In the Garden: Dividing Perennials
Dividing perennials (plants that remain or grow back each year) is a seasonal task and one that I love: you get to make new plants from existing plants. In essence, you cut up (divide) the plant into smaller sections that can be replanted or repotted to grow your stock and fill out your garden. Dividing is also a great way to control the size of plants or revive any that are starting to show their age. Perhaps best of all, it’s allows you to take time to observe your plants and get to know their growth habits intimately. The new rosettes pushing up from Catmint Nepeta racemosa 'Walker's Low' under a wiry nest of bare twigs tell you it’s time to trim back the old growth and get ready for the new. So, too, the colossal roots of the Dalily (Hemerocallis) hint to it’s native home in the Caucuses and Himalaya, where it sits stubbornly, unmoved by the icy winter winds.
Timing: It’s best to divide when the plant is not in active growth (although technically you can divide at any time so long as you keep your new plants well-watered). The usual convention is to divide spring-blooming plants in the fall and fall-blooming plants in the spring. As always, think like a plant: it wants the maximum amount of time to build up energy to flower, and also some warmth so that it can build new roots after division. When in doubt, wait a year to see when your plant begins to flower. You can also track back to when you first planted any bareroot specimens — they arrived in your garden in a dormant state and that’s a good time to divide. Another rule of thumb is that those with a tuberous bulb should be divided in spring and those with fleshy roots (like peonies) can be divided in fall.
There is no hard and fast rule about how frequently to divide — some plants require division every few years, others can wait a decade. Most are on a two-to-three year cycle. Look for signs that your plant is stressed from overcrowding, or has slowed or stopped growth and is ready for some rejuvenation. The following can indicate that a plant could use dividing: less flowering, smaller blooms, stems getting weak or floppy, or a “bald” spot appearing in the center of the clump.
A few to divide in the autumn: aster (Symphyotrichum novae-angliae), delphinium (Delphinium elatum), any iris (Iris germanica, I. siberica), ornamental grasses, peony (Paeonia spp.), Oriental poppy (Papaver orientale), salvia (S. clevelandii, S. greggii, S. guaranitica).
How to divide: If you can, dig when it’s overcast or outside of the hottest time of the day to avoid roots drying out. Gently loosen your plant from the earth with a garden fork and shake off enough soil so that you can see the root fibers. Take a look at the plant and see where growth is emerging from the root. These shoots will be separate plants once divided, the key being to maintain a connection of root-plus-plant in each of your divisions. You want to aim to have three to five shoots in each clump — any smaller and it will be difficult for the plant to survive or you’ll have to wait a long time for them to bulk up.
Separation can be done by hand, with a knife, or other tools. Small plants with fibrous roots can be pulled gently apart. A large, fibrous clump, like Iris siberica (Siberian Iris), might need to be levered apart using two forks: insert them at the crown, back-to-back, and then push/pull out from the crown to separate. You can break these pieces down further. Plants with a woody crown or fleshy roots can be cut with a spade or knife.
If your soil will be warm enough for a month or two after you divide, you can replant your divisions immediately, making sure to water them in well. Most will benefit from a little shade or sun protection when replanting. If not putting them in the ground, simply pot each division up in a separate container and keep them in a frost-free place to let them bulk up for replanting the following season. Here in California I will pot up divisions to protect them from hot, dry weather, too.
Let me know if you have questions about specific plants.
