Are they in the ground or in pots? Are there any other symptoms on the leaves or stems, such as spotting, streaking, or malformed, cabbage like leaves at the tips? If they are in pots, how long have they been in the same pots? Have you been feeding them, and if so, what with?

If they are in pots they need repotting into larger ones. Also, they may need more water than you think in hot weather. I'd repot, trim, water and feed! That should keep you busy!