Agapanthus, also known as Lily-of-the-Nile, is a flowering perennial that grows up from a fleshy underground rhizome. The plants are beloved for their tall blooms on upright, tensile stalks and their green, strap-like foliage that has a fountain form. According to the University of Wisconsin, agapanthus can be divided when the plants become crowded, roughly every 4 to 5 years, and dividing is best done after flowering in the late summer or fall.
Excavate the soil around your agapanthus root mass. Lift the root mass out of the soil and set on a stable surface like the ground or a work bench.
Cleave the root mass into several separate clumps of root mass and some top growth, if possible. The blade of a spade or a sharp garden knife with a long blade are ideal tools. Cut cleanly through the root mass and separate the divisions.
Plant the divisions immediately into moist, nutrient-rich soil at the same depth that they were previously growing. Water the plantings well until the surrounding soil is drenched. Allow at year for the division to produce bloom, though some will not take that long to recover.