Pruning is more art than science. It’s a relaxing practice that allows us to get up close and personal with our plants, do a close inspection and prevent problems later on. Think of pruning as tough love for plants. Plants are rarely harmed by the practice, and most will grow better and stay healthier with a proper bit of cutting back.
There are many reasons to prune, says Tracy DiSabato-Aust, whose book The Well-Tended Perennial Garden is the definitive text on the topic. (Her website also includes instructional pruning videos and plenty of helpful information.) “Much of it depends on your objective,” she says. “We prune to create more compact plants that don’t require staking. We prune to have plants that are fuller throughout the summer so they don’t develop that melt-out hole that sometimes develops in spring-flowering plants. We prune preemptively to delay flowering. We prune for pest and disease control, and we prune to do general garden cleanup. And we’ll dead-head for a repeat bloom. Sometimes we can prolong flowering for two months.”
Pruning Terminology 101
Pruning is an overall term that might be referred to as grooming, shaping, shearing or snipping. “Cutting back” or “shearing” generally refers to removing foliage and flowers to control height or flowering time. Minnesota garden consultant Lila Stanley cuts back plants as soon as there are more dead flowers than fresh ones. This pruning will cause the plants to branch out and increase the number of blooms even though they bloom a little later.
“Pinching” or “pinching off” is a technique that helps plants develop a fuller form by forcing them to focus on growing more new stems rather than taller ones. To pinch a plant, pinch off a main stem down to just above the leaf nodes you want to keep. This should cause the stem to replace itself with two stems. Stanley pinches the new growth on mums prior to July Fourth to create offshoots for more blooms and a rounder shape.
“Shearing” ornamental grasses usually means cutting them back to just above the ground. DiSabato-Aust shears ornamental grasses down to 2 inches off the ground in the spring.
“Deadheading” is the removal of old or spent dead flowers. “Deadheading refreshes a plant’s appearance, controls seed dispersal, and redirects a plant’s energy from seed production to root and vegetative growth,” DiSabato-Aust says. “I do it primarily to prolong the bloom period or encourage a second flush of blooms on some perennials. For most plants, however, all you need to remember is to prune spent flowers and stems back to a point where there’s a new lateral flower or bud. If no new flower is apparent, prune the stem back to a lateral leaf.”
She also prunes certain foliage plants such as artemisias or ‘Axminster Gold’ Russian comfrey to eliminate flowering altogether because flowering pulls the energy resources from a plant that is grown for its foliage. And she deadheads plants such as purple coneflower that might reseed too assertively.
Pruning to Control Pests and Diseases
Pruning the damaged foliage on a perennial affected by disease or pests can often be an effective method to control and prevent further pest invasion. Thinning stems on mildew-prone perennials, for example, can increase air circulation around the plant and decrease incidence of disease. Increased air circulation may also discourage the arrival of pests. Be sure to remove infected foliage from the garden rather than putting it in your compost.
Pruning Shrubs: University of Minnesota horticulturist David Wildung lists several reasons for pruning woody shrubs and trees: to keep the plant in bounds; to remove diseased or broken branches; for structural support (for example, fruit trees need strong crotch angles to support fruit); to open up the plant for better light penetration; and to develop form.
Wildung says most pruning should be done when trees or shrubs are dormant. “The exception is spring-flowering shrubs like lilacs,” he says. “With those, you wait until after they bloom.” It’s important to prune spring-flowering shrubs such as forsythia, honeysuckle, lilac, mock orange, spirea and weigela soon after their flowers fade. Shortly after this they begin to develop the new growth that will produce flowers next spring. If you wait to prune until winter, you’ll risk reducing the amount of blooms. Debbie Braeu, co-owner of Edelweiss Nursery in Duluth, Minnesota, prunes rhododendrons and azaleas right after they bloom as flower buds form in the summer. She says dwarf shrubs keep their shape and grow slowly so they need no pruning.
While most people tend not to do much pruning because they are afraid of harming the plant, Wildung says, “It’s better to do a little bit every year than to prune heavily one year and then wait five or 10 years.” Prune most deciduous shrubs such as lilacs, dogwood and honeysuckle by thinning them out. Thinning out is the removal of some branches completely to the ground. “Depending upon the age and size of the plant,” Wildung says, “remove no more than two or three branches per year, usually the oldest branches each time.” It opens up the plant to get better light penetration and air circulation and to remove older, less productive wood. It encourages new shoot development from the base of the plant. On most plants it should be done in early spring but on flowering shrubs, it should be done after bloom. Thinning out is especially beneficial to red- or yellow-stem dogwoods, which display the most color on young stems. “Think about these shrubs where you’ve got a lot of canes coming out from the ground,” Wildung says. “If you don’t prune every year, it’s going to get very thick in the center of that shrub. You want to get as close to the ground as you can and remove the oldest, least productive shoot or branch. Prune blackberries and raspberries the same way, cutting the old canes out at the ground to take out dead, diseased or thin, weak canes.”
A good hand pruner can be used for most fine cuts. Anything much bigger than half an inch should be cut with lopping shears. Anything much bigger than 1 to 1-1⁄2 inches requires a saw. When using a hand pruner, Wildung cuts down to an outside bud that points in the direction he wants the plant or tree to grow. He makes a slanted, 45-degree angle pointing toward the bud. “You want to make the cut as close to the main trunk or branch as you can so you don’t end up with any stubs,” he adds. “A stub leaves an area where infection can get in.” Wound dressing is not necessary, and Wildung never uses it.
Pruning Trees: Large trees are best pruned by professional tree specialists. Wait until late summer to have maple and birch trees pruned. They produce large amounts of sap in early spring and therefore bleed badly if cut then.
Fruit trees generally need some annual pruning. While the tree is young, it’s important to train it and develop a strong branch system so the older tree can support a heavy crop of fruit. “The main thing with tree fruits,” Wildung says, “is to develop structural strength by using wide crotch angles—never less than 45 degrees—and to remove crossing branches and develop a symmetrical shape.”
Prune most fruit trees during winter while the trees are dormant and before growth starts, which prevents the introduction or spread of disease. The trees don’t have their leaves yet, so it’s easy to see what you’re doing, and the trees are dormant, which limits bleeding of the sap. Pruning an apple tree helps shape it and give it a strong framework. The ideal tree has a central leader and six to eight well-spaced scaffold branches that come out from the main trunk at a wide angle and are uniformly spaced around the trunk. If any side branches grow upward and overtake the leader, cut them back. Remove branches that rub to avoid resulting wounds, decay and notches. Watersprouts and suckers, occurring at the base of the tree or inside the crown, are rapidly growing, weakly attached and upright branches. They use more energy than they return to the tree, so it’s best to remove them as soon as possible. Also prune out limbs that turn inward and those that extend beyond the natural outline of the crown.
Prune evergreen shrubs not grown for flowers or fruit, such as arborvitaes, junipers and yews, in spring or early summer, after new growth has begun but before temperatures get too warm. The natural form of an evergreen is most desirable, so prune only to correct growth defects and maintain the shape. Pruning after August can make these shrubs more susceptible to winter injury. When in doubt, don’t prune. It’s easier to correct an unpruned shrub than an incorrectly pruned shrub.
Coniferous trees should be pruned yearly to promote dense, compact trees. They must be pruned before the new growth, referred to as candles, becomes woody. Remove up to one-half of the new growth. Prune spruce or balsam fir at the same time.
Find the necessary tools for your landscaping projects in Tools of the Trade: Pruning.
Perennial Flower Pruning Chart
Garden consultant Lila Stanley divides landscaping plants into two categories: those that need hard pruning and those that need simple trimming. A third list includes plants that should never be pruned until they yellow in late summer or early fall.
Plants that can be cut back hard to basal foliage (which grows at the base of the plant) at the end of summer in preparation for winter:
Plants that benefit from trimming (deadheading and shaping), but not a drastic “haircut”:
Plants you should not prune (only deadhead) until they yellow in late summer, then cut to the ground at the end of the season:
Shrub Pruning Chart
Prune in early spring before flowering:
Rose of Sharon
St. John’s wort
Prune after they finish flowering:
Red twig dogwood
Avoid pruning these plants altogether:
Viburnums (if grown for berry display)
Tree Pruning Chart
Prune conifers and evergreens not grown for their flowers or fruit in late spring or early summer:
10 Reasons to Prune
1. Extend bloom period or promote repeat bloom
2. Stagger bloom times
3. Alter plant heights
4. Encourage new growth
5. Extend life of plants
6. Increase flower size or numbers
7. Prevent or control pests or disease
8. Enhance overall appearance of the plant
9. Remove unsightly or insignificant flowers
10. Clean up the garden