Fall is a good time to dig, divide and (if necessary) move garden perennials, since the high temperatures of summer associated with water stress have subsided. The cooler weather of September and October causes renewed root growth and, in some cases, top growth. Both better enable perennials to survive the ensuing winter. Don't wait too long, however, since plants will need about six to eight weeks after rejuvenation to reestablish themselves before cold weather arrives.
Some perennials, such as peonies, rarely need to be replanted as long as they are flowering well. Failure to flower is more often the result of excessive shade, improper planting depth or poor growing conditions. Healthy, free-flowering peonies which have not been disturbed for over 20 years can often be found in gardens. However, if flowering has been poor due to inadequate light, now is the time to move them. Select a location that receives at least eight hours of direct sunlight and has soil that is well-drained. Dig the fleshy peony roots carefully and reset them one or two inches below the soil's surface. Avoid planting any deeper, since this can cause poor flowering.
Although iris normally are reworked earlier in the fall, it is still timely to replant them. However, later transplanting might result in reduced flowering next spring. This also is a good time to remove perennial weeds and grasses that may have invaded the iris bed. In fall, there is less chance for diseases to invade roots or rhizomes that might have been injured accidently during the weeding process. Iris in the center of a clump become crowed and often fail to bloom. When this occurs, the clump needs to be divided. The sections around the outer perimeter of the clump have the most vigor and will redevelop most rapidly when divided and replanted.
Most daylily cultivars need to be replanted about every five years, although some can be left in one location longer. An exception would be reblooming daylilies such as 'Stella 'd Oro'. For maximum bloom production, clumps need to be divided more frequently to keep them young and actively growing. As with iris, if daylily plants are healthy and free-flowering replanting may not be necessary. Clumps can be divided into numerous sections before replanting. Larger plant sections (containing several fans) are better able to endure the rigors of winter. It is best to replant evergreen or semi-evergreen cultivars in summer, after the primary bloom period has ended.
While peonies, iris and daylilies are the "rock stars" among perennials, there are many others that can be dug, divided and replanted in the fall. Among them are astilbe, bleeding heart, butterfly weed, cone flower, hosta, perennial aster, Shasta daisy and yarrow.
When transplanting perennials in the fall, cut back top growth to within about six inches of the soil's surface. Making sure the soil is moist before digging, lift the clumps carefully. Try to move as much of the root system with the clump as possible. If division of the clump is warranted, do so with root preservation in mind. Species with spreading root systems most likely can be pulled apart by hand. Those having root systems that clump will likely need to be divided using a knife or other cutting instrument.
After being divided or relocated, most perennials should continue root growth in the fall and become established before cold weather arrives. Some species may develop modest amounts of new top growth, depending on how long favorable growing conditions persist into late-fall or early winter. Water newly-divided plants well and continue to water if fall rainfall is not timely.
Nearly all of our popular perennials require well-drained soils. Most perennials that do not survive our winters die because of root problems associated with poorly drained soil, rather than cold temperatures. Incorporating organic matter into the soil is a good way to improve drainage. Peat moss, compost or well-rotted manure represent good sources of organic matter for gardeners. In settings were soil is tight and very poorly drained, raised beds or berms should be considered.
Fall-planted perennials benefit from mulch, at least for the first year. Mulch not only conserves soil moisture, but it helps to prevent alternate freezing and thawing of the soil during late fall and early spring. The latter tends to cause "frost heaving" in perennials with fleshy roots, such as hosta. Mulches do not keep plants warmer. Rather they function to protect them from rapid temperature changes and drying winter winds.
Many people consider perennials to be "care free" plants. Such is most often not the case. Neglected perennials bloom more poorly, become unthrifty and tend to be more prone to develop certain diseases. Properly done, rejuvenation by dividing or relocating the clump can keep perennials young, actively growing and floriferous.
REVISED: February 21, 2017