Buckwheat

It grew quite vigorously in poor soil and provided the green manure we hope will help our crop of onions this spring.

It is still winter here.

I used this because I wanted something that would self-terminate. The tricky part is sowing it late enough so it doesn't go to seed before the first killing frost. I sowed it too early in the fall, so I had to mow it down because it was going to seed. I try it again next fall and hopefully, I'll time it better.