My wife's becoming more and more concerned about the electricity bill now that I've been keeping the warming light on for the cacti. She'd like it off for the better part of the day. Will they be okay with heat for only a couple hours a day and the rest of the time be in the high sixties?
Some truths in sci fi:
Han shot first
Even numbered Star Trek movies are better
HAL dosed Bowman
If it's cool, those guys will just grow a tad more slowly. Just watch out for rot, maybe ease up on the water a tad. By the time spring temps creep up higher you can start to push them again.