Really, I think I'd get them all planted sometime in August -- the bigger they get before the weather kills them off in probably mid-November (whenever the lows fall into the mid-teens for the first time), the more green manure biomass you'll have. Probably anytime from about August 15th to around Labor Day would work for all of the above, giving them plenty of growth time, but not so much time that they're already done and rotted down by time of freeze up. The goal is to have a LOT of plant matter to keep down the weeds, then have it freeze off and make a self-mulching bed until spring, when you can mow and then till it in.

I don't know how you would sow the peas over/through the oaks without a lot of hand work -- you could manually plant seeds through the oats, but that would be various laborious. If you scattered over the top and keep it very wet, they MIGHT root in and grow, but I suspect birds and animals would eat most of them. I'd just mix them together and sow at the same time if it were me.

The peas are a good idea because they fix nitrogen. You might want to mix a legume with the radishes, too, for the same reason. Other good ones that won't survive the winter in your climate are annual alfalfa and crimson clover, both widely available. Favas are good, too, but expensive, because the seed is very large, would take a lot of fava seeds, probably at least six or eight pounds, to do a green manure on a 1/4 acre of ground.

Denninmi, thanks for the quick response. My one concern about sowing the peas and radish is that i've heard they prefer cooler germination temps and August can be real hot and humid here. Also, how tall would they get 2-3 months? Thanks for suggesting the crimson clover with the radish, i'll see if i can still get seed for that.