Lily of the valleys have fleshy root structures called rhizomes. Because lily of the valleys self-propagate, about every 3 to 5 years, they will become overcrowded and bloom less than previous years. This is a sign that it is time to dig up the rhizomes, separate them and replant them again in new locations. Lily of the valleys should be separated--or divided--and replanted in the fall after the foliage has died back or in early spring before new growth begins.
Dig up your lily of the valley rhizomes, which should be planted just beneath the surface of the soil. Use a garden fork, spade or shovel and dig straight down around the perimeter of your plant, a couple inches from the stems. Then push down on the handle in several places to lift the clump of rhizomes out of the soil.
Shake off the excess dirt. You can even wash them, if necessary.
Separate rhizomes with your hands. Some will come off easily. If not, you will need to get a clean knife and carefully cut the rhizomes apart. Each piece should have a least one eye, also called bud.
Replant the rhizomes in full sun or partial shade, about 4 inches apart. Water well. The eyes should be facing up and just below the soil's surface. Consider planting smaller rhizomes in a separate area of your garden because they may not bloom for a couple of years. You can also mix in little rhizomes with larger ones, if desired.