The best time to prune most trees and shrubs is during the late fall or early summer, when the plants enter their dormant stage. Trimming during this period of rest protects the trees from using stored energy to form new growth before the spring warmth arrives. This is the time to shape your trees and shrubs, removing any long, overgrown branches that give these plants a lopsided or uneven appearance. While fall is best for most varieties, some flowering shrubs, such as certain varieties of roses, require trimming immediate after the flowers fade in the springtime. Pruning right after blossoming, rather than during dormancy, avoids removing excessive vegetation that supplies the old growth necessary for forming spring blossoms.
Even though annual pruning serves to retain the overall shape of shrubs and trees, some plants require severe pruning to restore healthy, attractive growth. Lilacs are one type of shrub that frequently require rejuvenation pruning to remove the old, woody growth that overwhelms the new growth and fails to produce healthy blossoms. Pruning off all but about 8 inches of old growth during the late winter promotes healthy new growth but also tends to eliminate the flower formation for one or two years.
Maintenance pruning is necessary throughout the growing season. This type of pruning involves trimming away dead, broken or diseased branches and limbs as soon as they appear. Cutting off the damaged vegetation slightly above the problem area will help discourage new damage due to disease pathogens and pest infestations.
Disinfecting your limb saw or pruning shears will help protect healthy trees and shrubs from infections due to contact with contaminated tools. Remove all the trimmed vegetation from the areas beneath trees and shrubs to discourage the growth of fungus and bacteria near the bases of your plants. Avoid placing diseased tree and shrub growth in your compost bin.