Perennials grace our gardens year after year with their variety of bright colors and unique leaf shapes. After a few years in the garden, however, these perennials may begin to produce smaller flowers, develop a “bald spot” in the center of their crown, or need to be supported to keep their stems from falling over. These are all signs that it’s time to divide the plants. In today’s article, we explain which perennials to divide in the spring and which ones to divide in the fall, and when the right time is in the fall.
Why should perennials be divided?
- To rejuvenate the plant and stimulate new growth.
Crowded plants compete for nutrients and water. Restricted air circulation can lead to disease. Dividing plants reduces this competition and stimulates new growth and more vigorous flowering.
- To reduce the size of the plant.
Because plants grow at different rates, division can be used to control perennials that spread quickly.
- To increase the number of plants
Dividing is an easy and inexpensive way to increase the number of plants in your garden.
Most perennials are best divided every three to four years.
Which perennials to divide and when?
Here’s the basic principle:
- Spring bloomers can be divided and replanted in the fall, a few weeks after blooming.
- Late summer and fall bloomers can be divided and transplanted in early spring, before bud break. These include asters, hosta perennials , coneflowers and fat hen. Perennial ornamental grasses should also be divided in the spring, as they flower or seed in late summer and fall.
The main goal is to leave the plants alone while they bloom and fruit, and schedule repotting or root division in the off-season.
But there is another indicator of the right time to divide plants – their roots. In general, perennials that have a tuber should be divided in the spring. Perennials with fleshy roots, such as peonies (Paeonia spp.), oriental poppies (Papaver orientale) and Siberian iris (Iris siberica), are best divided in the fall.
Which perennials should not be divided?
While most perennials benefit from being divided every few years, there are some perennials that do better when left intact. Avoid dividing these varieties:
- Indigolupine (Baptisia).
- Heart flower, watering heart (Dicentra)
- Silk plants (Asclepias)
- hellebore (Helleborus)
- Diptam (Dictamnus)
- Lavender (Lavandula)
When is the right time in the fall?
Divide perennials on a cloudy day, as sun and high temperatures can cause plants to dry out. Ideally, divide perennials in the fall when a few days of rain are predicted so that the new plants receive adequate moisture.
If you divide the perennials in the fall, the months of September and October are optimal for this. As a rule, you should do it four to six weeks before the ground freezes, so that the roots of the plants can become established. If you transplant the plants too late, such as in the winter, they will not have time to develop functional roots.
Basically, we talk about ground frost when a light layer of ice forms near the ground. This can happen even at air temperatures of 5 degrees. The humidity of the air plays a decisive role. If there is high humidity in combination with strong wind and fog, the chance of ground frost is low. If humidity is low, you can expect ground frost.
5 perennials that should be divided in the fall.
Peonies are popular for their showy blooms and sweet fragrance. They are low-maintenance, long-lived plants that can survive even cold winters. They need to be divided every three to four years in the fall. Each cutting of a peony should have three to six “eyes.”
Tall perennial phlox
This long-blooming perennial has large, showy clusters of flowers that please the eye all summer long. To keep the phlox blooming year after year, separate the clusters every two to four years.
If you divide the garden phlox in the fall, it is recommended that you mulch it after transplanting. A 10- to 15-inch layer of straw, pine needles or similar material should protect the newly planted offshoots from ground frost.
Divide iris perennials
Ideally, bearded iris should be divided six weeks after flowering (usually in August). However, fall is not too late either. After cutting back the foliage, dig up the iris rhizomes and cut them into sections with a sharp, clean knife. Each rhizome can be divided into three or four pieces. Take the pieces and plant them flat in compost-treated soil.
Divide lilies perennials
Daylilies enchant with their very long summer bloom and fascinating colors. Summer-flowering perennials like daylilies can usually be divided before or after blooming.
When dividing daylilies in the fall, it is recommended to cut the foliage back to 15 to 20 inches. Each division should have two or three pairs of leaves. Daylilies should be cut back in the fall even if they are not divided.
Dividing poppy perennials
Poppies are best known for their large, scarlet flowers, but there are many types of poppies in many colors. Divide these perennials when the above-ground parts die back in the fall.