Written by: Certified Arborist August Hoppe, WI-0477A
The definition of pruning is simple: removal of a part of a tree. How a tree reacts to pruning and why pruning can be beneficial is a more complex story. Pruning is a critical part in creating a successful long-lived landscape tree. Pruning is necessary because landscape trees grow differently than forest-grown trees.
The biggest difference is that forest-grown trees compete constantly with one another for light. They grow tall and skinny, expending all their efforts into growing upright to obtain light with single stems reaching skyward. This means less branching off and a more direct growth upwards. Little pruning is needed. With landscape trees, the opposite is true. These trees are planted with space to grow, lots of available sunlight and the ability to spread outwards, rather than just upwards.
This creates an entirely different canopy structure than their forest-grown cousins. Multiple leaders, long horizontal limbs with excessive end weight, rubbing and crisscrossing limbs can all be common occurrences as branches grow and develop throughout the tree in various places with little perceived order. These wayward branches can become significant defects in a tree and increase the chance of limb failure and thus shorter-lived trees.
Pruning for Structure
Pruning while young is the easiest way to set up your tree for success. The arborist will examine and prune the tree in a way to increase the strength and ability to hold up against snow, ice and wind. Competing leaders are removed or shortened to allow one leader to become dominant (just like a forest-grown tree). This can drastically reduce the chance of the tree splitting at a union where two limbs of equal size originate. Interfering branches can grow and rub on each other or weaken the main limbs of the tree. This often means removing branches that compete with these main scaffold limbs of the tree. Numerous smaller cuts are often better than one to two larger cuts at the trunk. These smaller cuts seal over faster with less chance of long-term decay setting in - yet another reason to prune while trees are young with smaller branches.
Pruning for Disease Control
Pruning also can promote better health for the tree! Opening the canopy by removing water sprout branches can promote better air flow and more sunlight penetration into the tree. This slows the spread of fungal diseases, such as tar spot on maple, rust disease on hawthorns and apple scab disease on crab apples. Fungal disease on evergreen trees can be potentially checked with good pruning as well. Raising up low branches can increase the air flow, thus drying out the foliage faster and slowing the spread of fungal diseases.
Pruning for Safety
Dead limbs can pose an obvious problem for the safety of people or structures below them. Cracked or damaged limbs may fail prematurely and cause undue risk. Limbs with heavy weight on the ends may not be able to be supported as well as more upright-growing limbs.
Every tree and situation is different, but on average, smaller young trees should be pruned frequently, every two to three years or so to create a good long-term structure. Medium-aged trees often need pruning every three to five years, while older mature trees can often go up to five to seven years before requiring pruning again.