Answer: Most hostas will grow to a mature size and then stop expanding and so can be left in place almost forever without needing to be divided. They are very low maintenance plants in this regard. The bluer hostas tend to be among the slowest growers and some varieties are actually very large plants at maturity, this is part of their appeal.
If you would like to make more hostas from the ones you have, you can divide them. It is best done in very early spring before they leaf out or in early fall so as to take advantage of the cooler and moister growing conditions.
On the other hand, hostas are very resiliant plants and can be divided virtually any time the ground can be dug. Water the plant the day before you want to do the job. Then lift the clump and separate it, by cutting straight up and down with a sharp spade or using a large knife, keeping sections of root and corresponding top portions intact. A six inch wide division will make a nice sized plant in one or two seasons, but you can make your divisions as large or small as desired.
Replant immediately into prepared soil, setting the plant at the same depth as it grew originally. Water well, mulch to help keep the soil moist, and keep the soil evenly moist but not sopping wet until the plants become reestablished, usually a full season. Ideally the transplanting would be done just before a rainy spell to help the plants recover faster; I would not recommend moving them in the heat of the summer or during a drought.
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