The best time to transplant hostas is in the spring when the soil is moist and frequent rainfall helps newly transplanted hostas thrive. It is more difficult to transplant hostas into hard, dry soil during the heat of summer.
Regardless of when they are transplanted, hostas do best when they receive plenty of water. Since transplanting is somewhat traumatic for the plants, no matter when they are transplanted, this need for water is especially important as they adapt to their new locations. For this reason, transplanting hostas in the springtime is typically easiest for the gardener. However, it is still important to make sure that hostas get a regular supply of water if insufficient rainfall occurs.