Spring flowering shrubs, such as the hydrangea, azalea or lilac, should be pruned immediately after they flower. This is because the plant flowers on old wood - that is, wood that grows and hardens during the summer. Pruning in the fall or winter trims off next year's flowers.
Summer flowering shrubs such as the butterfly bush, potentilla or the newer varieties of "everblooming" hydrangeas and lilacs can be pruned in late fall. Flower buds on these plants develop on new wood, or in the case of the Endless Summer hydrangeas, on both old and new wood, so an extensive pruning late in the season should encourage significant new growth during the spring and summer. In growing zones 4 through 6, for example, it's quite common for gardeners to chop butterfly bushes (Buddleia) all the way down to ground level and cover them with a thick layer of mulch. With the shallow roots protected from freezing, the plant springs up bushier and more floriforous the following year.
Once-blooming roses, such as the rugosas and many of the heirloom varieties, should be pruned fairly heavily after they bloom. Pruning, along with alfalfa tea, encourages a lot of new growth and basal breaks that result in flowering canes. Tea, floribunda and the "Knockout" roses should be pruned regularly during the growing season. Deadheading allows new flower buds to form all summer. Stop pruning roses in August, however. New growth produced in late summer or during the fall may not be hardened off enough to be able to withstand the rigors of winter weather.
Related to the rose family, raspberries and blackberries that are spring-bearing only should be pruned after they have borne fruit. Cane fruits bear their fruit on two year old canes; canes only produce one set of flowers and fruit. Old canes should be cut down to ground level, and the newer canes trimmed so that they produce side shoots that will eventually flower. Ever-bearing raspberries produce two crops each year - one in spring and a second smaller crop in the fall. They produce flowers on new growth, first at the tips and then from the side shoots. Ever-bearers should be cut back to about 2 feet after the first bearing; this will stimulate the production of side shoots and will result in a larger fall crop. Many growers prefer to have only one large crop per season, and cut raspberries down to ground level every fall after the fall crop has been harvested.
Broad-leaved evergreens such as holly or inkberry should be pruned in early spring, just before they begin their growth spurt. Needled evergreens should only have damaged or diseased branches cut out, and this should be done in June or July after their new growth has elongated. Pine trees can have their candles pinched in order to create a fuller appearance, but no conifer should have the inactive center of the whorl trimmed. Only the yews (Taxus spp.) can withstand hard pruning.
Using good sharp tools not only makes the job go faster, but it also minimizes damage to the tree or shrub. Clean cuts heal more quickly than those with ragged edges. Use good quality hand pruning shears that are ergonomically designed on branches under 1" thick; move to loppers or a pruning saw for branches thicker than an inch. If any of the plant material shows signs of disease, disinfect pruning tools with a solution of chlorine bleach and water or with an alcohol-based mouthwash such as Listerine before using them on healthy plants and prior to putting them away.