When soil is covered with plastic sheets or cloches in the spring, it warms up faster than uncovered soil and allows for earlier planting and sowing. This method is especially useful for gardeners and growers of vegetables in clay soils.
- Suitable for seeds that are best sown early, especially vegetables
- You can warm soils in winter to spring
The pre-warming of soils is especially beneficial for vegetables because it helps crops germinate earlier than usual and extends the cropping season. The soil will warm up during the day and lose less heat at night if it is covered.
Early crops that benefit from sowing in late winter or early spring into pre-warmed soil include, carrots, beetroot, onions, spinach, turnips and leafy salads including rocket, leeks and lettuce.
Tender crops sown in late spring or early summer into pre-warmed soil include courgettes, French beans, runner beans and sweetcorn. Seedbeds for ornamental plants can also be pre-warmed.
Due to their ability to retain water after the winter, clay soils are notoriously slow to warm up in the spring. On these kinds of soils, warming can be beneficial, but it may not be very effective without removing the excess water. In fact, for clay soils, raising transplants in pots and cell trays is more practical for early cropping.
When To Pre-Warm Soils
Although not difficult, pre-warming does require some preparation. Since the sun is weak in the late winter and early spring, cover up as soon as possible. Before covering, allow winter rains to restore soil moisture. Covering early crops from the beginning of the new year until the beginning of spring is ideal, but covering later, more tender crops until mid-spring can be effective.
How To Pre-Warm Soils
- Prepare your soil by cultivating to make a seedbed. Add fertiliser if required at the same time.
- Cloches or clear plastic film should be used as a cover. Fleece is less beneficial, but it does something. Black polythene works well as well, as long as it is stretched taut and close to the soil underneath. It works best when covered with another layer of clear polythene.
- Cloches are preferred for pre-warming clay soils because clay soil must lose water through evaporation before it will significantly warm.
- Cover until you are ready to sow, ideally for six weeks if you cover in the winter, but four weeks is fine if you cover later.
- Get rid of any weeds under the cover.
- Plant or sow, and cover again with fleece or cloches ideally
If they are not securely fastened, plastic films and cloches can move around in the wind, negating any warming effect. Bricks or a spade can be used to tuck the film’s edge into the soil or weight the covers around the edge to prevent this. When sizing the film, you’ll need to allow for this extra material.
Glass cloches need to be used with care because they are heavy and easy to break.
Frequently Asked Questions
“Seed germination, root growth, and the availability of nutrients all depend on warm soil. Soil temperatures of at least 10′ Celsius are ideal for cool-season crops like broccoli, cabbage, spinach, and lettuce. Beans, corn tomatoes, peppers, and other warm-season vegetables thrive in soil temperatures between 18 and 23 degrees Celsius,” says Chris Bonnett, a gardening expert for The Express.