Everything You Need to Know About Pruning and Dividing Plants
Don't be afraid to make the cut! Here's our best pro advice to help you snip back and multiply your garden staples with confidence
March is notoriously unpredictable. Shrubs can be crusty with snow on the first of the month, and then, a couple of weeks later, temperatures can warm up enough for flower and leaf buds to show signs of life.
Still, some early spring cleanup tasks are sure things this time of year. Pruning (for summer-bloomers) and dividing (for late-flowering perennials) are two such tasks. It's quick work, but pruning and dividing are among the most neglected tasks of homeowning. Why? Because for most, it's a black art. The risks of butchery seem high, and the rewards low. "But it isn't difficult," says This Old House landscape contractor Roger Cook. "And what you get in return is thicker foliage, more flowers, and healthier plants."
Keep reading for a roundup of our best how-to advice on the topic so that you can make a few cuts of your own with confidence.
Young flowering shrubs should be pruned lightly to make them grow fuller and bushier. With hand pruners, trim long, unbranched stems by cutting just above a healthy bud. This type of pruning, called heading, encourages lower side branches to develop and enhances the shrub's natural form. When selecting a bud tip to trim to, keep in mind that the new branch will grow out in the direction of the bud. Like most pruning, heading cuts should be timed to avoid disrupting the plant's flowering.
As a shrub develops, thin out old, weak, rubbing, or wayward branches where they merge with another branch. This opens up the middle of the plant to more sunlight, which keeps interior branches healthy, stimulates growth, and increases flowering.
For more simple pruning techniques, including how to prune aged shrubs, small trees, and when to prune, see How to Prune Small trees and Shrubs.
If the thought of cutting into what looks like a perfectly happy plant still makes you cringe, you're not alone. Even homeowners who know the benefits of pruning are often still confused about exactly the right time and right way to make the cuts, fearing they'll lop off next year's flowers, stunt the plant's growth, or kill it outright. But once you understand how plants respond to pruning, you'll realize how many problems a well-placed cut can solve.
In short, successful pruning involves mastering two basic cuts. Follow along with TOH landscape contractor Roger Cook in Shrub Pruning Dos and Don'ts to learn how to use them.
Let's set the record straight: You don't need to be a gardening pro to grow roses. But you do need to prune repeat-blooming varieties in early spring to get the best blossoms, says Peter E. Kukielski, a rose curator at the New York Botanical Garden. (Other types can be cut back in early fall.) Rounding up the right items to get you started will help you concentrate on your technique so that your shrub roses and climbers don't become, um, a thorn in your side.
See our guide to what you need to get the job done in Everything You Need to Prune Roses.
Here's what TOH landscape contractor Roger Cook had to say when reader Bev Nelson in Napoleon, Idaho, asked for the best way to prune lilac bushes so they'd bloom every year:
"Several things can cause lilacs to bloom poorly. An unusually cold winter or late cold spell could have killed your lilacs' flower buds, one of the shrub's more vulnerable parts. Or, your soil might have become too alkaline or acidic. Lilacs like soils whose neutral pH is between 6 and 7. A soil test will answer this question. Also, you should not overfertilize lilacs; that will cause them to produce more leaves than flower buds.
But the biggest reason lilacs don't flower is because of pruning at the wrong time of the year. Lilacs bloom early and set new flower buds early, so a pruning in June or later in the season takes away the buds for the following spring. The best time to prune is right after the flowers turn brown.
On large lilacs that haven't been pruned for a while, I'd prune one-third of the largest branches the first year, one-third the second year, and the remaining third of the older branches in the third year. Blooms form on stems at least three years old, so you'll never be without flowers. And the result will be a totally rejuvenated plant, with larger, stronger branches that produce more flowers."
For more of Roger's pro advice, see Pruning Lilac Bushes.
If your hydrangeas are sited correctly, with enough room to grow, the only pruning required is to remove dead wood—be sure to take it off at the base of the plant if the whole branch is deadand spent flowers. In Dirr's experience, all hydrangeas benefit from regular dead-heading to encourage more blooms. Left unpruned, they will produce fewer flowers because of a growth-inhibiting chemical released by the terminal bud at the tip of the stem. Don't prune past August, though, because any new growth is susceptible to an early-fall freeze.
For more on caring for these flowers, see 5 Tips for Growing Gorgeous Hydrangeas.
Proper pruning can enhance a tree's health and beauty. But tangled branches that don't look like the neat illustrations in books can be daunting. So where do you begin?
First, be sure it's a job you can handle. A small ornamental tree 12 to 25 feet tall—such as a flowering cherry, magnolia, crab apple, or Japanese maple—is one you can tackle yourself. (Bigger trees that require chain-saw and ladder work are best left to a pro.) Then be sure you understand a few key principles.
"Pruning stimulates growth," says Bill Pollock, co-owner of Arbor Services of Connecticut. "The goal is to encourage the tree to grow strong, healthy branches headed in the right direction." So Pollack makes health-promoting cuts first, removing competing, misdirected, and weak branches. Then he prunes to enhance the tree's natural form.
The best time for the kind of remedial pruning an established, long-neglected tree (like the one in this story) needs is early spring or winter, when active growth has not yet begun. Bare branches allow you to stand back and really see what needs to be removed.
Check out How to Prune a Tree for full step-by-step instructions.
Fill your yard with blooms for free! Late-flowering perennials are best divided in spring; wait until fall for early flowerers, but well before the thermometer dips too low. You want the roots to have time to recover before the ground freezes. This method works best on perennials with fibrous or fleshy roots, such as hostas, phlox, asters, and rudbeckia. Plants with tuberous roots, like peonies, or rhizomes, like bearded iris, should be cut apart with a knife.
See Dividing Plants for the three steps to getting the job done.
The daylily is one of the easiest perennials to divide. "Daylilies are bulletproof—they can take a lot of abuse," says Roger Cook, This Old House landscape contractor. According to Roger, the best time for division is either in early spring, as soon as new growth is visible above the ground, or in the fall, after they have finished blooming. Divisions bloom sparsely in the first year, but once they are established they grow in beauty and number of flowers.
For full step-by-step instructions, see How to Divide Daylilies and learn how you can double the blooms in your yard using the plants you already have.
Dividing perennials every three to six years is a great way to thin clump-forming varieties, like the daylily shown here (Hemerocallis), which blooms from late spring to late summer. This technique can also be used to control plant size, invigorate growth, and multiply the number of specimens in a garden.
A good rule of thumb is to split apart spring- and summer-blooming perennials in late summer or before the fall frost. Fall bloomers are best divided in the spring so that they can devote their energy to growing roots and leaves.
Before dividing, water the mother plant well for a day or two before you dig it up, and wait for a cloudy day to do the actual digging—hot, sunny weather stresses plants. Then follow these steps and you'll be rewarded with new, more vigorous plants to share with friends or add to your garden.
For full step-by-step instructions, see How to Divide Overgrown Perennials.