I planted winter rye in my soon-to-be-a-garden plot here in Western Mass; I bought 2 lbs of seed at my local gardening center. I have cardboard down over the whole plot, and a layer of dirt mulch over about 1/3 of the cardboard. The rye grew very well, and helped the cardboard break down pretty fast. My hens, and the wild turkeys that pass through the yard, liked it very much until the snow covered it up. If it grows tall in the spring, I'll just cut it and toss it in the chicken run. No nitrogen fixation, but at least it helped with the cardboard and should add some organic matter to the soil.