I can only speak from my own experience on this. The textbooks may disagree. If you really need to divide perennials during a hot spell, try to do it in the morning hours. Be sure to soak the plant to make sure the roots are moist. Cut back as much foliage as you’re comfortable with. Now you’re ready to dig it up. After you’re dug it up, divided and potted your plant, be sure to water well. Keep the plant out of full sun for about a week. The plant is already stressed and doesn’t need the full sun to add to it. Would love to hear your questions/comments.
Now don’t get me wrong. I think a lush, green lawn is beautiful. But, I am so tired of pulling grass out of my flower beds! Sick of it, I tell you! I now have sections of flower beds that need to be dug out completely and redone. Grass would be fine if it would stay where it belongs, but it never does. I could definitely go on about this subject, but I can’t spend a lot of time writing when there’s grass to be pulled.