Many trees, if left to their own devices, are more likely to look like shrubs at maturity rather than trees. The culprit is poor pruning when the trees are young. Many varieties of trees will send up shrubby growth if they aren't trained to a single stem. While some smaller ornamentals, such as a lilac, can be attractive when allowed to grow in this form, too many trunks can weaken the structure of other trees, like hawthorne. The following guide can help you properly prune a younger tree so it develops a strong trunk and upright form as it matures.
Choose a leader
Minimal pruning is required the first year after planting. Instead, you need to choose the branch that will become the leader, or the main upright "trunk" from which the other branches originate from. Choose the strongest, straightest, and most upright branch to be the leader. Insert a stake, taller than the tree, near the trunk ,and anchor this leader to the stake. This will help keep it straight as the tree grows. (Just remember to loosen the ties periodically so they don't restrict growth.) Then, trim off any other upright branches that try to grow so there is only one leader on the tree.
Trim at the right time
Pruning at the right time is vital, or you can kill the tree. As a general rule, trees should be pruned in late winter as the buds begin to swell but before they actually break and unfurl. This way the tree is poised to grow, so it will heal from the wounds quickly, but pests and disease pathogens aren't yet active to attack the cut. There are some exceptions, as some flowering trees are better pruned right after their bloom cycle. Check the requirements for the specific tree species to be sure.
Remove any suckers
Suckers are the main cause of shrubby growth. These whiplike branches originate near the base of the tree, with some growing up directly from the roots. Spring is typically the worst time for suckers, but they can emerge at any time during the growing season. Cut off suckers just below soil level with a sharp knife. You may have to remove a sucker several times before it stops growing back.
Maintain the branches
Finally, maintain the branches. You need to trim out any damaged or crossed branches. Also, cut off any weak branches that try to grow from beneath the lowest level of branches in the canopy. These can become whiplike and shrubby over time, ruining the form of your tree.
Contact a tree service if you need further help with training your young tree to a single trunk.