I think they will grow and be fine. Often, down here in our somewhat warmer zone, they are planted in the fall, overwintered and tilled under in early spring. But, I think you can plant them as late as May, especially since your're in zone 6.

I'm find and link the Seedland website because it always has great planting info and maps. It is one of my favorite websites for planting info on cover crops and helps me keep straight what to plant in the warm season vs. cool season, and when to plant them. I don't know why I didn't think of their website the other day when you asked about a cover crop/green manure crop.

Dawn

Here is a link that might be useful: Seedland Planting Info