10 Plants You Should Never Divide in Spring
When dividing perennials, most plants do well when divided in spring. However, there are a few that will do better if you wait until late summer or fall to divide them. Others, particularly semi-woody perennials, don’t take well to dividing at all. This guide explains which perennials aren’t good candidates for spring division so you can take those off your list of spring garden chores.
When to Divide Perennials
If you consider what happens when we divide perennials, the importance of proper timing is easier to understand. The process of digging up a perennial plant, disturbing its roots, cutting the plant into pieces, and then replanting it is quite traumatic to the plant. It may take the divided plant weeks or months to recover.
For that reason, plants should not be put through this type of stress before flowering, which takes a lot of energy out of the plant. Similar to pruning, dividing perennials at the wrong time risks disrupting the upcoming flowering or weakening the plant before adverse weather conditions such as heat or freezing temperatures.
In general, the rule is that spring- and summer-flowering plants are best divided in the fall, and fall-flowering plants are divided in the spring. Dividing perennials after blooming allows the plant to focus all its energy on getting reestablished.
The lists below are not exhaustive, but they include several popular and commonly grown perennials. You’ll also find general tips to help you decide which types of plants to divide in spring, which ones to divide in fall, and which ones to leave alone altogether.
When dividing perennials in fall, do so at least 4 to 6 weeks before the estimated first frost date in your area. For northern locations, that means dividing should be done in late August or September. For gardeners farther south, it can be as late as the end of October.
Plants to Never Divide
There are some plants you shouldn’t divide regardless of the season. Woody perennial shrubs, such as roses and some herbs, fall into this category. They’re better reproduced from stem cuttings or by layering, a process by which plants sprout roots directly from their stems upon contact with soil. Most plants with taproots also don’t take well to dividing.
Perennials not suitable for dividing include:
Plants to Avoid Dividing in Spring
The following spring-flowering perennials are best to divide in fall rather than spring:
Peony
Whether you’ve got herbaceous peonies (Paeonia lactiflora), hybrid Itoh peonies, or maybe even fernleaf peonies (Paeonia tenuifolia ), these plants are all best to divide in fall. If you’ve got tree peonies, these are on the never divide list.
Oriental Poppy
Kindra Clineff
Oriental poppies (Papaver orientale) are best divided in late summer after the plants have finished blooming. Use a soil knife to cut through the root mass, ensuring each section has at least one eye, plus some roots. Oriental poppies should be divided every 4-5 years.
Siberian Iris
Kritsada Panichgul
Siberian irises (Iris sibirica) grow in a large clump. Eventually, they become crowded, and flowering decreases, which signifies it’s time to divide. Divide Siberian irises in late summer or fall. Dig up the entire clump, cut the iris foliage back to 6-8 inches, and divide the clump into several new pieces.
Bearded Iris
Bearded irises (Iris × germanica) can quickly become overcrowded and usually need dividing every 3-5 years. Late summer is the best time. Cut the leaves back to about a third of their height, dig up the clumps, and cut the rhizomes apart with a sharp soil knife.
Allium
Also called ornamental onions, alliums grow from bulbs that multiply each year. Divide them in fall by digging the entire clump and pulling it apart by hand where it naturally wants to divide. Mark their location in summer with a stake or flag if they typically lose their foliage and are hard to find in autumn.
Globe Flower
Matthew Benson
When the center of a clump of globe flowers (Trollius chinensis) starts to thin out, or if flowering seems to be declining, it’s time to divide them. New transplants may take a year to recover from division and resume full flowering.
Jack-in-the-Pulpit
Bob Stefko
Divide spring-blooming Jack-in-the-pulpit plants (Arisaema triphyllum) after they’ve entered dormancy in autumn. Wear gloves because the sap can be a skin irritant. Break off any offsets that have formed by hand and replant.
Marsh Marigold
Denny Schrock
Marsh marigold (Caltha palustris) is native to much of the U.S. and loves damp areas. Lift the entire plant in late summer or fall when flowering has finished. Separate clumps by teasing them apart; don’t hack it with a tool.
Prairie Smoke
Denny Schrock
A gorgeous wildflower, prairie smoke (Geum triflorum) forms large clumps. While it doesn’t get divided growing out in the prairie, in a garden you can split it up every few years to keep it flowering and growing vigorously. Wait until the plant has finished flowering and setting seeds. When the seed heads are drying is a good time for division.
Lily
Bob Stefko
True lilies, plants from the genus Lilium, are planted or divided in the fall. This group includes Asiatic, Oriental, and trumpet lilies, but not daylilies. Wait for the foliage to yellow and die back, and wear gloves; the sap can be a skin irritant.
Dig as deeply as you can and lift the clump. Look for natural divisions where the bulbs seem ready to pull apart by hand.
