August 16th - 22ndWhatever else they may sound like, "green manures" are crops that are grown solely to be dug back into the soil to improve its structure and to add to its fertility. Use them where you have empty patches of earth after uprooting vegetable crops, or where you have done heroic weed clearance and you're not sure what to plant next.
Garden centres stock a variety of green manures costing from around £1.50 to £2.50, and depending on the particular plant or plant-mix, a packet of seed will cover from two to 60 square metres.
Some crops, such as mustard and buckwheat, are suitable for short-term green manuring and should be dug back into the soil between four and 10 weeks after sowing. Others, and these are leguminous plants (of the pea family), should be left in the ground for some months, during which time they form beneficial, nitrogen-rich nodules on their roots. Sow the following now, and over-winter them if you like: trefoil, winter tares, winter field beans (for heavy soils) and crimson clover (for light soils).
Crops which form thick stems will take longer to break down after they are dug into the soil, so it is advisable to wait some time before sowing or putting in new plants. If you need the ground immediately, cut the crop down at soil level and compost it.