Fall is the best time of year for division of spring and summer blooming perennials – they’re finished putting energy into blooming, 
and new divisions will re-establish quickly during the cooler, wetter fall weather. Dividing helps control the size of plants, rejuvenates them, and increases the number of plants for your garden.
It’s easy to divide perennials, just follow the simple steps outlined below.
For fall blooming perennials, it’s generally better to wait until spring to divide. By dividing plants when they’re not blooming, they can put all their energy into root and leaf growth.
What Plants to Divide
Most perennials should be divided every three to five years. It’s best to divide plants when they still look good, but they should definitely be divided when the center of the plant has smaller leaves, fewer flowers, or weaker blooming stems than the outer portion of the plant – these are all signs of overcrowded roots.
Some common varieties of perennials to consider for fall divisions are astilbe, asiatic lily, beebalm, bellflower, black-eyed susan, bleeding heart, cone-flower, coreopsis, cranesbill geranium, daylily, hosta, Japanese iris, lambs-ear, lily-of-the-valley, oriental lily, peony, phlox, siberian iris, veronica, and yarrow.
There are also a few perennials that it’s best not to divide; these are alyssum, candytuft, carnation, delphinium, euphorbia, foxglove, garden sage, lavender, rose campion, rosemary, russian sage, sea holly, silvermound, sweet pea, and trillium.
When to Divide
Plants should be divided in the cooler fall whether, but they should be given about 6 weeks in their new location before the first hard freeze occurs in your garden. This gives them time to settle in and helps to ensure they’ll survive the winter.
Preparing for Divisions
Plants that will be divided should be watered a day or two before you plan to divide them, and the stems/foliage should be trimmed to about 6 inches from the ground to minimize moisture loss.
Prepare the area where the new divisions will go before lifting the parent plant by removing the soil (at least 8 inches deep), removing debris (rocks, roots, etc.), and adding compost.
Digging the Parent Plant
Dig all around the parent plant at the drip line with a sharp shovel, and then lift the plant, soil and all, from the hole.
Separate the roots into smaller divisions by cutting with a shovel or sharp blade.
Replanting the Divisions
Place a small mound of soil in the middle of the prepared hole and spread the roots over the soil so that the crown sits at the soil line. Fill the hole back in with soil and firm around the plant crown.
Water the divisions in and keep them moist until a hard freeze. Once the ground freezes, it’s a good idea to apply a layer of mulch – 3 inches deep or so is usually good. Next spring the plants should emerge with renewed energy and vigor.
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