This undated photo shows a clump of irises being divided in New Paltz, N.Y. Overgrown clumping perennials such as iris can be rejuvenated and brought down to size by dividing the clumps into smaller ...

As clump-forming perennials mature, they often become overcrowded, competing among themselves for light, water and nutrients. This typically results in fewer blooms, said David Trinklein, state ...