Mine are under 6 mil plastic cover.. One near the top had begun to wilt a little.. Been hot as hell here in the PNW.. Near 100 everyday the last couple of weeks.. I placed a fan on them and within an hour they had totally recovered.. I think circulation is an important factor to consider. Hot, still air is bad news.. Just my opinion for what it's worth.. I think they can withstand excessive heat as long as they are getting good air movement.. Oh BTW my girls began flowering!! I'm so excited..