With its verdant green, sword-like leaves and purple-blue spherical flowers, the agapanthus (Agapanthus spp.) stands out in garden plantings. Also known as lily-of-the-Nile, this clumping perennial grows from underground rhizomes, or swollen fleshy stems. Depending upon the variety, agapanthus grows in U.S. Department of Agriculture plant hardiness zones 6 through 10. Every three to six years, an agapanthus may become crowded and require thinning by division to reinvigorate blooming. This process is most successful during the spring or fall, in the morning on a cloudy day.
Water the agapanthus in the morning, one day prior to dividing it. Apply 1 inch of water from a garden hose to the ground surrounding the plant.
Prepare a new planting site before digging up the agapanthus, if you wish to plant the divisions. Pull weeds and remove rocks from a site that receives full to partial sunlight and contains fast-draining soil. Spread a 4-inch layer of compost of the area with a rake. Mix the compost into the top 10 inches of soil with a rototiller or garden fork. Smooth the soil's surface with the rake.
Dig in a circle around the agapanthus with a shovel or garden fork, at least 8 inches away from its base. Dig down to a depth of 12 inches. Push the shovel or fork horizontally underneath the plant to separate it from the ground.
Lift the agapanthus from the ground using the shovel or fork. Brush the soil from around its rhizomes. Cut the rhizomes into sections with a knife to divide the plant. Each section should have four to five white, healthy roots and at least one eye, or bud, from which foliage grows.
Place one section back in the hole where the agapanthus was previously growing. Add or remove soil to the hole's bottom to position the pointed top of the rhizome 3 inches below ground level, or at the depth that it was previously growing. Fill the hole with soil, tamping it firmly around the rhizome. Water the area thoroughly.
Dig one hole in the new planting site with the shovel for each section you want to replant. Make each hole twice as wide and three inches deeper than the height of the section's rhizome. Space individual holes at least 1 to 2 1/2 feet apart.
Place the section in the center of the hole with its roots facing downward and its pointed top facing upwards, 3 inches below ground level. Backfill the hole in the same manner as before. Water the planted agapanthus thoroughly. Repeat this process to plant each section.