Hey Kevin,
I'm asking the same question! Colemans book says to plant first planting end of July and 2nd planting the first of August( this is in Maine and Zone 5 with ave high temp in july of 78)...In my area of SE Missouri we are still hitting the high 90s and probably continue till end of August. I'm going to try it the first of Sept in hoop house and see what happens..according to the book they must be up and growing before the "Persephone Days" (days that have less than 10 hours of daylight)because all plants slow way down and don't grow until the day lengthens again...It's not temp but day length they respond to. Now I'm trying to find out just when the Persephone days are in my area.
Lextra