Buying enough plants to fill all the empty spots in your landscape can quickly become expensive. One way to minimize the number of plants you need to buy over the long term is to purchase perennials. Perennial plants return from the same roots each year. Most multiply and spread, and can be divided every few years to make new plants. Most perennials are not temperamental and, if properly divided, will fare well whether they are divided in spring or fall.
When to divide perennials
A good rule of thumb for dividing perennials is to do so in the season opposite of when they flower. So spring and early summer bloomers such as Coreopsis and Iris are best divided in fall, while late summer and fall bloomers such as Mexican bush sage and Joe Pye weed are best divided in early spring. The idea behind this is that the new plants will be able to put all of their energy into root and leaf production, rather than flowering, and therefore have an easier time becoming established. Spring division is ideally done as soon as the new growth emerges, which for many perennials begins in mid February or early March.
How often to divide perennials
Perennial plants are healthiest and most productive when they have room to spread. Most perennials should be divided every three to five years. Some vigorous perennials like daylilies and black-eyed Susans (Rudbeckia) may need to be divided every other year or they may crowd themselves into non-flowering clumps of leaves and roots.
To determine if your perennials are ready to divide, take a look at the size of the clump and amount of room they have available to grow. Most perennials will spread and run out of growing room over time. When this happens it is time to divide. Some plants resent being divided and should be left alone if possible. These include butterfly weed (Asclepias), euphorbias, Japanese anemones, false indigo (Baptisia), columbines (Aquilegia), as well as Lenten and Christmas roses (Helleborus).
How to divide perennials
Experienced gardeners know that plants are pretty tough. So when it’s time to divide your perennials, be brave. All you need to do is sink your shovel in the ground, loosen the soil deeply around the perimeter and grab the plant. It’s going to be fine.
To move a perennial with minimal root damage, begin digging several inches beyond the outer edge of the clump. The roots will generally extend that far, so digging further out allows you to lift the plant with as many roots intact as possible. Dig a trench around the clump, cleanly severing any long roots, then cut at an angle down and under the clump from various points around the outer edge until you can lever the plant out of the hole. For large plants you may have to first dig a trench, then slice straight down through the center of the plant as if it was a piece of pie, halving or quartering the clump before undercutting and lifting.
It is always best to divide perennials on a cloudy day. Aim to divide plants so the new pieces are 20 to 25 percent the size of the original clump. Protect divisions from drying out by covering them with a damp piece of burlap or placing them in the shade immediately after digging. Never leave them exposed in the hot sun. Plant divisions as soon after digging as possible. If you cannot plant the new divisions immediately, pot them up in containers of potting soil or temporarily heal them in somewhere in the garden.
Before replanting, take the time to improve the soil. If you remove a wheelbarrow full of perennials, then a wheelbarrow full of compost should be added back into the site before replanting. This will renew the soil and build the site back up to ground level. When the soil is ready, dig a hole that is at least as wide as the division’s roots when spread out. Place the division in the hole and cover with soil so it is planted at approximately the same depth the original clump was growing. Water new plantings thoroughly and apply a two inch layer of mulch. Keep in mind that spring divided perennials often bloom a little later than usual the year they are divided.
If you have questions about lawn and garden care, visit ces.ncsu.edu where you can submit questions via the ‘Ask an Expert’ link, or contact your local Cooperative Extension center by phone: in Pender County, call 910-259-1238; in New Hanover County, call 910-798-7660; in Brunswick County call 910-253-2610.
Susan Brown is the consumer horticulture agent with the New Hanover County Cooperative Extension of N.C. State University, College of Agriculture & Life Sciences.