When to sow
Green manures, especially autumn sown ones, are effective in mopping up nutrients remaining after crops, preventing them being washed away by rain. Italian ryegrass, and rye sown in September are very hardy, growing all winter before being dug in during spring to release nutrients as they rot. Clay soils are best dug in the autumn. Fast growing fodder radish or mustard sown before mid-September can be incorporated in October, or their frosted remains left as a mulch.
Summer grown green manures, buckwheat and fenugreek for example, consume useful space and leave soil very dry. However, they can form dense foliage that is ideal for smothering weeds.
Legumes accumulate nitrogen with help from bacteria in their root nodules. These function best in summer, and although field beans and vetches can be sown in autumn for incorporation in spring, summer crops of lupins, clovers and peas fix more nitrogen.
Other benefits of green manures include winter protection of the soil from compaction by rain and shelter for beneficial insects such as ground beetles. Slug control may be needed after green manuring.
Tagetes minuta is claimed to suppress eelworms and weeds however it supplements, rather than replaces other controls.
Sowing and digging in
Green manure seeds are sown broadcast and raked into the soil. They are dug in when lush and leafy, before flowering. Flowering green manures such as clovers and phacelia benefit helpful insects. But after flowering plants become woody and can temporarily use up soil nitrogen when dug in, so add extra nutrients before sowing or planting. Decaying green manures can suppress plant growth; allow at least two weeks between incorporation and planting or sowing