By dividing the plant when it is not flowering, all the plant's energy can go to root and leaf growth. Fall division should take place between early September in the uppermost Piedmont and mid- to late October on the coast. Allow at least four to six weeks before the ground freezes for the plants to become established.
Transplant three to four weeks before the ground freezes. Allow time for the roots to establish and not be heaved out during winters freezing and thawing weather. “Also allow the plants to become established before fertilizing. If they have been moved in the fall, fertilize the next spring.
September is the best month to divide and transplant perennials, but you can still be successful in early October.
Divide fall blooming perennials in the spring because
Plants have stored up energy in their roots that will aid in their recovery. Rain showers that generally come along with the early season are helpful. Plants divided in spring have the entire growing season to recover before winter.
At a slight angle, trim the plant nearly all the way down to the ground, leaving about a 1-to 2-inch stub above ground. Spray your pruners with alcohol or Lysol in between cuts to sterilize them.
Also, do not cut back hardy perennials like garden mums (Chrysanthemum spp.), anise hyssop (Agastache foeniculum), red-hot poker (Kniphofia uvaria), and Montauk daisy (Nipponanthemum nipponicum). Leave the foliage. It's important to protect the root crowns over winter.
Cutting old and diseased foliage in the fall helps a perennial jump right into new growth come spring. But some plants need their foliage for protection over the winter and instead should be pruned in the spring. Know your plant's particular care requirements for the best results.
Don't Divide These Perennials
Some plants resent being divided and it should be avoided if possible. These include butterfly weed (Asclepias), euphorbias, oriental poppies, baby's breath (Gypsophila), gas plant (Dictamnus albus), Japanese anemones, false indigo (Baptisia) and columbines (Aquilegia).
Dig up the plant you're splitting with a spade or fork on a cool, cloudy day. Lift the plant to remove any loose soil and to tease the roots apart. And separate the plant into smaller divisions, making sure each piece has growing shoots and roots. For perennials with finer root systems are among the easiest to divide.
You can transplant perennials anytime until the ground freezes in the fall, or wait to transplant them in the spring. Fall is an excellent time to transplant herbaceous perennials because your plants will then have three seasons to establish a good root system before hot summer weather sets in next year.
If your perennial blooms in the spring or early summer, it should be transplanted in the fall. If your perennial blooms in the late summer or early fall, transplant it in the spring. Remember that when transplanting in the fall, the perennial needs to be in its new location about six weeks before the first hard freeze.
Lupine. In areas with cool summers, lupines are prized perennials that attract butterflies and hummingbirds. The secret to beautiful lupines is rich, slightly acidic soil that drains well. This deep, tap-rooted perennial doesn't respond well to division or disturbance, so let plants set seeds instead.
Cutting back perennials in the fall is not only a great way to keep flowerbeds looking neat and tidy through winter, but it also aids greatly in keeping plants vibrant and blooming the following year. Removing spent foliage and blooms helps a plant focus on recharging it's roots and resources.
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.
While most perennials benefit from being divided every few years, there are a few that thrive better if they're kept intact. Avoid dividing these varieties: Baptisia. Bleeding heart (Dicentra)
It's common to think that everything should be chopped down to the ground in the fall, but some perennials actually need their foliage to protect new shoots through the winter. Other varieties offer up important habitat for local wildlife and some perennials provide height and interest through the winter months.