The time to prune depends on the kind of tree and the desired results.

Light pruning can be done anytime.  The removal of unwanted growth while it is small is easier and will have less dwarfing effect than if done later.  The removal of broken, dead, weak, or heavily shaded branches will have little or no dwarfing effect on the tree no matter when they are removed.

Rapid plant development can best be maintained if the required pruning is done before the period of rapid growth usually occurring in the spring.  Most deciduous trees can be pruned during the dormant period between leaf fall and spring growth.  Evergreen plants will be set back the least if pruned just before spring growth starts.  A few broadleaved evergreen pants make their most rapid growth after the weather warms later in the season.  Pruning of these plants can be delayed.  Pruning just before the period of most rapid growth will keep the most leaves productive for the longest time.  Also pruning cuts will be quickly concealed by new growth.

To retard plant development, prune when growth is about complete.  The pruning should not be so severe nor so early as to encourage new shoot growth.  For many plants, the time to prune for maximum dwarfing usually would be in late spring to middle summer.

Leaf area will be reduced for the longest period of time.  Pruning cuts should be made so they are not easily seen.

Directing the growth of a young tree can be done effectively during the growing season.  Branches in desired positions can be encourage by pinching back or by removing competing shoots in less desirable positions.

Corrective pruning may be easiest during the growing season.  Branches that are too low because of the weight of leaves and fruit can be partially or completely thinned.  Dead and weak limbs are easily spotted for removal.

Time of pruning to maximize flowering depends on the flowering habit of the tree.

Plants flowering on current-season's growth (e.g. crape myrtle, Japanese pagoda tree and jacaranda) should be pruned during the winter before growth begins.  Moderate to severe pruning will favor larger blossom clusters.

Plants flowering in the spring from buds on 1-year wood, particularly the flowering fruit trees, should be pruned at or near the end of the bloom period.  The blossoms can be enjoyed and then removed before they set fruit that may compete with new shoots.  Vigorous growth will be encouraged o which to bear next year's bloom.

Bleeding of pruning wounds can be heavy on mature trees such as maples and elms.  Bleeding of susceptible trees can be minimized if the cuts are small (less then 3") and made in the fall or early winter.  Bleeding is much more likely if severe pruning is done just before growth begins in the spring.  Bleeding usually is not harmful to the tree.  However, if it is heavy and persistent, it may cause bark injury below the pruning cut.

Cold injury may be increased by pruning.  Some plant (e.g. roses, subtropicals) may be stimulated into new growth by pruning in the fall or early winter.  A pruned plant may begin growth during a warm period in the winter only to be injured when it turns cold again.  These plants should be pruned close to the time growth begins in the spring.

At high elevations where temperatures below 0° F may occur, it is best to delay pruning until just before growth begins in the spring.  Even though growth is not stimulated, pruning may reduce plant hardiness somewhat.