Beat the freeze so your spring- and early-summer-blooming plants can re-establish strong root systems.
AUTUMN IS THE best time to divide perennials that will bloom next spring or early summer. Dividing now will allow sufficient time to re-establish strong root systems before the flowering cycle begins again.
Divide them now, before freezing weather sets in.
Perennials need dividing because they grow outward from the center. If one of your plants dies out in the middle, produces fewer blooms or seems less vigorous, it might need dividing. Even if your plant is vigorous and healthy, you still might choose to divide it in order to control its size and start over with a smaller plant. Another reason to divide is to create starts of prized perennials to give to friends and neighbors.
Some perennials need dividing more often than others. Achillea (yarrow), Anchusa (bugloss), Campanula (bellflower), Crocosmia (montbretia), Hemerocallis (day lily), Siberian Iris, Lobelia, Monarda (bee balm) and Phlox are spring and summer perennials that generally bloom more reliably if they are divided every one to three years.
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Some perennials are almost impossible to divide and are better left undisturbed. Actaea (bugbane), Aruncus (goatsbeard), Gypsophila (baby’s breath), Eryngium (sea holly) and Platycodon (balloon flower) have woody, ropelike roots that tend to break easily, causing severe dieback when divisions are attempted.
Others, such as Aconitum (monkshood), Aquilegia (columbine), Euphorbia (spurge) and Pulsatilla (pasque flower), simply resent root disturbance and show their displeasure by going into major decline. Fortunately, these perennials rarely die out in the center or stop blooming with age. Give them room, and enjoy the fact that these perennials will thrive with little or no effort on your part.
One spring-blooming perennial that never needs dividing is herbaceous peony. As long as they are happy in their location, these long-lived plants flourish and bloom for years with little care.
There is a misconception, however, that they can’t be divided. Nothing could be further from the truth. Peonies are easy to divide once they die back in fall, and it’s a great way to make new plants. Just make sure that each division has three to five growth eyes (buds), along with three or more thick, healthy roots. When replanting, choose a sunny location with well-drained soil, and work plenty of compost into and around the planting hole. When you plant the divisions, make sure the buds are facing up and protrude just above the soil surface. Don’t get bummed if your new plants don’t bloom for a couple of years. Peonies live for more than 100 years, so they aren’t in any hurry to flower and raise a family. Once they begin blooming again, they’ll continue to put on a star performance every spring.
Dividing most perennials is straightforward. Begin by cutting the stems back to the ground to make it easier to pull, or cut the roots apart without having to deal with tangled foliage. Then dig and lift the clump.
When cutting the root mass into sections, look for natural divisions, and take sections from the vigorously growing edges of the root mass while discarding the bare middle. If sections can be easily teased apart, simply use your hands to pull off pieces from the edges. If, on the other hand, the roots are thick and hard to break, it might be necessary to use a sharp knife or an old pruning saw to cut off sections. Most of the time, I simply use my digging spade to slice off a healthy chunk of roots from the side of the root ball.
Always work compost and organic fertilizer into the planting hole before you replant the divisions. By the way, it never hurts to remind your neighbors how much you love chocolate-chip cookies when you give them a division.