The heat does not cause them to bolt. Broccoli is supposed to bolt, it is the seed heads that most of us want. At high temperature the large leaves lose water rapidly and wilt to the ground. The plant can't do much growing at that point. Yes I do sheet mulch and it helps. Seldom shows in photos here is a row of cauliflower which illustrates the way I do it. I use yard waste ( leaves and grass clippings).
