F/-10 C. I can grow some hardy crops outside all winter long. But in order to grow more crops year-round, I have an unheated polytunnel which typically remains frost-free.
Protecting the soil in my polytunnel is particularly important, since good fertility is crucial for growing areas that are used all year round. Mulches are important in maintaining fertility throughout the year.
I typically top-dress the polytunnel beds with homemade compost/leaf mold in spring, and again in early autumn when summer crops come out and winter crops go in. Additionally, I mulch fruiting plants like tomatoes with comfrey and other dynamic accumulators during the flowering and fruiting period. I add autumn leaves as protective mulch around crops like overwintering onions, for example.
To protect the soil, I practice crop rotation, especially of nitrogen-fixing legumes. Over the winter in the polytunnel, crops such as fava beans and winter peas are important within this scheme. They aid in adding nitrogen for the brassicas and other crops that crave it, which follow them in spring or early summer.
In addition to the overwintering legumes, I also keep the polytunnel soil covered by growing a range of other crops—Asian greens, winter lettuces, mustards, daikon radishes, etc. These not only protect the soil and maintain a living root, but also provide food over the winter months. Onions and garlic varieties for overwintering are also integrated into my year-round crop rotation plans.
Winter Cover Crops or Green Manures
In outdoor annual production beds, I typically do not grow edible crops year-round. While some brassicas, leeks, onions, and garlic can survive over the winter months (the latter with protective mulch), I usually use cover crops or green manures in most areas to preserve fertility and protect the soil.
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 grown in the area.
One useful green manure for the winter months which I find useful is field beans. These show better cold tolerance than typical fava beans; and yet, like the fava beans which I grow primarily for eating in my polytunnel, they also fix nitrogen. I sow these in September or October, sometimes between rows of edible crops such as kale or winter cabbages.
Field beans are often grown (at a sowing density of around 20g per square meter) as a winter cover crop alongside winter rye (at a sowing density of around 17g per square meter), which improves ground cover and weed suppression. Rye is good at taking up nitrogen and can then release up to 90% of the nitrogen it lifted for the use of the next crop.
An alternative legume to consider in a winter cover crop or green manure are vetches, or winter tares (Vicia sativa). Note however, that this is not ideal for dry or very acidic soils, and is beloved of slugs, snails, and birds like pigeons. It is also important to note that, after chopped and dropped, seeds should not be sown in the location for a month or so, as it releases a chemical which can inhibit the growth of some small seeds (such as carrots, parsnips, and spinach).
Clovers can be good cover crops to protect soil over the winter months. I use clovers as perennial ground covers in my forest garden; but they can also be useful as part of cover crops or green manures within annual growing systems.
One final green manure that I use is mustard. This member of the brassica family adds plenty of organic matter to improve soil texture and moisture retention. Mustard gets damaged by frost where I live, but the frost-damaged foliage can be left in place as a soil-covering mulch. So if you live in an area with a similar climate, you don’t even have to worry about chopping and dropping it in spring. Planting mustard before potatoes can reduce wireworm damage and suppress nematodes and pathogenic fungi.
Finding the right cover crops and green manures for your specific site is important. What works well where I live may not be the best solution for you and your location. But perhaps learning about how I protect the soil in my garden over the winter months will help you begin to formulate a sustainable winter management plan for your own property.