Garden lilies (Lilium spp.) grow from a bulbous root structure, which stores nutrients for these large and fragrant summer flowers. The bulbs are perennial in U.S. Department of Agriculture plant hardiness zones 4 through 9. Digging and separating the bulbs every few years increases flowering and provides a supply of new plants. Lily bulbs are toxic, so keep them away from children and pets if you don't get them planted immediately.
Dividing lilies every three years prevents overcrowding while giving the plants time to form new, viable bulbs. You can divide them sooner if the plants become crowded and begin flowering poorly. The bulbs must replenish their nutrient stores during the summer, so wait until early fall to dig and divide. Replanting the bulbs about six weeks before the first expected frost allows the bulbs time to re-establish before they go fully dormant in winter.
Lily bulbs usually sit between 4 and 6 inches beneath the soil surface. Loosening the soil with a handheld garden fork around the exterior of the lily clump allows you to lift the bulbs without damaging them. You can rinse the excess soil from the bulbs with a gentle stream of water, but avoid breaking the delicate scales that cover the bulbs. Dispose of any bulbs that are cut during digging or that show other signs of damage such as soft, rotten spots.
Larger lily bulbs will bloom the following summer, but the smaller bulbs may take two years before they produce their first flowers. Each lily bulb is formed of scales, which are actually separate bulbs. Carefully separating the larger scales results in more lilies for the garden. You can leave the smallest scales attached to the main bulb to grow larger. Break apart the scales carefully because a damaged or snapped scale won't grow into a new plant. Only save the healthy, whole bulb scales for replanting.
A well-drained garden bed that receives all-day sun usually provides an excellent location for your lilies. Before replanting, amend the soil with a 2-inch layer of compost. The compost aids drainage and provides some nutrients. Lily bulbs require planting with their pointed top end sitting about 4 inches beneath the soil surface, but you can plant smaller bulbs as shallow as 1 to 2 inches deep. Clustering bulbs together in groups of five, and spacing the clusters 36 inches apart, prevents overcrowding. Space the bulbs in the cluster about 12 inches apart, and mix small bulbs with larger ones to ensure even blooming. Water the lilies immediately after planting, supplying enough water to moisten the soil to a 6-inch depth, and cover the soil with a 2-inch mulch layer to protect the bulbs over the winter.