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Project Difficulty:
Moderate
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To the beginning gardener especially, pruning may seem a daunting task fraught with peril for the plant and the gardener’s conscience. Why remove a branch when it looks perfectly healthy? It’s taken all that time and effort to grow, why should I chop it off? Can pruning harm the plant? Relax. Although there are many types of shrubs with differing pruning requirements, a relatively small number of general principles and practices will serve a wide variety of plants. You just need to find out what pruning category your shrub belongs to. The best way to do this is to ask the salesperson when you purchase a new shrub. Or ask an experienced gardener to take a look at your plant.
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1.
Why Prune?
There are many reasons to prune. Four of the most important are: 1) to improve general plant health 2) to control plant growth 3) to stimulate flowering and fruiting and 4) to rejuvenate the plant.
Removing diseased or damaged branches and stems improves both health and appearance. Removing branches that are too densely packed (called thinning) admits light and air.
Pruning to control and stimulate growth is up to the gardener’s sense of aesthetics. Some people train, trim, and shear shrubs into all sorts of shapes. Others try to maintain the appearance nature provides. Whichever style you prefer, eventually you’ll need to prune to correct or encourage bushy or compact growth or to direct a branch to grow in a certain direction.
Pruning flowering and fruiting plants wisely can improve flower and fruit production. Eliminating older or diseased wood and improving light and air circulation will help increase production. You'll need to be careful not to prune out next year's developing buds, however.
If you have a shrub that looks like a lost cause--overgrown, crowded, weak, unproductive…you get the picture--drastic pruning may revitalize it, if it’s a type that can handle this severe treatment. Shrubs like boxwoods, forsythias, and privets will respond well to severe pruning. Pruning stems to 1 or 2 feet above ground may seem drastic, but in a few weeks, vigorous new growth will appear.
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2.
What Makes a Plant React to Pruning?
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Why prune a growing tip to make a bushier plant or to affect the direction a plant grows in? It's all because of auxins. These are growth hormones produced in the tips of growing shoots. Auxins stimulate growth at the tip and suppress growth along the sides of the shoot. Removing the growing tip and its supply of auxins, releases some of the buds along the shoot, called lateral buds, from dormancy, allowing them to form side shoots (Fig. 1). Although the strength of auxins varies among plants, in general, removing a growing tip encourages growth on the shoot, stem, or branch that remains. (Some conifers--junipers, arborvitae, hemlocks, and many pines--won’t generate new growth if cut back to bare wood or branches that are too old.)
So, consciously or not, we put this principle into practice by shearing a yew or privet to create a dense hedge (encourage bushy growth); or we head-back roses to outward-facing buds to stimulate an open framework of lateral canes (choosing growth direction).
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3.
When to Prune
Remove dead, damaged, or diseased stems as soon as possible. If you’re just snipping a healthy stem or branch here or there, you can do this almost anytime. More extensive pruning of healthy growth is best done at specific times of the year.
For flowering shrubs, the timing depends on whether the plant blooms on new or old growth. Spring-flowering shrubs, such as forsythia and lilacs, usually bloom on growth formed the previous year, so it makes sense to prune them after they bloom. Plants that flower later, such as potentilla and crape myrtle, bloom on the current season’s growth; prune them in late winter or early spring. For plants that bloom more than once, you should wait until they’re dormant to prune them.
Prune non-flowering shrubs in winter or early spring before they leaf out. Prune them in the spring if you wish to encourage a bushy habit, to stimulate or direct growth; this allows a full season for the new growth to develop.
If you live in a cold-winter climate, avoid pruning in the fall, as any new growth induced will be more susceptible to damage from cold, wind, and snow.
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4.
Light Pruning
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Many shrubs thrive for years with a minimum of pruning. Yet, the following techniques can help most any shrub look and feel their best.
1) Pinch or cut off growing tips in the spring to make them bushier and more compact.
2) Pinching off spent flowers, called deadheading, can encourage more flower buds to form for next year.
3) Dense inner growth and branches that rub together can invite plant disease. To allow more air and light to reach inner branches, thin or head them back. Also remove any stems that touch each other, so that they don’t end up damaging themselves when the wind blows.
Note: Heading back means cutting more than just the growing tip. Stems are usually headed back to a lateral bud. Pruning whole stems back to the plant’s crown or severing lateral branches at the crotch where they arise is called thinning (Fig.2).
4) You can head back stems or branches to encourage branching into a particular area. Find a bud facing the direction you want a branch to grow and cut just above it. (Fig. 3)
5) New shoots arising at the base of stems or from roots are called suckers. A few may be desirable; many can produce a thicket. Cut them off flush with the stem base; pull up those sprouting from roots.
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5.
Heavy Pruning
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You can prune some shrubs heavily each year in order to produce robust new growth the following growing season (Fig. 4). Spirea, forsythias, and others are often cut back almost to the ground. Other shrubs are cut back to old wood or to just a few buds on new wood. [Some shrubs, such as scarlet rose mallow (Hibiscus coccineus), will die back to the ground in colder climes but renew themselves each spring. Cut these to the ground in the winter or early spring.]
Heavy pruning can also bring new life to overgrown or tired shrubs. You can rejuvenate some shrubs, such as mahonia, by cutting them almost to the ground. Lilacs and cotoneaster, on the other hand, can be renewed by cutting stems to several feet or longer. You can renew a shrub in stages, cutting one-third of the stems back hard one year, another third the next year, and the remainder in the third year. Sometimes you can nurture selected suckers as replacement stems. Heavy pruning can be risky. Certain plants, including a number of conifers, do not renew from old wood. Before embarking on rejuvenation, consult with an expert to determine the best method.
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