Get Out the Shears: When to Prune Shrubs and Why
DK - Learn to Garden , 2008 Dorling Kindersley Limited
Mophead hydrangeas will flower on stems produced the previous year. Prune old stems by up to 12 inches down to pairs of fat, healthy buds.
Keep an eye on your shrubs throughout the year, cutting out dead, diseased, and damaged stems to keep the plants healthy. Remove any stems that are crossing and rubbing against each other, all-green shoots on variegated plants, and suckers on grafted plants. Cutting out badly placed stems and reshaping plants will improve their appearance. Some pruning methods increase growth, fruiting, and flowering, as well as stimulating the production of young, decorative stems.
When to Prune
As a general rule of thumb, prune late-summer and fall-flowering plants, which usually bloom on the current year’s stems, in early spring. Spring and early-summer flowering plants bloom on the previous year’s wood, and should be pruned after flowering, so that the new stems have time to ripen before winter. To renovate most deciduous trees and shrubs, prune in winter when the plants are dormant. For the majority of evergreens, prune in spring, just as the new growth resumes. Bear in mind that pruning deciduous plants in winter results in vigorous regrowth in spring. Pruning in spring and early summer produces much thinner growth, while summer pruning reduces the leafy canopy and the plant’s food resources, resulting in relatively weak regrowth.
If you are not familiar with your shrubs, prune them lightly, and see how they respond during the growing season. Prune newly-planted shrubs cautiously, as they need to grow to develop their natural beauty—although you may wish to encourage a good shape with a little pruning. For example, boxwood (Buxus) or yew (Taxus) can be clipped lightly into a sphere. Other plants, like heathers, just need shearing over to remove spent flowers, and to promote neat growth. Tip pruning, where the shoot tips are removed, can be carried out on young shrubs to encourage a bushy habit. Many plants that should be pruned cautiously will respond to renovative pruning when they become too large for their site, their stems have become congested, or flowering is poor. In these cases, cut back the stems to near ground level when the plant is dormant, or buds are just breaking, in late winter or early spring.
Cutting shrubs back to half their size works for those that resent being pruned to a low framework, but flower on new wood, such as sun roses (Cistus). Prune shrubs that flower on shoots produced the previous year, such as broom, after flowering. Remove the old flowering stems and shorten new stems to encourage more sideshoots—they will bloom the following year.
Cutting back to soil level works for plants that send up shoots from below the ground, such as brambles (Rubus). The resulting canes are either colorful, or will flower and fruit prolifically. Leave a low framework of stems, 8–12 in (20–30 cm) high, from which new shoots can arise. Plants that produce young, colored stems, such as dogwoods (Cornus) and willows (Salix), can be pruned in this way in late winter.