Whilst the use of polytunnels and glasshouses can help extend the six months, or so, of growing season we get in the UK, crops grown in open ground often have to make the best of whatever weather comes along. The relatively simple process of prewarming the soil before sowing or planting can provide an earlier start, especially for those growing on heavy soils.
Begin by cultivating the soil to seed-bed quality and excavate a small, shallow trench around the area. Cover the area with clear polythene and bury the edges in the trench. The ‘greenhouse effect’ will now occur with solar radiation being absorbed by the soil and the radiation of that energy back into the atmosphere being restricted by the polythene. The physical barrier to rainfall, provided by the polythene, will also keep the soil dryer and easier to work prior to sowing and planting.
Use black or clear plastic sheets to warm the soil for earlier crops