General Pruning Instructions: Pruning trees and bushes is part art and part science. You need to take care of the structural needs of the plant first and then prune for the aesthetics you want to achieve.
Newly planted Plants: New plants should be left to grow for 2 or 3 years before regular pruning is started. A new plant needs to establish its root system to become established and it needs the energy that photosynthesis provides to expand its root system. This is why you should only prune out branches that were broken in transport and branches that are growing into the center of the tree or are already rubbing on other branches and creating wounds in the bark. Otherwise let the plant get established before you begin regular pruning.
Structural Health: You create a healthy structure by removing branches that are growing into the center of the tree and any branches that are rubbing together causing damage to the bark. You should also remove any branches that are coming off the trunk at a 90 degree angle or more or less than 20 degree angle since roughly a 45 degree angle is structurally the strongest and will resist breaking under heavy snows. There are several exceptions to this advice where the branches on certain types of tree always grow off the trunk at near a 90 degree angle, such as Pagoda Dogwoods, spruce trees and certain weeping trees, and where certain types of trees (specialty cultivars) always keep their branches close to the trunk and will have less than a 20 degree angle.
Disease Prevention: Thinning the branches of larger bushes, roses, and trees allows better air flow and will help prevent fungal diseases that attack many plants leaves.
Aesthetically Pleasing: The final stage of pruning is to create a shape and size that is pleasing to you when you look at your garden.
Maintaining an appropriate size: The most common goal for small bushes is to keep them at a size appropriate to their setting.
Disease prevention: is also a goal for some bushes susceptible to fungal diseases such as roses.
To reinvigorate the bushes: Renewal pruning will bring struggling or overgrown bushes (that can be renewal pruned) back to vigorous health if they have not been left to struggle too long.
Times to Prune: Winter, and especially late winter (February 15th to March 31st) is a good time to prune bushes and trees (i.e. second half of February and all of March). The leaves are off and you can see the branching structure of the plant and find the dead wood.
Summer and late fall: are also good times to prune.
Times not to prune: Early fall, You should not prune during the second half of September through November. Pruning in the early fall while the weather is still warm will stimulate new growth that will freeze off in the winter.
Spring, April through the first of June is a poor time to prune because the plants is pushing out the new growth and the plant will bleed excessively. Wait till the leaves have hardened off to their darker summer green.
Note: There are restrictions on pruning oak trees from March 31st to October 15th because the picnic beetle that spreads oak wilt is active between these dates.
Evergreens are better pruned in late winter (March) or the summer, if they are pruned in late fall or mid-winter the cut tips tend to wind burn.
Note: Flowering trees and bushes: You should make sure you prune flowering trees and bushes within a month after they bloom to make sure you are not pruning off next year's flowers.
How to Prune: Start by removing any dead wood. Then remove branches that are growing into the center and will eventually rub on other branches. Cut them right back to the trunk so you are not leaving stubs that will open the tree up to latter infections. Each branch has what is called a collar. This is a bulge in the bark where the branch meets the trunk. This is a special type of bark that will quickly grow over the wound and protect the cut off branch from infection. Do not damage this collar but cut right up to it.
Remove any branches that are rubbing on each other (less of a problem for bushes than for trees). Remove the branch that looks the weakest or is growing in an undesirable way.
Thin the interior branches of trees and to some extent the exterior branches so that air can move easily through the tree. This will help prevent problems with leaf funguses. Be careful to not open up gaping holes that will be unsightly.
Artistic Part of Pruning: After the first steps, step back from the tree or bush and look at the overall form. Here is where the artistic part comes in. Prune to shape the tree or bush in a way that is attractive to you. When pruning the tip of a small branch leave a bud at the tip that will grow in a direction that you want. Never top off the leader (the main trunk growing tip) on a tree. This will lead to all sorts of problems in the future.
Opposite branched or budded plants: When pruning trees and bushes where the branches grow out directly opposite each other, clip off the branch down near the buds or smaller branches and then clip off one of the buds, leaving the bud/branch that will grow in the direction you want that stem to grow in. Otherwise you will have a branch end that looks very bushy and clumsy especially in the winter when the leaves are gone.
Alternate branched or budded plants: Pruning the tip of plants where the buds/branches alternate up and down the stem is easier. Clip the branch tip back to a bud/branch that is growing in the direction you want the overall branch to grow in.
Note: Clipping a branch randomly without picking a direction to grow in will result in a bunch of growth at the end which looks like a fuzz ball at the end of a stem.
Renewal Pruning: Certain types of bushes need to be renewal pruned. Examples are lilacs, bush types of dogwoods, viburnum bushes, forsythia, raspberries and more. To renewal prune you take out up to 20% of the old bush stems a year once the bush is full grown. Cut stems down as low as possible to minimize the chances of rotting fungus setting in.
Hedges: Certain bushes can be pruned with a hedge pruner to keep them small or to create a hedge. Spirea and Potentilla are the easiest bushes to prune. At the end of the year they can be chopped in half every year if you want to keep them roughly the same size or cut down to one third to shrink next year's size.
Other bushes such as yews, privets and boxwoods can be clipped to form hedges with a hedge shear. After hedging the outer branches make sure you go into the bushes and tip back some of the inner branches to make the bush fill out.