I think it is pushing it now and I am here in SE Iowa. I planted on July 27th and still have a ways to go before they mature. They may eat the leaves on brassicas if you plant now but I am not sure if the turnips would grow fully. But, DBLTREE is the one to ask but that is what I have noticed when planting them. If you plant to early, like we did 2 years ago, they get TOO mature by fall. I think Late July to Mid August is the time so you might try it? What about rye??