A properly pruned shrub is a work of art and beauty and does not look as if it has been pruned. Pruning cuts should be hidden inside the plant where they will be covered by remaining leaves. The first step in pruning a shrub is to remove all dead, diseased or injured branches. Remove branches that cross or touch each other and those that look out of place. If the shrub is still too dense or large, remove some of the oldest branches.
Prune Plants by Either Heading Back or by Thinning
When shrubs are headed back or sheared routinely (random cutting of the ends of twigs or young branches to a bud or node), a lot of dense, thick new growth is produced near the outer portions of the canopy. As a result, less light reaches the interior portions of the plant, leaves within the canopy becomes sparse and the plant appears stemmy and top-heavy. To avoid this problem, head back the shrub’s shoots to several different heights. When heading back, make the cut on a slight slant one-quarter inch above a healthy bud. The bud should be facing the direction preferred for new growth.
Thinning (cutting selected branches back to a side branch or main trunk) is usually preferred over heading back. Thinning encourages new growth within the interior portions of a shrub, reduces size and provides a fuller, more attractive plant.
Homeowners and inexperienced landscapers will sometimes make the mistake of planting large-growing shrubs along the foundation of a building or home. As the plants mature they overgrow the site, crowd other plants, hide windows and appear out of scale with the building. When this occurs, it may be necessary to prune severely, called renewal pruning, to bring the plants within bounds.
Renewal pruning means cutting the plants back to within 6 to 12 inches of ground level. In this instance, timing is more important than technique. The best time to prune severely is before spring growth begins. Pruning in late fall or midwinter may encourage new growth which can be injured by cold. Renewal pruning results in abundant new growth by midsummer. Once the new shoots are 6 to 12 inches long, prune the tips to encourage lateral branching and a more compact shrub.
Most broadleaf shrubs such as azaleas, camellias, privets, glossy abelia, nandina and cleyera respond well to renewal pruning. Boxwoods, junipers, pines, cypress, cedar, arborvitae, yews and other narrow-leaf evergreens do not respond when severely pruned and may decline. Transplanting, instead of pruning, may be better for these plants when they overgrow a site.
An alternative to the drastic removal of top-growth on multiple stem shrubs is to cut back all stems at ground level over a period of three years. At the first pruning, remove one-third of the old, mature stems. The following year, take out one-half of the remaining old stems and head back long shoots growing from the previous pruning cuts. At the third pruning in yet another year, remove the remaining old wood and head back the long new shoots.
Shaping Tree-Form Shrubs
Common landscape shrubs, like crape myrtle, yaupon holly, wax myrtle and wax-leaf privet are often pruned as tree forms (shrubs shaped like a tree with one or more main trunks). The best time to begin a tree form is in late winter before spring growth begins. It is easiest to start a tree form from a 1-year-old plant, but you can also use older, mature plants. Select one to three of the most vigorous growing trunks or upright branches (depending on the number of main trunks desired) and prune all other upright (vertical) branches to ground level. Remove lateral branches that are less than 4 feet off the ground along the main trunk and thin the canopy by getting rid of inward growing branches or branches that cross one another. Avoid shearing unless you want a high-maintenance topiary.
A multiple-trunk tree-form can be developed by pruning back to ground level, selecting three to five of the most vigorous new shoots during the growing season to serve as main trunks and removing all others. You can easily remove undesirable shoots by hand while they are young and succulent in the spring. It may take three to five years to shape a tree-form plant, but the interest and accent it lends to the landscape may be worth the extra effort.
Pruning & Training Hedges
Hedges are used primarily as privacy screens. For this reason, it is best to begin pruning plants while they are young to encourage a compact growth habit. Prune young hedge plants (one to two years old) to within 12 inches of ground level. This procedure is called heading back, where stems are shortened and new growth is promoted below the cut. Prune new shoot tips during the growing season to encourage branching. To develop a dense, compact hedge that provides privacy, prune regularly while the plants mature.
Once a hedge reaches the desired height, select an informal or formal pruning style. An informal style is best for a low-maintenance landscape. Informally pruned hedges assume a natural growth habit. Prune only “as needed” to remove dead or diseased wood and head back just enough to maintain the desired height and width.
Formal or clipped hedges require specialized pruning, which can be a continuous job during the growing season. A formal hedge has a sharply defined geometric shape. Follow these two important factors when pruning formal hedges: (1) Clip hedges while the new growth is green and succulent, and (2) Trim plants so the base of the hedge is wider than the top. Hedges pruned with a narrow base will lose their lower leaves and branches because shading from the top growth will not permit sufficient light to penetrate. This condition worsens with age, resulting in sparse growth at ground level and an unattractive hedge that does not give desired privacy.
Flowering hedges grown formally should be sheared after they have bloomed as more frequent shearing reduces the number of blooms. If the blooms are not very important, prune at any time.
More information on pruning is available in the fact sheet HGIC 1003, Pruning Trees.
Suggested Pruning Time for Common Flowering Trees, Shrubs, & Vines
Prune before spring growth begins (produce flowers on current season’s growth)
Beautyberry (Callicarpa species)
Butterfly-bush (Buddleia davidii)
Camellia (Camellia species)
Chastetree (Vitex agnus-castus)
American Cranberrybush Viburnum (Viburnum trilobum)
Crapemyrtle (Lagerstroemia species)
Fragrant Tea Olive (Osmanthus fragrans)
Gardenia (Gardenia jasminoides)
Glossy Abelia (Abelia x grandiflora)
Goldenraintree (Koelreuteria species)
Japanese Barberry (Berberis thunbergii)
Japanese Spirea (Spiraea japonica)
Mimosa (Albizia julibrissin)
Nandina (Nandina domestica)
Rose-of-Sharon (Hibiscus syriacus)
Sourwood (Oxydendrum arboreum)
Anthony Waterer Spirea (Spiraea x bumalda ‘Anthony Waterer’)
Sweetshrub (Calycanthus floridus)
Prune after flowering (produce flowers on previous season’s growth)
Alternate-leaf Butterfly-bush (Buddleia alternifolia)
Azalea (Rhododendron species)
Beautybush (Kolkwitzia amabilis)
Bigleaf Hydrangea (Hydrangea macrophylla)
Bradford Pear (Pyrus calleryana ‘Bradford’)
Bridalwreath Spirea (Spiraea prunifolia)
Clematis (Clematis species)
Crabapple (Malus species)
Deutzia (Deutzia species)
Dogwood (Cornus species)
Doublefile Viburnum (Viburnum plicatum var. tomentosum)
Flowering Almond (Prunus species)
Flowering Cherry (Prunus serrulata)
Flowering Quince (Chaenomeles species)
Forsythia (Forsythia species)
Indian Hawthorn (Raphiolepsis umbellata)
Japanese Kerria (Kerria japonica)
Japanese Pieris (Pieris japonica)
Mockorange (Philadelphus species)
Oakleaf Hydrangea (Hydrangea quercifolia)
Pyracantha (Pyracantha species)
Redbud (Cercis species)
Saucer Magnolia (Magnolia x soulangiana)
Star Magnolia (Magnolia stellata)
Thunberg Spirea (Spiraea thunbergii)
Weigela (Weigela florida)
White Fringetree (Chionanthus virginicus)
Winter Daphne (Daphne odora)
Wisteria (Wisteria species)
Witchhazel (Hamamelis species)
Excerpted from the South Carolina Master Gardener Training Manual, EC 678