What’s the one spring garden chore that costs you nothing, and yet earns you free plants?
If you answered dividing perennials, then give yourself a nice pat and keep moving. It’s go-time with your shovel.
Simply said, dividing a plant into two or more vegetative pieces or clumps is the best way to stimulate new root growth for plants that are no longer growing and blooming with vigor like they previously have.
Sometimes, plants poop out in the center (ornamental grasses, I’m talking to you). Plants can also crowd out surrounding plants or outgrow their zip code and spread to Kansas (culinary mint comes to mind).
Try as we might when gardening, sometimes we plant perennials in a landscape location that just isn’t suited to the growing conditions they prefer. They might need more sun or shade or less root competition. Move the plants this spring where they’ll be happier; just follow the same directions below but skip the division steps.
When to divide
An easy-to-remember rule of thumb is to divide fall-blooming perennials in the spring and spring bloomers in the fall. Mid-summer bloomers can be divided in either spring or fall.
As days lengthen and soil warms up in the spring, the newly divided plant’s roots will take off and you’ll practically never know it was tampered with. The same goes for fall division when the plant is focused on root growth since it already bloomed in the spring. Divide plants in the fall a good six weeks before frost is predicted. They need time to settle in before the soil freezes.
Try to avoid dividing in the heat of summer; the exception is bearded iris, which mostly go dormant in the summer and prefer being divided after blooming (July-August).
How often to divide is up to you and the plant; some experts say every three to five years or longer. Try to divide before you see the plant going downhill. It’s OK to divide a plant that’s thriving and healthy if you know it will need it eventually.
The goal is to divide plants when you see new growth starting in the spring. Early spring through mid-May is the sweet spot. Soil conditions matter, too: The soil shouldn’t be too wet or frozen.
How to divide in spring
First, determine your plant’s root system type. Learning the way the plant roots grow will help with the right procedures to divide (types described below).
Water the plants to be divided a day or two prior to dividing if it’s been dry. Choose a cloudy day; if rain is expected after completion, that’s a bonus.
Prepare new planting holes where you will replant the divisions. With extra divisions, consider sharing with others or pot them up and bring to a plant swap event.
Use care when digging new planting holes and avoid digging into underground sprinkler pipes and drip lines. It’s OK to plant over underground pipes; sometimes it can’t be avoided. Dig the new planting hole location wide enough for roots to spread out but not too deep.
Cut off any spent or dead foliage from the previous year to the top of where you see new growth emerging. If dividing in fall it’s OK to take off some of the taller growth to make it easier to move.
If needed, move mulch away from the plant being divided.
Dig down deeply on four or more sides of the plant, approximately 4 to 6 inches away from the plant. If growing right next to other plants in the landscape, you may have to dig closer to the parent plant. Use a clean, sharp shovel or spading fork. Lift the entire parent plant and as much attached root as possible upward and out of the ground and out onto a tarp or wheelbarrow. Remove excess or loose soil from the rootball.
Sometimes the plant is just too large or heavy to lift all at once. It may also be a groundcover plant where only sections need to be moved. Either way, you can cut the parent plant into several pieces or manageable clumps while it is in place in the ground. Using your shovel, dig what you need before lifting out portions with attached roots.
Separate the parent plant into smaller clumps or divisions by one of these methods. (Be sure to keep the divisions shaded and moist until replanted in the ground or container if not planted right away):
1. Gently pull or tease apart the roots with your hands; this is easier on small sections of the root ball.
2. Use a clean, sharp knife or spade to physically cut the plant root from the top of the roots (crown) downward into sections. A Sawzall tool works too (make sure you’re proficient using one).
3. More forcefully separate large plants using two forks back-to-back inserted into the center of the root ball. Carefully move the handles away from each other so that the divisions each contain three to five vigorous root shoots (bud eyes or growing points). Be sure to toss or compost any visible weak and small divisions including the center of the plant which may be dead and needs removal (common on ornamental grasses).
4. After digging up cut rhizomes with a clean, sharp knife, toss away the parts damaged by insects or disease. Be sure each section contains a few inches of rhizome and a fan of leaves and replant with the top of the rhizome just showing above the soil level.
Before planting the new divisions, add some quality soil amendment or well-aged compost to the planting hole if the soil is not well draining. Avoid adding too much amendment; no more than approximately one part amendment to three parts existing soil. Use less soil amendment for native plants as they like leaner soil, but still appreciate good drainage.
Plant new divisions in their relocated spot at the same depth as the original plant or with the crown just slightly above the soil level. They will settle a bit and you don’t want them too low in the ground. It’s OK to replant divided plants in the original planting hole.
After dividing, fill in with soil the gaps around the root ball of the parent plant and new divisions. Water plants well. Additional soil may be needed after watering; then apply a couple of inches of mulch if needed.
Avoid fertilizing your newly divided plants. Instead, focus on giving them time to grow new roots. Water when needed, not letting them dry out. Provide temporary shade such as a cloth clipped over a tomato cage or old umbrella if there is unexpected hot sun or hail.
Let the root types guide you
Spreading fibrous root systems are very intertwined and spread outward from the plant’s crown. The short list includes aster, lamb’s ear (Stachys), tickseed (Coreopsis), beebalm (Monarda), black eyed-Susan (Rudbeckia), bugleweed (Ajuga), goldenrod (Solidago), blanket flower (Gaillardia), sedum, yarrow (Achillea) mums (Chrysanthemum), creeping germander (Teucrium aroanium), and perennial salvia. Use one, two or three of the division methods depending on the size of the parent plant.
Clumping root systems are more compact. The root system grows more downward than out and originates from a central clump with multiple growing points. These include daylily (Hemerocallis), hosta, phlox (Paniculata) coneflower (Echinacea), and many ornamental grasses. Use dividing methods two and three.
Rhizomes grow along or underneath the ground on horizontal stems. Nodes grow on the stems, where other stems grow from. Bearded irises are the most common in this category. Use dividing method four.
Some plants are not good division candidates, such as perennials that grow like woody shrubs or have long taproots. These include lavender (Lavandula), Russian sage (Perovskia) and butterfly weed (Asclepias).
Other plants resent being disturbed and prefer to be left in place; it’s important to read plant tags or visit with experts at independent garden centers who can guide you on the best location for plants. Keep these in mind to leave alone: false indigo (Baptisia), euphorbias, lupine (Lupinus), clematis, columbine (Aquilegia) and baby’s breath (Gypsophila).
The easiest herb perennials to divide in spring include culinary chives (both onion and garlic), oregano, tarragon, chamomile, sweet woodruff, bee balm, catmint, feverfew and lemon balm. If culinary mint becomes more aggressive than you can handle, consider growing it in containers that are sunk into the ground. They may still edge over into nearby soil, but youi can just harvest the escapees and make more tea!
Don’t forget the pat on the back for another finished garden chore!
Resources
Handy list of dividing perennials commonly grown in Colorado from University of Minnesota Extension: https://drive.google.com/file/d/1S6yqog4CnN_zqwG02uiOo28HT-is_LlP/view
Betty Cahill Plant Divide Denver Post Video https://www.youtube.com/watch?v=n0yr9qRPKZE&t=43s
