Till in or cut cover crops before the seed heads mature. If you till in the whole plants, allow 2-4 weeks for them to decompose, as raw biomass ties up soil nutrients to the detriment of newly planted seedlings.
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Cover crops provide the primary benefit of preparing your soil for further vegetable cropping.  If you choose to allow your cover crops to go to seed so you can harvest the grain, be aware that their root mass can be extensive and difficult to turn over. That said, your own oats, rye or buckwheat straight from your own garden are really a treat and can aid the determined 100 Mile dieter.