Q: When is the best time to divide overgrown perennials and make new plants from them? —Maria-Teresa Turner, Leesburg, VA.
Roger Cook replies: Dividing perennials every three to six years is a great way to thin clump-forming varieties, like the daylily shown here (Hemerocallis), which blooms from late spring to late summer. This technique can also be used to control plant size, invigorate growth, and multiply the number of specimens in a garden.
A good rule of thumb is to split apart spring- and summer-blooming perennials in late summer or before the fall frost. Fall bloomers are best divided in the spring so that they can devote their energy to growing roots and leaves.
Before dividing, water the mother plant well for a day or two before you dig it up, and wait for a cloudy day to do the actual digging—hot, sunny weather stresses plants. Then follow these steps and you'll be rewarded with new, more vigorous plants to share with friends or add to your garden.
Pictured: Landscaper Randy Leland of Groundworks Landscaping cradles the root ball of a daylily that's ready to be divided.
Dig up the plant
Rake back any mulch covering the base of the stems, then set a spade or shovel 6 to 12 inches from the center of the plant and push it down vertically into the soil. Work the shovel this way around the plant until you've formed a circle. Now slip the shovel blade under the root ball and pry it out of the ground.
Separate the stems
After plucking out any thin or weak stems, gently separate the remaining healthy ones into clumps of three to five shoots. That number helps ensure that the plant will recover quickly after being divided. If the center looks dead compared with the outside edges, cut it away with a soil knife and leave the outside edges.
Make the cut
For smaller plants with six to 10 stems, place a soil knife between the separated stems and saw straight down through the thick, fleshy roots that form the crown. Continue cutting all the way through the root ball. For larger, heavier root balls, use two garden forks placed back-to-back to pierce the center of the crown. Push the forks apart until the clumps separate.
Tease and plant
Gently tease the roots out of the bottom and sides of each new clump with your fingers. Now dig the holes; they should be 1 to 2 inches shallower and 6 to 9 inches wider than the teased root ball. Plant each root ball, and backfill with soil mixed with a root-promoting fertilizer, watering as you go. Water every two or three days for the next couple of weeks to get them established.