The Why and When of Pruning - Brooklyn Botanic Garden
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The Why and When of Pruning

Down the road from me is a sugarhouse that neighbors long ago abandoned. It is encased in Boston ivy (Parthenocissus tricuspidata). Left to its own devices, the vine has wound and wrapped and crisscrossed and crept until the shack has disappeared. What's left is a sugarhouse made solely of leaves, green in summer and maroon in autumn. Only in winter are the supporting wood walls and slate roof visible.

Every time I pass I am reminded of the delicious description by the French novelist Colette of the exuberant but sinister Wisteria that grew on the garden walls where she was born (Flowers and Fruit, 1986). As she sits in her former home, the wisteria strikes:

The sudden shattering of a windowpane made me shudder, and decided it: a vegetable arm, crooked, twisted, in which I had no difficulty recognizing the workings, the surreptitious approach, the reptilian mind of the wisteria, had just struck, broken, and entered.

That's what happens when you leave some plants to their own devices. Gardeners soon learn that they must exercise a bit of authority over what they grow. What Colette—and my neighbors—needed was a pair of sharp pruning shears. Every gardener needs a pair, and a pruning saw as well.

Nature has its own ways of keeping plants in check. Fire reorders landscapes for decades, sometimes forever. Competition between plants allows the stronger and taller and longer-lived to conquer or hamper the weaker and smaller and fugacious. Shallow-rooted trees topple when the ground becomes saturated from rain, while leaders—the main, or terminal, shoots—die back when there is too little moisture. Limbs bend under the weight of ice and break in high winds. Diseases and insects take their toll, as do larger animals: elk, moose, porcupines, beavers, rabbits, mice, birds, and whitetail deer, the plant pruners par excellence in my region of New England.

It's our bad luck that nature doesn't always do what we want, especially on small city and suburban lots. I live on 15 wooded acres and have room to let most things grow as they will, but I don't want the climbing hydrangea (Hydrangea petiolaris) to shroud the guest-room windows, nor do I want the red maples (Acer rubrum) to shade out the tree lilac (Syringa reticulata). I do want the summersweet (Clethra alnifolia) by my patio to stay small and compact, and I'd like flowers covering the entire 'Comtesse de Bouchaud' clematis in my atrium, not just the top four feet. California friends treasure their marbled bamboo hedge (Chimonobambusa marmorea), but they don't want it to take over their yard.

Many gardeners never think about training or cutting back plants until the neighbor is bleeding from a head-to-limb encounter with the crabapple (Malus) someone planted too close to a path or it's impossible to get past the eastern white pine (Pinus strobus) and through the front door. These are pruning SOSs. Reminders, too, that many pruning jobs are avoided if you choose the right plant for the site.

Beginning a new garden? Do yourself a favor and think waaay ahead when buying woody plants. Those five-foot blue spruces that perfectly frame the front door now can't be kept that small forever. Boston ivy won't just cover a wall, it will cover the neighborhood. Rhododendrons the size of laundry baskets get to be the size of garden sheds. Like the national debt, most plants just keep getting bigger. If you must prune a plant drastically once or twice a year to make it fit its spot, it's the wrong plant—or the right plant in the wrong spot.

Gardeners with zillions of acres who want shade can plant a pin oak. But if you have a small yard and also want to grow cosmos and Shasta daisies, Quercus palustris isn't for you. The happy news is that your choices aren't wholly limited just because your acreage is pinched. Breeders have been busy miniaturizing just about every species known to humankind, as well as grafting standards on dwarfing rootstocks to keep them small. We gardeners, to paraphrase slightly, can have our plants and keep them too.

But if there is a huge and spectacularly fragrant Syringa vulgaris growing too close to the back door and you can't bear cutting it down, there is still a solution. Pruning. Bonsai—which involves grooming a woody plant that would normally tower over the gardener to be 11 inches tall and look as if it predated Moses—is as much art as craft, but everyday pruning can be learned. And put an accent on "everyday." I once lived in a house owned by a small midwestern college. That college's approach—carried out just before the alumni and parents arrived for commencement—was to top every shrub and vine and remove all dangling tree limbs. Come May, anything green between 4 feet and 15 feet off the ground was history. English garden writer David Joyce calls this the "vague notion that at some time in the year all plants need to be tidied up."

Examples of dreadful pruning are everywhere: You have only to look under the power lines of any city to explain why so many of us are timid about taking a sharp blade to a plant we have spent years encouraging to grow. But any gardener can learn to prune properly. The secret is that pruning must be planned and ongoing. The outcome is that anyone looking at your work won't see it. Good pruning is like good acting: It is invisible.

The Reasons Why

Pruning is removing parts of a plant, either above or below ground. Training, an associative term, usually refers to shaping a plant to a desired form. Gardeners prune and train to ensure safety; to maintain a plant's good health; to limit or promote growth; to shape; to encourage flowering and fruiting; and to renew and repair. Put more simply, you prune to keep yourself safe, to keep plants hale and hearty, and to make them do—as far as humanly possible and sensible—what you want them to do. Don't assume that frequent pruning is necessary for every woody plant in your landscape. Desultory pruning is a mistake too. Always know why you're sawing, snipping, and shearing.

Pruning for Safety    Making sure woody plants don't injure people or structures should be the first item on anyone's pruning agenda. Pruning for safety includes removing branches that threaten to fall and do damage; trimming limbs that may interfere with utility lines; cutting back growth that blocks the line of sight at driveway and street intersections; curbing thorny plants that endanger passersby; lightening vines that threaten to bring down trellises and other supports; and taming plants that are growing into someone else's yard. In addition to protecting yourself and your family, the objective is to be a good neighbor. And to avoid being sued.

Pruning for Good Health    For good health, begin with the two D's: Prune wood that is damaged or dead; it always needs to come off. Also remove branches that rub against each other, as well as branches with weak, unnatural branch-union angles, as they are vulnerable to breaking. Remove suckers and thin out water sprouts, or epicormic shoots, which are weakly attached vertical shoots that emerge from latent buds.

Pruning to Limit Growth    Owners of Lilliputian lots have a special interest in hog-tying plants. The math is simple: Smaller equals more. One standard pear tree dominates a tiny garden, but espalier that tree—train it against a vertical surface—and there is space left for a spicebush (Lindera benzoin) and a firethorn (Pyracantha coccinea), and you'll still have fresh pears for the picking.

Everyone who gardens will want to constrain something sometime. Most species are cooperative if you don't wait until they are shading the roof of your house or have secondary limbs the size of a boxer's neck. Eastern arborvitae (Thuja occidentalis), for instance, stretches to 50 feet on its own, but you can keep it at 5 feet or reduce its girth if that's what will safeguard your view of the mountains. If cooling a plant's size is the aim, follow the country wisdom: "Weak growth, prune hard; strong growth, prune light."

That adage comes from the ostensibly illogical fact that pruning can stimulate growth, especially severe pruning undertaken in early spring. Cut back a healthy main shoot, and it will sprout secondary shoots. (More about this below.) Pruning can be a way to promote growth as well as to curb it.

Pruning to Shape    The last word in plant shaping is topiary—an art invented some 2,000 years ago by the Romans—but even gardeners who don't want boxwood peacocks and yew sheep grazing on their lawn may have an interest in tailoring their plants. Most of the work can be done with hand pruners, and the usual goal is aesthetic.

Unless you yearn for green obelisks to frame the front door or you're maintaining a hedge, it's smart to follow the plant's lead. Most species are inherently graceful. Forsythia, for example, is far more beautiful when just the old stems are removed and the remaining stems are allowed to cascade freely than when it's cut back severely. Different shrubs and trees have distinct forms. Trying to change those natural patterns, especially with large trees, is a time-consuming and often frustrating undertaking.

You may want to prune off low limbs to reveal attractive bark or interesting stem and trunk forms—a technique called lifting—or remove errant shoots, or shear a conifer to encourage symmetry. And pruning and shaping doesn't have to mean artificial. Japanese gardens are the epitome of controlled growth: Every tree and shrub is painstakingly trimmed in order to appear natural. Trees with gardener-encouraged multistem trunks or asymmetrical shapes may be exactly what your "natural" landscape needs.

Pruning to Encourage Flowering and Fruiting    Pruning is a fundamental tool of fruit growers, who shape and control their trees to enhance their crops. But pruning shouldn't be left to the orchardists. Flowers and fruits (as well as leaves and stems) are important in the ornamental garden. Their colors, size, and number can be enhanced by strategic pruning.

For example, pruning young plants stimulates vegetative growth and delays the production of flowers and fruits. Rosarians who want jumbo blooms for the annual garden show prune their plants heavily, but they do less cutting back if more flowers is the goal. Fruit-bearing ornamental shrubs, such as winterberry (Ilex verticillata), American cranberry bush (Viburnum trilobum), and Rosa rugosa, bear more fruits if pruned only lightly or not at all.

Many gardeners regularly cut back Kerria japonica, red osier and Tartarian dogwoods (Cornus stolonifera, C. alba), and other shrubs grown for their brightly hued stems to promote new shoots, which are more intensely colored than older ones. Leaf color of other plants, such as purple-leafed filbert (Corylus maxima 'Purpurea') and redtip (Photinia x fraseri), can be augmented if not magnified by well-timed pruning.

Pruning to Repair or Renew    Repairing or renewing a mature plant can be ugly—there's no instant gratification in this kind of pruning. But when a tree, shrub, or vine is damaged or allowed to outgrow its space, drastic surgery may be required. Severe pruning, which may mean topping (removing most or all of a plant's crown) or cutting plants to the ground, carries risks. Some species grow back successfully, but others, such as arborvitae, cedar, juniper, pine, and other conifers, respond less successfully—or not at all. If a mature tree needs to be topped, remove it instead.

Before the First Cut

A southern California cousin of mine was surprised to learn that Vermonters don't set out tomatoes in March. Climate doesn't only affect when gardeners plant, it affects how and when we prune. Trees in warm, humid regions need open canopies; in hot, arid conditions, heavy, compact growth is an asset, not a liability. In some parts of the country, late winter means February, in others April. Like politics, all gardening is local, so consult neighborhood gardeners, local growers, nursery owners, and other authorities for advice about when to prune.

It's also important to know something about how plants grow before you head for the garden armed with loppers and saw. In a nutshell, pruning affects plants' size and the way they grow, and it alters their carbohydrate-to-nitrogen ratio.

Size reduction seems straightforward enough: You remove growth and the plant is smaller. But it's not quite that easy, for pruning also stimulates growth. Confusing, yes, but the bottom line is that pruning, done correctly, yields a plant smaller than it was before you pruned.

All new growth comes from buds. Plants grow up and out from the tips of their shoots. Woody species, the subject of this book, have lateral, or auxiliary, buds that are arranged along branches in different ways—either opposite or alternate—depending on the plant. These buds, which arise where leaves attach, at nodes, are separated by sections of branch called internodes. Woody plants also have latent buds, less visible dormant buds that lie under the bark.

Both lateral and latent buds take their growing orders from the apical, or terminal, bud that is located at the tip of the branch. Apical buds produce auxins, hormones that suppress the growth of the lateral buds below the tip, an effect botanists call apical dominance. Cut off the apical bud, which stops the production of auxins, and the lateral and latent buds closest to the cut are signaled to grow. (In addition to affecting lateral budbreak, apical dominance also influences the length of lateral shoots and the angle at which they are joined to the limb.)

The potency of apical dominance not only varies from one species to another but is influenced by orientation: Lateral buds on horizontal branches are less restrained by the apical bud, while lateral buds on vertical growth are strongly suppressed. Train a crabapple's limb to grow horizontally, remove the apical bud, and the lateral buds will erupt in water sprouts that you'll need to remove. Age, too, has an effect. Apical dominance is pronounced in young plants but less strong in older plants, which is why many conifers lose their Christmas-tree shape and develop rounded crowns when they mature. Similarly, if you head back a year-old stem, the effect on the lateral buds is more pronounced than if you head back a branch that is ten years old.

Finally, the vigor of the new growth is also influenced by where you cut. The farther back you cut a shoot, the more robust the new growth. (The plant is attempting to keep its root system in balance by regrowing its top.) Pinch out the growing tip of a stem and the effect is modest; cut back a shoot by two thirds and the result is an onslaught of sprouting lateral buds.

Pruning a plant also changes its carbohydrate-nitrogen balance. Removing vegetative growth reduces both stored carbohydrates and their manufacture (carbohydrates are produced in the leaves). As a result, the higher level of nitrogen in the plant stimulates vegetative growth at the expense of reproductive growth. So pruning usually means more shoots and leaves and fewer flowers and fruits. Severe pruning, especially of young plants, can mean no flowers and fruits for several years. (Alternatively, root pruning lowers the nitrogen level and encourages both root and reproductive growth, as nutrients, including nitrogen, are taken up by the roots.)

When to Cut

The traditional wisdom "Prune when the knife is sharp" is only good advice part of the time. You can prune any time of year without killing your plants, but you may weaken them if you do so over and over again. For some woody plants, when you scissor and saw isn't crucial, but that isn't true for many species, especially if flowers or fruits are your aim. If you want to realize your plants' "ornamental potential," as current gardenspeak puts it, you need to prune at the appropriate time.

Right now is always the best time to remove dead or damaged wood. Responding quickly reduces the chance of more difficulties later. And don't wait to remove growth that is dangerous or poses a liability, such as a large limb that threatens to fall on your neighbor's new Jaguar XK8 convertible. Don't delay pruning new shrubs to eliminate crossing branches, multiple central leaders, and other structural problems. But let newly planted trees grow for at least a year before you prune.

In tropical zones like those of southern Florida and southern California, gardeners prune throughout the year (although even there it's a good idea to observe plants' growth cycles). For the rest of us, residents of temperate climates, the "when" of pruning is consequential. The worst time to prune is in spring immediately after new growth has developed, because you're removing the foliage that is replenishing the plant's food supply.

The late dormant season—just before new growth begins—is the best time for pruning most woody plants. With exceptions, of course. Severely cut back a lilac (Syringa) or an American cranberry bush (Viburnum trilobum) in late winter, and come late spring there will be few blossoms to enjoy. Pruning flowering species at the correct time of year is especially important.

Fall is prime pruning time for gardeners who have very mild winters. Since cutting back promotes growth, cold-climate gardeners should limit their autumn pruning to avoid winterkill.

Winter, specifically late winter when days begin to lengthen but before new growth begins, is the time to prune summer-flowering plants that set buds on new growth in spring, such as Camellia, crape myrtles (Lagerstroemia), goldenrain tree (Koelreuteria paniculata), and mimosa (Albizia julibrissin). This is also the time for much general pruning and for renewal pruning, as new growth is only weeks away and wounds will be exposed only a short time before they seal. In some tree species like maples (Acer) and birches (Betula), sap will flow, but this is normal and not harmful to the tree, provided that pruning cuts are made correctly. (Another option is to prune these trees in late summer, when less sap will be lost.) Late winter, when leaves are down, is the easiest time to evaluate the "framework" of deciduous shrubs and trees that may need reshaping.

Spring, after growth begins, is an acceptable time to cut out damaged or dead wood; to remove small shoots heading in the wrong direction; to pinch back new growth; and to shear conifer hedges. In general, pruning in late spring strains deciduous plants and is not recommended.

Summer—early summer—is the time to prune spring-flowering plants that set buds the previous fall and have just finished blooming. Crabapples (Malus), Forsythia, lilacs (Syringa), and Wisteria are prominent examples. Because new growth slows in late summer, that's also a good time to remove suckers and thin out water sprouts, to trim hedges, to basal-prune evergreens, to pinch back stems after flowering, and to prune trees that have outgrown their location.

Avoid pruning plants when they are especially open to attacks by insects or diseases. Oaks, for instance, are vulnerable to oak wilt if they are pruned before late summer; plants susceptible to fire blight, such as apples and flowering crabapples (Malus), pears (Pyrus), mountain ash (Sorbus aucuparia), hawthorns (Crataegus), and Spiraea, should not be pruned in spring or summer, or during warm, humid conditions when fire blight spreads freely. There's always less chance of transmitting diseases if you prune when the weather is dry rather than when it's wet.

Maybe a rose is a rose is a rose, but you can't assume that all cultivars of the same species can be pruned at the same time or the same way. Some Clematis cultivars flower on the current season's growth, or new wood; other cultivars flower on old wood, shoots produced the previous year; and still others flower on both old and new wood. Each group is pruned differently. Generalizations are helpful, but generalizations are not particulars. No single rule works for all plants.

Pruning Roots

In addition to growing up from the tips of their stems, plants grow down (and out) from the tips of their roots. Root pruning, which removes many of those tips, reduces a plant's water and nutrient uptake, changes its carbohydrate-nitrogen balance, and slows its growth.

Exposed roots of healthy, young shrubs and trees can be removed. If you're worried about how much root to cut off, err on the side of caution. Girdling roots, roots that wrap around the base of a young plant, should be removed as soon as you detect them. Never remove large girdling roots of mature trees.

Root pruning—slicing into the soil with a spade at the plant's drip line—is one way to slow growth and is often used to prepare field plants for transplanting. The technique is more effective with shrubs and vines than with trees, although espaliered trees respond to root pruning.

Remember the Risks

While many species are altogether forgiving, sawing and slicing always carries some risk, especially severe sawing and slicing. The best way to minimize the dangers is to prune at the correct time and in the correct way. The golden rule of pruning is "When in doubt, don't." Many trees, shrubs, and vines do fine without interference. Too much pruning may destroy their natural grace. Palms, rhododendrons, magnolias, and many conifers, such as Norfolk Island pines (Araucaria heterophylla), are just a few plants that should be approached carefully. If you've placed them where they have room to spread, leave them alone except for maintenance pruning and a shaping cut here and there.

Karen Davis Cutler, who has edited five previous BBG handbooks—Essential Tools, Salad Gardens, Tantalizing Tomatoes, Flowering Vines, and Starting from Seed—gardens on 15 acres in northern Vermont. A frequent contributor to national garden magazines, her latest book is The New England Gardener's Book of Lists (2000).

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Image, top of page: Elizabeth Peters