Most perennials are purchased in 4-inch or 1-gallon containers; mail-order sources often also ship them bare-root.
Soil, water, and fertilizer
In general, these plants prefer soil well amended with organic matter, but a surprising number do well in ordinary or even poor garden soil. Some thrive in full sun; others need some shade, especially in hot-summer climates. Water needs differ, too: some perennials are thirsty, while others succeed with little water. Most perennials appreciate an annual feeding, either in the form of organic amendments worked into the soil in spring or fall or with a complete fertilizer applied in spring. Some, however, are heavy feeders and need regular fertilizing throughout the growing season.
It's also important to find out how wide and high a perennial will ultimately grow. Some reach 7 to 8 feet tall, while others are almost as low as ground covers; and plants that are only a few inches across when brought home from the nursery may eventually form mounds 3 to 4 feet wide. To avoid crowding problems, space plants and choose planting locations with an eye toward plants' mature size.
Deadheading and trimming
Perennials look their best with regular maintenance during the growing season. Deadheading (removing old flowers) keeps the garden looking neat and can prolong bloom for several more weeks.
For many flowering perennials, trimming and pinching also improve appearance. After a spring-flowering plant's bloom period ends, cut back all stems and foliage by one-third; a healthy mound of new growth soon fills in and remains throughout the growing season. To prevent lanky, floppy growth on some summer- and fall-blooming perennials, control growth early in the season. Pinch individual terminal buds to encourage bushier growth; to make plants bushier still, cut back entire branches by a few inches rather than just pinching the top bud.
Even with assiduous pinching and trimming, some perennials will be topheavy when in bloom and will require staking to stay upright. You'll find a variety of stakes at nurseries and garden centers. The most common are simple wood, bamboo, or plastic stakes. Others are circular grids that fit over the growing plants; still others are flexible "fences" or frames that can be positioned where they're needed.
Gardeners divide perennials for at least two reasons: first, to improve the health and flower production of overgrown, crowded plantings; and second, to gain new divisions to increase a planting. Note that division is usually feasible only for perennials that grow in clumps with an expanding root mass. It is not practical to divide those that grow from a taproot; if you attempt to divide the taproot, you'll probably kill the plant. Such plants are usually increased by root cuttings or from seed.
Though there are exceptions, fall is usually the best time to divide plants that bloom in spring or early summer, while those that bloom in late summer to fall should be divided in spring.
Once divided, a large clump may yield several dozen divisions (or even more), but keep in mind that the smaller the division, the longer it will take to mature and bloom well again. For faster results, divide plants into fewer, larger sections.
To divide most perennials, follow these steps:
1. Wet the soil thoroughly a day or two before dividing to make the clump easier to dig.
2. Cut into the soil around the clump with a spading fork or shovel, digging 6 to 12 inches beyond the perimeter of the clump. Then dig under the roots to the depth of the fork or shovel, working around the perimeter until the entire area is loosened.
3. Remove excess soil from the clump (rinse it off with water from a hose, if necessary) so you can find natural dividing points.
4. Now begin the actual division. The best tool to use depends on the size of the clump and the type of roots it has. Some perennials have such thick, tough roots that a shovel (or even an ax) may be the only practical dividing tool. Others have mats of small fibrous roots that are easily sliced with a knife or pruning saw. Sometimes hand-held pruners or a trowel will do the job easily; if clumps are very loose, you can even separate by hand.
5. Cut foliage of large plants back to 4 to 6 inches once you've made the divisions. Keep the divisions' foliage and roots damp while you prepare the planting area; place them in a shady spot if the day is sunny and warm.
6. Amend the soil with organic matter, whether you are replanting in the same area or in another part of the garden. Many gardeners also work in a dry granular fertilizer high in phosphorus and potassium to promote healthy root development.
7. Plant divisions and keep them well watered while they're getting established. You can also plant divisions in containers and hold them for planting later or for sharing with fellow gardeners.