Contrary to popular opinions, hostas do not need to be divided, unless the plant outgrows its space if you want to multiply your hostas, then dividing them will give you more plants for no cost.
Sometimes after being in the same location for many years, some Hostas may die out in the center. If this happens, dig up the plant, remove the actively growing plants and discard the dead areas.
When dividing a hosta plant, the entire plant should be dug up. Remove as much soil as possible from the roots by either shaking or washing off the roots. Exposed roots make it is easier to determine each individual plant. The clump may be twisted, tugged, and pulled or forked apart, separating the individual plants.
If the plant is tightly congested, use a sharp knife to cut each crown apart. Carefully pull the roots apart so as not to break them off. The crown is the portion of the plant where the leaves join the roots, and there may be many on a large clump. The separation of each individual crown with roots will result in one new plant. If larger clumps are desired, then several crowns may be left together in a clump. There may be damage to some crowns, but the majority will develop into a larger plant. It may not always be necessary to dig the entire clump from the ground.
Hostas can be divided any time the soil can be worked. The important thing to remember is after replanting to supply enough water to keep the plant actively growing. Early spring divisions can be done before leaves emerge. The plant may take the rest of the season to recover from the shock of dividing. Late summer/early fall, damage still may occur to mature foliage, but the plant will have time to reestablish before winter sets in, and will emerge as though undisturbed in the spring. Also if dividing after the foliage matures, existing leaves may flop over and wilt, but new leaves will emerge from the crown. If leaves flop open, just cut off the old leaves, which makes the plant look better.