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