Learning when to prune a particular plant is every bit as important as learning how to do the actual job. The timing is easier to understand if you know a little about plant metabolism. Most plants produce new leaves and stem growth from some point in spring through midsummer. Photosynthesis proceeds most intensively during this time, producing food (in the form of sugars) for the plant. As full summer heat sets in, the sugars are gradually transferred to the plant's woody parts and its roots, where they're stored during winter's dormant period. When spring arrives, the stored sugars are used to start new growth. Pruning is timed to harmonize with this cycle; it is typically done either late in dormancy or during summer. For some plants, a combination of both late-dormancy and summer pruning often yields the best results.
Note: The following guidelines are most pertinent to climates with four distinct seasons and definite winter chill. In warmer-winter areas, timing will vary depending on the particular plant's native climate. If you have any doubts about the best time to prune a particular plant, ask knowledgeable nursery personnel or your Cooperative Extension Office for advice.
Pruning in late dormancy
Many plants, especially deciduous trees and shrubs, are best pruned in late winter or early spring, just before they break dormancy. Heavy frosts have abated, so the plants are less likely to suffer cold damage at the point where you make your cuts. Sugars are still stored in larger branches, trunks, and roots, so little food will be lost to pruning. Deciduous plants are still bare, so you can easily spot broken and awkwardly growing branches and decide how to direct growth. And because growth will soon start, your pruning cuts will stimulate new growth in the direction you want.
For flowering trees and shrubs, you'll need to know if the flowers are produced on old or new growth. If early spring flowers come on last year's wood ― as in the case of forsythia, flowering trees such as peach and plum (Prunus), and flowering quince (Chaenomeles) ― you'll lose many flowers by pruning before plants break dormancy. It's best to wait until flowering has finished, then prune. But plants that bear flowers on leafy new growth formed in spring, such as cinquefoil (Potentilla), can safely be pruned while dormant.
Pruning in summer
A second time to prune is in late summer, when sugars needed for the next year's growth are moving into large limbs, trunks, and roots and will not be seriously depleted by pruning. Some gardeners like to thin plants in summer, since it's easier to see how much thinning is really needed when branches are still thickly foliaged. And because growth is slower at this time of year, pruning is less likely to stimulate new growth ― an advantage when you're thinning. In cold-winter regions, don't do summer pruning later than one month before the first frost; if you do, an early frost may damage the plant at the point of the cuts.
Some notes on pruning evergreens
Though evergreen trees and shrubs do not drop their leaves, they approach a near-dormant state during the winter months. The group includes broad-leafed evergreens ― such as boxwood (Buxus) and camellia ― and conifers, among them spruce (Picea), pine (Pinus), and many others (a very few conifers are deciduous; the larch, Larix, is an example).
Broad-leafed evergreens are usually best pruned in late dormancy or in summer, as outlined above. For flowering broad-leafed evergreens, however, timing is a bit more precise; you'll need to prune with an eye toward preserving flower buds. Prune after bloom for evergreens flowering on last season's growth; prune before spring growth begins for those that bloom on new growth.
Most conifers are pruned only in their first 2 or 3 years, in order to direct their basic shape; from then on, they're best left alone. Some of the most badly botched pruning you'll see is on conifers that have been pruned too severely, usually to keep them confined to a too-small location ― though a few conifers, including arborvitae (Platycladus and Thuja), yew (Taxus), and hemlock (Tsuga), lend themselves to shearing into hedges. When you do need to prune a conifer, the timing will depend on whether the plant is a whorl-branching or random-branching type.
In whorl-branching conifers, the branches radiate out from the trunk in whorls. Members of this group include fir (Abies), spruce (Picea), and pine (Pinus). These trees produce all their new growth in spring; buds appear at the tips of new shoots as well as along their length and at their bases. On pines, the new shoots are called candles, since that's what they look like until the needles open out.
Prune whorl-branched conifers in early spring. To induce branching, you can pinch or cut anywhere along the new growth, being sure to do so before the shoots harden. When the tree is still relatively small, you can nip back the pliant new growth of the leader (the central upward-growing stem) and all side branches to make a denser, bushier plant. If you cut into an older stem, however ― even at a point where it bears foliage ― no new growth will sprout from ― below the cut.
Unlike whorl-branching sorts, random-branching conifers have branches that grow randomly along the trunk. These plants don't limit their new growth to spring, but grow in spurts throughout the growing season. Trees of this type include cedar (Cedrus), cypress (Cupressus), dawn redwood (Metasequoia), redwood (Sequoia), giant sequoia (Sequoiadendron), bald cypress (Taxodium), and hemlock (Tsuga). These can be pruned much as you would deciduous and broad-leafed evergreen trees. New growth will sprout from below your pruning cuts as long as the remaining branch bears some foliage; in general, no new growth will develop from bare branches (hemlock is an exception). It's best to prune random-branching conifers right before new growth begins in spring, though they do allow you a little more leeway in timing than whorl-branching types do.