Are your perennials looking a bit overgrown?  Are some of your plants making sinister overtures to their neighbors?  It’s not uncommon to notice things like that occurring when gardens gain maturity. I have a dusty miller, for instance, that’s threatening to swallow up a nearby row of coral bells.  Across the yard a monstrous sage is pushing against an equally aggressive costmary. Unless I do something soon, neither one will win out. They’ll both end up looking like has-been prizefighters in this ringside garden of mine. 

OK, so sometimes it’s my fault.  What gardener isn’t guilty of planting things too close on occasion?  It’s the fast-food equivalent of home landscaping - instant gratification gardening.  Other times, though, plants simply don’t act the way we expect.  They live longer or grow larger than the gardening books or plant labels say they will.

So you have to get out the garden spade and divide those perennials into smaller, more manageable sizes.  In addition to solving the problem of overcrowding, some perennials benefit from being divided because they begin to die out in the centers after a period of time. 

Perennials can be divided throughout the year, provided you give them good conditions, including plentiful moisture; but fall is the foremost season for dividing. Cooler temperatures and reliable precipitation are favorable for a good root system to develop before the next bloom season rolls around.

Dividing perennials is a cost-effective way to increase landscape plantings with specimens you already know will grow well in your garden conditions; and it's a good way to make new friends by sharing divisions with neighbors and other gardeners.

First, prepare planting holes that are large enough for division roots to fit into without being crowded. Do this before you dig up the plant that’s being divided. That way the roots won’t be exposed to drying sunlight and air.

The soil at the new location might have to be amended.  If the plant being moved is in heavy, clay soil, and the new location has medium soil, amending isn’t required.  If it’s the other way around, though, amend the soil by mixing in one third organic matter, such as compost, peat moss, or old rotted sawdust to create a transition zone.  Don’t simply dig a hole and fill it with a different type of soil.  Creating zones of dissimilar soil inhibits moisture movement, and it’s a shock to plants when roots that have been happily growing in one type of soil reach an entirely different type.

For good root development and disease resistance, add a fertilizer that’s high in phosphorus and potassium. Use less nitrogen, which only encourages the growth of foliage that will be susceptible to wintertime freezing.

Prune the "mother" plant back to several inches from the ground to minimize water loss through the leaves.  Dig carefully around the plant, leaving as big a soil ball as possible around the roots, and trying not to damage the roots while digging. Then lift the plant gently from the ground. 

If the soil ball is too large to carry, wrap it with burlap to keep the soil ball from breaking up.  Cut back the burlap to expose roots to the surrounding soil when planting.
 
Divide plants by pulling them apart at obvious separation points.  Plants that prove tough to pull apart can be cut with a sharp knife or pried apart with two garden forks back-to-back and pushed away from each other.  A successful division requires both root and crown sections.  Discard woody centers and cut off unhealthy or broken roots.

Spread the roots and plant the divisions at the same depth as previously planted.  Fill the hole with more amended soil, firm it down, and keep it watered until winter precipitation takes over.

A few of the many plants that are especially amenable to dividing in fall are plants that bloom in spring and early summer, such as columbine, peony, daylily, Shasta daisy, thyme, rosemary, and coral bells.

Don't shy away from dividing later bloomers, however. Some that divide easily include alchemilla, anemone, chrysanthemum, coreopsis, erigeron, gaillardia, geum, hosta, monarda, rhubarb, Jerusalem artichokes, rudbeckia, saxifrage, sedum, sempervivum, solidago (goldenrod), spirea, stachys (lamb’s ears) and viola. 
 
Conifers, hardy broadleaf evergreens, and deciduous plants still in leaf
must be moved with a soil ball around the roots to survive transplanting.
Dormant deciduous plants that have lost their leaves can be transplanted bare-root.