I can't answer your main question.

But if you do put black plastic, make sure it is good and damp before you cover it. That will help all the weed seeds germinate.

--McKenzie

The idea of cover crop. I think. is that it will provide green top that will be tilled/ turned over to make green manure and amend the soil. That is why you need to plat it at the right timing. I have not done it for fall but I know some gardener who plant rye or wheat in the fall and tilled in the spring , before planting out.

Buckwheat will grow now. When it flowers it is very attrative to many types of bee and wasp. But you probably don't want to let it set seed. It is quite easy to till in also.

so if i grow buckheat for a month and 1/2 and till it in - it will still have time to do something good for the soil?

Or perhaps a layer of compost & leaves and then plastic?

mckenziek - great info on the black plastic! i have lots of yard i'm going to be planting so this is good to know!

Why not cover the area with cabbage? It will close canopy and limit weeds, and in November you can harvest the whole plants and keep them in bins in the unheated garage for the winter, or until tey are all eaten.

I plant oats in mid Sept. for a cover because the seed is very cheap for me, it establishes quickly, it doesn't mind the heat at all and by or before killing frosts it has at least grown to the point I want it to (not quite set seed). For you, I'd broadcast and incorporate the seed heavily now to get a dense, young stand in weeks time, it may or may not get you what you want but give it a try and find out. It took me a few years to get the oat cover system worked out well for my system of planting and growing to full cover with minimal intervention, cost and effort.

happy planting.

I think the best thing you can do for your soil is to bring in as many earthworms as possible. A good compost will do this and veggie peels from the kitchen. These critters are natural tillers and will aerate the soil and keep it loose and fertile; everything a plant needs to be happy.

Another cover crop is FENUGREEK. It is edible with many benefits, its root is a nitrogen fixer and its gree will be also rich in nitrogen. You can buy the seeds real cheap from Indo-Pakistani grocery store, by the pound, half a pound ..
You can plant it as a companion plant too.

I use buckwheat as a short-term summer cover all the time. In late July, it doubles as a nurse crop, because I pull out square-foot holes and plant fall seedlings amidst the shelter of the buckwheat. Buckwheat plants pull out really easy, so instead of turning them under I gather them and compost them.