Timing Your Pruning Jobs
You can tell a lot about gardeners by watching them prune. I tend to whip through the garden like a cyclone, leaving a wake of clipped branches and stems behind me. Others are meticulous in their efforts, making a careful study of the plant before and after each cut and cleaning up debris as they go.
Whether you are a Tasmanian Devil, Nervous Nelly or fall somewhere in between, pruning does not have to be daunting. As long as you use good, sharp tools, make a clean cut and consider the growth habit of the plant you will be safe. And when in doubt you can't go wrong by just removing dead wood and branches that cross one another. I think the hardest aspect of pruning is deciding when it is the appropriate time to tackle the job.
Here is a general list of plant groups with some tips on when they should be pruned. As additional resources I suggest you read Lee Reich's The Pruning Book (Taunton Press 1997) and Peter McHoy's Pruning - A Practical Guide (Abbeville Press 1993).
Clematis is broken down into 3 pruning groups. Group A Early Flowering clematis blooms on the previous season's growth and should be pruned immediately after they flower.
Group B Large Flowering Hybrids often bloom twice in a season. Lightly prune in February or March to remove dead and diseased wood, but wait until after the first flowering to do any heavy cutting to manage size.
Group C Late Flowering clematis such as "Sweet Autumn Clematis" should be cut back to 12 to 24 inches from the ground in February or March.
Most hardwood trees should be pruned in winter while they are dormant. This allows you to see the branches and make cuts that will maintain the tree's natural shape. It also gives the tree a full growing season to heal. Cut the branches off right above the branch collar. This is the area at the juncture of the limb and the tree. You can identify it by the whorls of wrinkled bark. Cutting just above this area rather than flat against the tree will ensure quicker healing. This area of the tree contains special anti-microbial chemicals and phenols, which help inhibit decay. If the cut is made here it's not necessary to use pruning paint, nature will take care of it.
Hybrid tea, old-fashioned and climbing roses should be pruned right before the leaf buds break. In my zone 7 garden, I do this in late February. If you live in a cold climate, pruning should be done when you remove winter protection and the danger of frost has passed. An exception to this time frame is the old-fashioned roses that flower once each growing season, such as Damasks and Mosses. These varieties bloom on old wood and should be pruned in the summer after they have flowered.
The rule of thumb for pruning flowering shrubs is if it flowers after May the 15th, prune it in late winter or early spring for lots of bloom in summer. Prune shrubs like forsythia, quince and azaleas that flower before May the 15th as soon as the plant finishes flowering. This is because summer bloomers flower on new wood while spring flowering shrubs produce flower buds the previous growing season.
For the best bloom production follow the form of the plant rather than shearing it into a box or a ball. I think you will also find that the shrub just looks better when allowed to retain its natural shape.
Flowering trees follow the same rule as flowering shrubs. Summer bloomers like crape myrtles should be cut back in late winter, but spring flowering crabapples, redbuds and dogwoods should be pruned immediately after they bloom.
When it comes time to prune spring flowering trees, they will often already be leafed out. To make the job easier, mark the branches that you want to cut with paint or a ribbon while the tree is still dormant and you can clearly see the branch structure.
In general, broadleaf evergreens such as hollies and boxwoods don't require much pruning. I find that a light pruning with a sharp pair of shears in spring before new growth begins and then again in summer works best. These broadleaf varieties will produce new growth where the cuts are made.
Caution should be used when pruning needle type evergreens such as pine or spruce because they don't bounce back from a bad "haircut." These types of evergreens should only be pruned to remove diseased/damaged wood. This can be done any time of the year except when temperatures are below zero.