Can hostas be divided after they have already started to grow in the spring, and if not, when can they be dug and divided?

Hostas can be divided almost anytime, but very early spring before they have started to grow and then again in the fall are preferred. The reasoning is to allow the roots time to reestablish before the hot weather sets in. Make sure to water them well the day before you dig them and water them in well afterwards. Keep the soil moist but not soggy until they are reestablished.