As the sun sets on summer, it’s time to start thinking about dividing your perennials.

The end of July through September is the best time to divide most spring and summer blooming perennials, like irises and peonies, because it gives them a chance to develop good root systems before cold weather sets in. Given proper care, these plants should bloom again next season, but some will take two years to bloom well.

The best method for dividing is to use a sharp garden spade to dig a few inches out from the outermost stems and lift the clump out of the ground. Use a large knife, or two digging forks, to cut the clumps into sections.

How often you divide the plant depends on the type of flower. Peonies, for instance, don’t need to be divided on a regular basis. If they are flowering well, leave them alone, unless you’re looking to move the plant or increase the number of plants.

Bearded iris and perennial asters should be divided every three to four years. When dividing clumps, only keep the young, healthy rhizomes and discard the older ones in the center of the clump. When re-planting, spread the roots out to make sure the rhizomes are slightly above the ground, and don’t cover completely with soil. Water the area well after planting to promote growth.

Hostas, bleeding hearts, and coral bells may also be propagated at this time. Cut clumps into two to four clumps, depending on size, and replant at the same with ample compost or peat moss added.

Fall flowering perennials should be divided in spring.  These include sedums, goldenrods, and asters. Ferns are also best divided in the spring, when the young fiddleheads just begin to push out of the ground, as are bulbs. Divide bulbs after flowering, when foliage is almost completely yellow.

For more gardening tips, a monthly calendar of gardening tasks, and other helpful advice, visit the Kemper Center’s Gardening Help page.

Catherine Martin
Public Information Officer