Cabbage, broccoli, lettuce, and collard plants like cooler soil than I have now, but the literature suggests setting them out now for a fall crop.
How Do You Shade Cool Weather Plants Set Out Now?
Certified GKH Gardening Expert
Really, the trick is to keep their roots cool. Hot roots are what cause cool weather plants to bolt or grow poorly in hot weather.
A heavy mulch around them along with making sure they are well watered will help keep their roots cool until the weather cools down.