All types of shrubs and bushes benefit from annual pruning to keep them healthy, shape the wood and promote new growth. Novice gardeners may be nervous about improperly pruning their shrubs, and shrubs that are poorly pruned can weaken and die. Avoid improper pruning of shrubs by avoiding poor timing and poor technique.
Timing for Summer-Flowering Shrubs
No matter what cuts you make when trimming back shrubs, any cut can be considered improper if you prune at the wrong time of year. Pruning shrubs late in the fall can encourage new growth right before winter, leaving the plant vulnerable to cold damage. For shrubs that flower in the summer, such as hydrangea, butterfly bush and most spirea, the best time to prune is in late winter or early spring when the plants are still dormant.
Timing for Spring-Flowering Shrubs
Spring-flowering shrubs should not be pruned in late winter or early spring. If you prune them then, you will remove flower buds and greatly decrease the performance of your spring-flowering shrub that season. For spring-flowering shrubs, which include lilacs, forsythia, and most azaleas, wait until the plant finishes its display and prune in late spring to early summer.
Removing Too Much Growth
Shrubs benefit from thinning out, but removing too much wood in one year can harm rather than help your shrub. The Arizona Master Gardeners' Manual advises removing no more than 1/3 of the old growth wood in a given year. When trimming evergreen shrubs avoid cutting all needles off the branch. Most evergreen shrubs, like juniper, have a dead zone in the center of the plant, and cutting too much growth off the stem means that part of the plant will not regenerate needles.
Keep those hedge clippers in your garage since most shrubs dislike shearing. Shearing cuts all the shrub branches at the same level, using a flat cut. All shrubs have a natural shape, which shearing ignores. Only use hedge clippers to prune shrubs that are part of a hedge, and be sure to shear hedges the correct way. For the hedge to grow properly all branches must receive light, so a hedge should have a pyramidal shape. Hedges that are trimmed in a straight vertical line or are wider at the top than the bottom have been improperly pruned.
Improper pruning includes neglecting to prune shrubs when they need it. Shrubs benefit from annual pruning to remove dead and diseased branches, and neglecting shrubs could exacerbate the spread of disease in your garden. You may not need to prune newly planted or young shrubs for the first couple of seasons, but after that they should be pruned regularly at the correct time of year.
Tools and Their Use
Avoid improper pruning by having the correct tools on hand. Avoid hedge clippers except for hedges, as noted. Use pruning shears to make small cuts and lopping shears or a pruning saw for cuts through branches thicker than 3/4 inch. Don't cut a branch off flush against the parent branch; instead cut it off just above the branch collar, which is a swollen area at the point where the branch attaches.
When removing dead or diseased branches, spray your pruning tools with a disinfectant such as denatured alcohol after every cut to sanitize them and avoid spreading disease. Use gardening gloves when pruning shrubs with thorns or prickers, such as rose bushes.
Care of Tools
Maintain your tools to avoid problems with pruning. Keep them sharp and rust free so that the cuts they make are smooth and clean. Disinfect them before you put them away after each pruning session. As mentioned above, disinfect tools after each cut when pruning away dead or diseased branches.