What does that mean?

• Training means pruning young trees for form during the first few years of their existence.
• Good nursery stock already has had quite a bit of training by the time it reaches the market. In fact, that is an important trait to look for when you buy.

• Trees are trained 
• to direct growth
• to correct structural weakness
• to adapt the tree to its human environment
  
• In the long run, a trained tree will be stronger, healthier, safer--and cheaper to maintain.
• Early training is better because the extent of infection from wounding depends greatly upon 1) the size of the wound, and 2) the age of the tree.  In general, the smaller the wound and the younger the tree, the less decay will result.

                                                                      Amur maple after being trained for scaffold branches

• Select a single central leader if the young tree does not have one, and prune out any competitors. One central leader, or stem, is almost always preferable in street trees.

• Some species (such as many conifers) usually produce a single leader and need no training, others (such as sugar maple) often produce multiple leaders and need quite a bit; most species are in the middle, and do well with moderate training.

• Select what will be the lowest permanent ("scaffold") branch, and prune out nearby competitors. Choose a branch that 1) is vigorous, and 2) has the right clearance.

• Remember that once a branch is formed at a given level, it never gets any higher.

• Select and prune for other scaffolding branches.

• Choose vigorous branches less than half the size of the central leader that are 
• well spaced apart (at least 18" for large trees)
• well distributed around the trunk.

• Finally, make temporary branches by cutting back branches you will not be keeping that are below or between the scaffolding branches.

• do not remove these branches for now, just reduce them to a few buds
• the leaves on temporary branches produce food for the young tree, and protect the young bark from the sun; they can be removed later when shaded out.
• Don’t take off more than 25% of the leaves and buds at any one time.

• Prune nothing the first growing season after planting except broken, rubbing, or misshapen branches. The transplanted tree needs all its leaves to reestablish its roots.
• Many communities schedule the first training for the third year after planting, a time when remulching can also be done
  
• Most species are best trained in late winter, though badly resprouting species (such as lindens or crabapples) are better pruned soon after leaves have fully expanded.

ISA, "Pruning Young Trees."  For other information, advice and help on this topic, call offices of your State Urban Forestry Coordinator or University Extension service, or visit urban forestry web sites.