When do you cut back hostas?

In a warm temperate climate you would do it in winter.

Another answer:

If a plant is in bloom, aka the mating season for plants, wait for the last bloom to fall, then add two weeks. I would wait till fall to transplant, that way the plant has recovered from its sexual cycle and had time to grow. soak the root/dirt area for at least 24 hours, then separate and replant into ground or pots. This is recommended for Daylilies, iris's, and other tuberous plants. As stated above, most plants are dormant in the winter and can be dug up and the bulbs separated and stored in a cool dry place till spring.

Why you people stop giving questioners bum steers the only time to transplant Hostas is in the Spring as they start to sprout. Split them up and produce more plants. If you don't want more plants leave them alone.