Fall is a great time for transplanting and dividing perennials. The cooler weather for plants to root in. I would recommend doing it in the cooler temperatures and not really do division in the middle of the summer when plants are actively going. We happen to work in a sunny location today, but ideally the best time to do would be on cloudy day when temperature are cooler in the 70's. We're up to the 80's today, which isn't the ideal situation, but for demonstration purposes we're doing it on a sunny day. Basically, you want to keep your roots from drying out if you have to do it on a hot day. We have them in here soaking in the B1 hormone to get as much energy as possible to that root system for when we actually go and plant them.