If you’ve never divided a plant, the thought of digging one up and chopping the rootball in half may be a scary prospect. But with astilbe (Astilbe spp.), division is the best way to both invigorate and propagate it, instantly doubling your enjoyment of this perennial beauty. Astilbe thrives in shady, moist sites and does best in U.S. Department of Agriculture plant hardiness zones 3 through 9. Although different species grow to varying heights, the process of dividing them is the same. The best times to divide astilbe are in early spring before it comes out of dormancy or after it finishes flowering in the summer.
As astilbe grows, its roots spread and form large clumps. After two or three years you may notice that the center dies back or the plant isn’t flowering as it should. These are both signals that the plant needs to be divided. Once its roots are separated and the divisions replanted, the astilbe will take off growing as if it were a new plant. As a general rule, divide the astilbe every three to four years.
A day or two before digging up the astilbe, determine where you’ll plant the division and water the area until the top 5 to 6 inches of soil is moist. Water the astilbe deeply as well. Disinfect the shovel, pruning shears and a pruning saw by soaking them for five minutes in a bucket containing a solution of household bleach and water, using 1 part of household bleach to 3 parts of water. Rinse the tools in clear water and allow them to air dry.
Dig up Plant
At the plant’s dripline, insert the tip of a shovel's head pointed toward the astilbe as deeply as possible into the soil. Remove the shovel and insert it again, immediately next to the first spot. Continue this procedure all the way around the plant and use the shovel to carefully pry the astilbe from the ground, gently shaking soil from the roots.
You will need sharp pruning shears and a pruning saw to divide the root clump. Use the pruners to cut back all the stems to about 3 inches from the rootball. Once all the foliage is gone, cut down the middle of the clump with a pruning saw or a sharp knife. You may need to tear the rootball apart to separate it.
Dig the planting hole to the depth at which the astilbe was previously growing and twice the diameter of the rootball. Scrape the inside walls of the hole with a garden fork. This loosens the soil so roots won’t have to struggle to penetrate the walls of the planting hole as they spread.
Set the rootball in the hole and backfill with the removed soil. Tamp the soil around the base of the astilbe with your hands. Water the astilbe slowly, until the soil is damp to the bottom of the root ball.