What to do when a perennial gets too big for its bed
In a thriving perennial garden, whether it's mostly in sun or shade, older specimens will eventually outgrow their allotted space and send out fewer flowers. That's when it's time to divide the plants. Done properly, division multiplies your botanical assets—you get two or three plants where you only had one—and stimulates growth, which pays off in a stronger plant and more blooms next year.
Timing is everything with this technique: Late-flowering perennials are best divided in spring; wait until fall for early flowerers, but well before the thermometer dips too low. You want the roots to have time to recover before the ground freezes. Note: This method works best on perennials with fibrous or fleshy roots, such as hostas, phlox, asters, and rudbeckia. Plants with tuberous roots, like peonies, or rhizomes, like bearded iris, should be cut apart with a knife.
1. Dig it out. Use a spade to cut around the plant and lift it out of the ground.
2. Divide. Insert two garden forks back to back in the middle of the root ball. Pry apart the root ball by pushing the fork handles apart. Divide the pieces again, if necessary, but leave at least 6 shoots on each division. Prune the dead or woody shots.
3. Replant. Set each new plant in its hole so that the crown (where the stems emerge) remains just above the soil level. Fill the soil in layers around the roots, then water thoroughly.