I have seeded it as late as August here in North Texas and have done ok. I've done a lot of pit seeding after grubbing mesquites with a dozer. Timing hasn't been as big an issue as moisture. When it rains, everything works. When it doesn't rain, nothing works. March and April work a lot better here for that very reason.

It really takes 3-5 years before you know what you've got when seeding native grasses. Bluestems more so than switchgrass.