While you’re weeding and seeding and planting and pruning, there’s one more thing to put on your plate: dividing.
Dividing is a matter of digging up plants, prying them apart with your hands, spades, knives or an ax and replanting them in new holes, said Weston Miller, a horticulturist for Oregon State University’s Extension Service. As perennials grow from the crown and the clumps get larger, plants decline in vigor and have fewer flowers. If you wait too long to divide, flowering can stop altogether.
That’s not the only reason to take out the spade and start splitting up your prized plants.
“I divide plants when I want more of them,” Miller said. “I’m looking outside and seeing some chives. I’ll hack those into quarters. I divide things like yarrow a lot because I want to populate large areas.”
Gardeners love to dig and divide plants and share them with friends and neighbors. However, with new threats in Oregon from invasive species like Japanese beetles and the so-called “crazy snake worm,” it is no longer recommended to move plants or soil from one location to another. Also, OSU does not recommend plant swaps or sales based on dug-up plants at this time. Refer to guidelines for Best Management Practices for plant sales.
How many more plants you end up with depends on the size and vigor of the original plant. Hosta, grasses and daylily can produce six or more, while hollyhock and coral bells may only yield three. Be sure to pay attention to how many growing points there are and leave at least two per clump when you cut plants apart.
As a general rule, Miller said, divide summer- and fall-blooming plants in spring and spring-blooming plants after they’ve bloomed or in the fall. Don’t cut apart a plant when it’s blooming.
You’ll know your plant needs dividing if it’s dying in the middle or becoming lackadaisical, he said. Even if a reference says to divide a plant every three years, there’s no need to do so if it’s growing well.
One thing people don’t think to divide are culinary herbs such as sage, thyme, oregano and marjoram, Miller said.
“You’ll get much more supple growth and better culinary quality if they’re divided every couple of years,” he said.
Miller’s advice on how to divide plants:
- Saturate soil where you plan to plant divisions a few days ahead of time.
- Prepare soil in the new planting area with organic matter and saturate again.
- Dig holes where you will plant divisions. Make them wide enough to accommodate the roots and a bit more. Dig deep enough so crown of divisions will be level with top of soil.
- If the soil is dry, water the plant a day or two before dividing it.
- Cut the plant down to 6 inches from the ground.
- Use a spading fork or sharp shovel to dig around the plant.
- Pry or dig out in one big chunk.
- Once on the surface, don’t allow root ball to dry out. If it’s a warm day, cover with plastic sheet, cloth or even plant clippings.
- Make sure there are growing portions in each division.
- For smaller plants or those with fibrous roots systems like small ornamental grasses, pry apart the roots with your hands or saw through them with a serrated knife or weeding tool such as a Hori-Hori knife. Use a sharp spade, shovel, handsaw or ax for larger plants. With really tough roots, pry them apart with two spading forks placed back to back.
- Shake or hose off loose soil.
- Discard dead center of plant.
- Plant divisions with crowns even with soil line.
Miller recommends dividing these plants in spring
Divide every 1-3 years
- Shasta daisy
- Coral bells
- Bee balm
- Fountain grass
Divide every 3-5 years
- Blanket flower
- Black-eyed Susan
Divide every 5-10 years
- Siberian iris
- Asiatic lily
- Lady’s mantle
- Meadow rue