Answer: Green manures are excellent for improving the garden soil, and there are a number of types available. Buckwheat, in my experience, is one of the easiest for home gardeners to use because it grows quickly and is relatively easy to turn under when the time comes. However, it is not frost hardy so late November would be too late to try to grow it for this year. Although quite hardy, winter wheat would also need to be planted earlier, probably in August, in order to be well established before winter. At this late season, it is even risky to try winter rye as a cover crop. For future reference, all of these are available through a number of catalogs such as Johnny's Selected Seeds (207) 437-4301 (http://www.johnnyseeds.com).
Since you would like to try a cover crop over the winter, I think your best bet this late in the season might be ordinary annual rye which should still be available in the lawn grass seed section at the garden center. Should this fail due to falling temperatures, another soil building alternative would be to dig in organic matter such as fallen leaves and possibly straw so that they may decompose over the winter.
Good luck with your soil building -- it's the best thing you can do for your garden!
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