The scaffold limbs and the main structure of a tree usually have been selected by the third or fourth year depending on the kind of tree and its growing conditions.  If the scaffolds are well placed, the tree may need little or no pruning for several years.

Staking to support the tree should not be necessary by this time.  Use short stakes to protect the trunk if there is danger of mover, car or other damage.  Place the stakes so the trunk or branches do not rub against them.

Mature trees may need to be pruned for several reasons.

Tree health and appearance can be improved by removing limbs that are dead, weak, diseased, and insect-infested.  Sources of future infection and infestation also can be reduced.  Many species of insects more readily attack weak trees and limbs than vigorous ones.  Some diseases, too, are more serious on weak trees than on healthy ones.

Pruning can remove new or holdover sources of some diseases.  For example, pruning can reduce the spread of fireblight, a serious disease of many landscape apple, pear, and hawthorn trees as well as pyracantha and cotoneaster.  Be careful to make the cuts in healthy wood well below the infection-12 to 18 inches if possible.  If many trees need attention, the shears should be disinfected after each cut with common household bleach, sodium hypochlorite.  Dip the shears in a 1 part bleach to 9 parts water solution for a couple of seconds.

Household bleach will corrode tool metals with prolonged use.  To minimize corrosion, rinse the tools in running tap water after each day's use, then dry and oil all surfaces.

Anthracnose is often a problem on 3 to 4 year old limbs of London planetrees.  Pollarding or heading these trees back to knobs each year usually reduces the disease.

Remove broken, low, and crossing limbs for appearance and safety.

Open the top of the tree to light so interior leaves and branches can stay healthy and function properly.  High light intensity is necessary for active and productive leaves.

The structural features of a tree may be emphasized by moderate thinning to open the tree to view.  Just another tree many be transformed into a picturesque feature in the landscape.  Pittosporum, dogwood, olive, ginkgo, and many other are particularly suitable.

To open up a medium to large size tree (40 to 60 ft), moderate-size (1 to 2 inches in diameter) thinning cuts of limbs are effective.  Somewhat smaller cuts for smaller size trees are appropriate.  These should be made in the top and around the sides of the tree.  Remove branches that are close to others. In some large trees, cuts may remove limbs up to 6 inches in diameter.  However, such large cuts indicate the tree has not been properly pruned or that its use in the landscape has changed.

Size control of plants is commonly attempted by pruning.  You can most effectively control size by pruning the plant as it begins to reach the desired height.  Delaying pruning until the tree is much larger than wanted make pruning more difficult, cuts harder to hide, and encourages excessive regrowth.

Thinning-out pruning can be used to reduce the height and spread of a plant.  Cut branches to lower laterals (drop crotching).  Some limbs may be removed completely.  A thinned tree retains its natural shape and is less subject to vigorous watersprouts than a headed tree.

Heading or stubbing is the most common way to reduce tree size.  While more rapid than thinning, the results are in most cases much less desirable.  Regrowth is vigorous and upright from the stubs.  The new branches form a compact head, cast dense shade and are weakly attached to the older ones.

For pruning of equal severity, regrowth following heading is more vigorous than that after thinning.  The influence of thinning is spread throughout the tree, while that of heading is concentrated near the cuts.

Pruning for size control should be done while the tree is small so that not more than 25 percent of the leaf area must be removed.  Otherwise, no matter how carefully the tree is pruned, excessive growth results.

Chemically controlling the size of plants offers promise of more efficient tree maintenance, particularly for trees growing under or near utility lines.

Maleic hydrazide has given control of shoot growth in certain species of plants.  Best control with the least injury resulted when trees were sprayed after the basal leaves had fully expanded but before there was much shoot elongation.  The terminal growing point is killed.

Prune the trees the fall or winter before the first spray application.

Spray effectiveness depends on the species, tree condition, and stage of growth.  Low humidity will reduce spray effectiveness.  Spray concentration and application method also influence spray effectiveness.

In certain situations, this may be a desirable growth control procedure.  Use it wisely, however.  Consult your county Agricultural Extension Service for more detailed information.

Crowded trees can result in misshapened trees with branches weakened by shading and rubbing.  In many situations you can remove the smaller, more deformed, or less desirably located trees.  The remaining trees will grow into the new space with improved health and beauty.

Trees are often planted closer together than would be desirable when they are mature size.  As these trees begin to crowd, those that are to be removed should be pruned back more severely each year until they are removed.  This will allow proper development of the more permanent trees while still retaining the value of the temporary trees.

Flowering and fruiting response to pruning depends on the flowering habit, age, and vigor of the plant.

Severe pruning delays the onset of flowering in trees that flower on 1-year-old spurs (e.g. crabapples, pear, cherry).  In fact, no pruning may be needed in some cases.  After the tree has begun flowering, an annual light thinning (10 to 15 percent of the leaf area) to reduce crowded or weak branches will usually be enough to maintain a balance between tree vigor and flowering.

Plants flowering on current-season's growth (e.g. crape myrtle) or on 1-year-old shoots (e.g. peach) usually lower at a young age and profusely when vigorous growth is maintained.  Pruning is one way to stimulate vigorous growth.  Plants with such flowering habits should be pruned more severely than those flowering from 1 year spur buds.

Pruning is not enough to ensure adequate fruit size or prevent limb breakage on trees setting heavy crops of fruit (e.g. plums and apricots).  For these, fruit thinning may be necessary as well.

Some flowering trees set beds on 1-year wood but do not need pruning to maintain vigor and good flower display (e.g. magnolia, eucalyptus, aracia).

Conifers (narrow-leaves evergreens) may present a special problem.  Many do not have latent buds in older wood that will develop after severe pruning.  Some can be pruned more severely than others.  Conifers may be divided into three categories based on the kind of growth they make and these dictate the kind and degree of pruning that can be used.  None of them grow well when pruned to wood lacking leaves.

1) Determine growth - all new growth is from preformed initials in the bud laid down the previous growing season:

Pinus - pines (most but not all)
Abies - firs
Pseudotsuga  - false firs (Douglas-fir and allies)
Picea - spruces

Visible dormant buds must be left.  Soft pinching can be practiced before leaves have reached full size.  New terminal and lateral buds will form.

2) Indeterminate growth - vegetative growth continues as long as conditions are favorable:

Podocarpus - plum yew
Chamaecyparis - false cypress
Thuja - arborvitae
Cupressus - cypress
Calocudrus - incense-cedar
Juniperus - juniper
Taxidium - bald cypress
Sequoia - coast redwood
Sequoiadendron - grant redwood

These trees can be pruned as long as foliage is left below the pruning cut.  New growth usually will arise behind the pruning cut.

3) Intermediate growth - these often make a single flush of growth but can continue growth if favorable conditions exist.

Some Pinus - Monterey, Canary Island, etc.

These will usually respond to pruning if foliage is left below the point of pruning.  They should not be pruned as severely as conifers of indeterminate growth habit.