The best time of year to divide hostas is in late summer (August or early September). But, don't worry if you forget: You can divide hostas anytime from spring to fall.
Here's a hint: If you divide your hostas in summer, be sure to keep them well watered for a few weeks to help them get through the shock of being transplanted. And make sure you allow at least three or four weeks for the hostas to become established before the soil freezes solid if you divide them in fall.
You'll know your hostas have to be divided when they get too crowded and the center of a clump starts to die out. As a general rule, count on dividing the plants every three to four years to keep them at their healthiest. Some slower-growing varieties may need longer before they're ready for division. And you may be able to divide faster-growing varieties every two or three years.
If your hostas aren't too large, dig out the entire clump. One way to do this is to dig around the clump in a circle, then use your shovel like a lever to lift the clump out of the ground. Once it's out of the ground, you should notice that the clump is made up of many individual plants. (If there's still a lot of soil around the plant, wash it off so you can see the hosta crowns.) Carefully break apart the clumps into divisions made up of at least three sets of shoots coming out of a crown.
If your hostas are too large to lift out of the ground, use your shovel to cut the clump into divisions. Carefully dig out the sections and replant them.
Here's a hint: Many gardeners find that it's easiest to divide hostas using a garden fork or flat spade.
Plant your hosta divisions in the ground at the same depth they were growing before. Water them well.