Vegetables in particular benefit from pre-warming of soils, helping crops to germinate earlier than usual, increasing the length of the cropping season. Covering the soil will warm it during the day and slow down heat loss at night.

Early crops that benefit from sowing in late winter or early spring into pre-warmed soil include:

• Calabrese
• Carrots (early cultivars)
• Beetroot (bolt-resistant cultivars)
• Broad beans
• Kohl rabi
• Leafy salads including rocket
• Leeks
• Lettuce
• Onions
• Parsley
• Parsnips
• Peas
• Radishes
• Spinach
• Turnip (early
  Artificially speeding up a plant’s growth, flowering or fruiting, usually by manipulation of temperature and light. Rhubarb and chicory can be forced in late winter by covering with a forcing jar or bucket, or by placing potted plants in the dark, to produce earlier, paler, sweeter growth. Hyacinths and other spring bulbs are often forced, so they flower earlier indoors.

  forcing cultivars)

Tender crops sown in late spring or early summer into pre-warmed soil include:

• Courgettes
• French beans
• Runner beans
• Squash
• Sweetcorn

Seedbeds for ornamental plants can also be pre-warmed.

Clay soils are notoriously slow to warm up in spring, due to their retention of water after the winter. Warming can help on such soils but without ridding the surplus water it may not be very efficient. In fact, raising transplants in pots and cell trays is a more practical way to accomplish early cropping for clay soils.

Pre-warming is not difficult but does require a little forethought:

• Cover as early as possible, as late winter and early spring sun is weak 
• Allow soil moisture to be replenished by winter rains before covering
• Covering from early in the New Year until early spring is ideal for early crops, but for tender later crops, covering can be effective even if delayed until mid-spring

Just a few simple steps are needed to pre-warm soil:

1. Prepare soil by cultivating to make a seedbed, adding fertiliser if required at the same time
2. Cover with clear plastic film or with cloches. Fleece is less good but has some effect. Black polythene is also fairly effective, provided it is stretch taut and in close contact with underlying soil, and is best when covered by another layer of clear polythene 
3. Clay soil must lose water by evaporation before it will warm significantly and for this reason cloches are preferred for pre-warming clay soils
4. Leave covered until sowing, ideally for six weeks if covered in winter, but four weeks is sufficient if covered later
5. Remove cover and eliminate weeds
6. Sow or plant and, ideally, cover with fleece or cloches

Plastic films and cloches can blow around in the wind if not securely fitted, negating any warming effect. To avoid this, weight the covers around the edge with bricks or tuck the edge of the film into the soil with a spade. You will need to allow for this extra material when cutting the film to size.

Glass cloches can be heavy and easily broken so care is needed when using them.