PRUNING: WHEN, WHY, AND HOW?
Pruning can be done at any time of the year. When you should prune is really a question of why you are pruning your plant.
TO PRUNE OR NOT TO PRUNE?
One of the things you should figure out before you start pruning is: Do I have to do this? Remember that plants are not pruned regularly in the wild. Many species of plant actually look better in a natural shape. If you prune a flowering shrub before it flowers, you may cut off the reason you bought your shrub in the first place. If you cut the hard wood of an evergreen instead of the fine foliage, you may create an unsightly hole. So be sure that you have to prune and why you are doing it before you proceed. Your plants will look all the better for it!
PRUNING DEAD, DISEASED, OR DAMAGED MATERIAL
If you are pruning dead, diseased, or damaged material out, any time of the year is fine as it won't hurt the plant as much as leaving this material in. Dead material can begin to rot on the tree or shrub, and this can lead to disease.
PRUNING TO CAUSE GROWTH
If you are pruning to cause growth, pruning in early spring is best as you remove new buds. The buds that remain will be fewer in number and all the growth will be forced into these buds. Evergreens should be pruned lightly as the foliage is almost all new buds.
PRUNING TO SHAPE
If you are pruning to shape a plant, you should prune in early spring. This allows you to remove the branches and foliage you don't wish to grow before they get large. If you are pruning to maintain the shape of a plant or to keep its size the same, pruning in summer (June to July) is best (see SUMMER PRUNING below). This cuts down on the amount of food the plant will have stored for next winter and the spring burst of growth won't be as great.
Pruning in the summer shouldn't be done after mid-August in our region as our summers are relatively short. The plants will need some time to recover before the long winter sets in. NOTE: Pruning in summer should never be heavy. It forces the tree or shrub to produce new wood. This wood is not very hardy and may be killed by even mild winter weather.
Thinning a plant so that it appears more open is often very heavy pruning. It should be done early enough so that the plant can recover or after the growing season is over. If not, the new growth caused by the heavy pruning may die during the winter.
GENERAL PRUNING TIPS:
1. Pruning shears should not be of the "Hammer and Anvil" type. These are shears in which the cutting blade stops in a groove. The shears crush the limb instead of slicing through it. While this may work for awhile when the shears are sharp, as they become dull the wounds will become jagged and torn. If the wound isn't clean and smooth disease can take hold. Use shears in which the cutting blade passes next to the opposite bar instead of into it.
2. When pruning woody material, do not cut into the area where the branch connects to the rest of the shrub. Cutting into this raised area (that is called a collar) can cause problems. Limbs should be cut as close to the collar as possible without actually cutting it.
3. Foliage pruning (as with evergreens) shouldn't be very heavy. The heavier the cut, the less leaf there is to make food for the plant. No matter how formal the shape of the plant it always looks better if the foliage appears rich and full.
4. Most evergreens will look better if pruned or sheared each year. Cedars, Junipers, Hemlocks, and Chamaecyparis should be given a light trimming in late May or early June. Pine and Spruce should be pruned to shape when the candles (new growth) have nearly reached full growth - early to mid June for most varieties; Colorado Spruce a bit later - perhaps the third week of June.
5. Fruit tree blossoms should be picked off for the first year, to direct the plant's energy into growth, rather than fruit production.