Trimming for tree health

In a tree pruning workshop at Montana State University's Northern Agricultural Research Center March 19, Research Associate Roger Hybner offered tips on pruning practices for healthy trees.

Trees and shrubs are pruned for safety, plant health and aesthetic reasons, Hybner said.

In other words, sometimes branches are trimmed to keep them from damaging property or people, or blocking a path or view. Sometimes the plant itself needs the help because a branch is growing in a direction that will end up weakening the structure or because a part of the plant is diseased or dead. Other times, a tree or shrub is trimmed to make it looked more balanced or to shape it.

Pests, Mother Nature and poor management can kill trees, Hybner said, but given their regenerative properties, "a tree is a plant that should never die."

Two of the most common mankind-generated problems that threaten a tree's longevity are choosing the wrong plant species and choosing the wrong location.

Hybner said planting trees and shrubs that are native to the area is the best aid to keeping plants healthy and thriving. But, when buying nonnative trees and shrubs, people should buy those rated for Winter Hardiness Zone 3 weather conditions, though he added that some Zone 4 species can thrive if they have adequate shelter from the harshest elements.

While people can do well getting plants that are tolerant of the generally alkaline, clay-based soil most common in north-central Montana, he said other localized conditions can affect plants, as well. A nearby saline leach, a septic system's drain field, or run-off from a feedlot can alter the chemical properties of the soil and the amount of moisture sitting in the soil.

Two of the best ways to help choose the proper plant for an area are to look at what is growing well in the neighbors' yards, he said, and ask local county Extension offices and USDA Natural Resources Conservation Service for suggestions and resources.

The location a tree or shrub is planted might stunt or kill the plant, such as planting a spruce tree, which likes drier soils, near a septic system drain field.

Trees planted under power lines or too close to a house or other building may need to be cut back in ways that are detrimental to the health of the plant, Hybner said. Trees that might otherwise be healthy are weakened because the top half, one side, or even a big V-shape in the middle, are lopped off to protect a power line or building. People should be aware of how tall and wide a tree or shrub has the potential to grow and plant them accordingly, he added.

A dwarf variety of tree might be able to be planted under a high power line where a towering cottonwood would be too tall, he said.

A shrub planted next to a house, sidewalk or fence should be a variety of low-growing plant tolerant of pruning and training - which should be done from the beginning - to keep the plant from blocking a porch or window view or invading the walkway or fence line.

Hybner said he sees junipers and lilacs most often planted too close to buildings and walkways.

It is better to err on the side of caution by planting farther away from structures than seems necessary, Hybner said, adding that his rule of thumb for a tree is to measure or step off the appropriate distance away from the nearest structure, then take two more steps away.

The First Pruning

Pruning begins prior to planting a new tree or shrub, Hybner said, adding that this is the only time pruning includes the roots as well as the limbs.

If the tree has a double leader - a trunk that splits into two branches, both growing upward - then one of the leaders has to be pruned off to give the tree a single, strong trunk, he said.

The hole for the tree should be twice the diameter of the pot in which the tree has been growing, he said, because the roots need to be spread out in the hole.

Preparing the tree for healthy root growth should include pruning the outside layer of the root ball - where thread-like roots grow around the inside of the pot - and the remaining stout roots spread out in the bottom of the hole. Any roots that cross another and can't be straightened need to be pruned, and the largest root should be pointing toward the prevailing wind to give the tree the most strength against the stress of repetitive and high winds.

If the young tree has been grafted to a root structure, as apple trees are, the graft needs to be above ground, he added.

Any branches growing in the wrong direction, as described in the side bar at right, should be trimmed, but Hybner cautioned that if the young tree has branches low on the trunk, those should be left on the tree.

"I've never been comfortable with pruning a tree to a stick in this area," said Hybner, who grew up north of Rudyard.

Those lowest branches gather nutrition from the sun while the tree grows above. Each year after the first or second year, one layer of lower branches, starting with the lowest, can be trimmed off until the lowest branch is the desired height off the ground.

Tree trunks always grow in height from the top of the tree, so the height a branch grows out from the trunk will always remain the same. A low-growing, 3/4-inch tree limb that is hitting people in the face will never get farther off the ground, but maybe one day it will be a 3" limb that really hurts to be run into. Keeping on top of the pruning will save hassles and save stressing the tree later by taking a major limb.

Pruning for Health

Dead limbs and suckers can be trimmed from trees and shrubs any time of the year, but a few rules of thumb will help make pruning helpful rather than hurtful, Hybner said.

• Pruning of deciduous trees and shrubs, including fruit-bearing, should be done after the worst cold of winter has passed and before the leaves bud out. The exception to this rule is that flowering shrubs that do not grow fruit, such as lilacs, should be trimmed after the flowers have died. Evergreens are trimmed after the first growth has started because the tree needs the pitch to be running to seal the wounds from pruning. Formal hedges have their own development and maintenance needs that homeowners will have to research.

• Do not prune more than 25 percent of a tree's live growth in any year - and less if the tree is already stressed from damage or disease.

• Cut limbs at a 45-degree angle 1/8 to 1/4 inch past a node or bud. New growth will come from the bud or node, and any limb left past that bud will turn into a dead stub and might get diseased or cause more limb death.

• When trimming a limb from the trunk, don't leave a stub. Cut the limb 1/8 inch to 1/4 inch from the branch collar, which is the fatter, swollen part of the limb where it connects to the trunk. That branch collar will grow, or form a callous, around the open wound left after a limb is cut off.

• Be careful not to cut into or shave off the branch collar.

• Do not rip the bark - it doesn't grow back. Keeping tools sharp is crucial, and when using a saw to cut large limbs, use a three-step method in which the first two cuts remove the majority of the limb and the third cut prunes the limb near the branch collar.

The first cut is across the bottom side of the limb, out away from the branch collar. The second cut is across the top, at a point slightly farther down the limb from the first cut - a half inch is sufficient - and it cuts through the limb. The third cut trims the limb at the branch collar, without the weight of the limb to cause problems.

• Frequently step back and view the tree from different sides to make sure the pruning is creating a balanced tree.

• Limbs can be trained to a certain extent. A limb growing at less than a 45-degree angle away from the trunk should be removed or trained to grow out lower by bracing it away from the trunk or weighting the branch to make it droop.

Old Growth, Unkempt Trees

Whether it's a single tree in the yard, or rows of trees in a wind break, sometimes the tree growth gets away from the property owner.

All is not lost, though.

Hybner said even trees that have a lot of dead limbs and stubs can be revived by pruning.

He also emphasized the importance of sticking to the limit of pruning no more than 25 percent of growth in a year. In cases when it is hard to tell which limbs are alive and which are dead, he recommended waiting to prune

"When in doubt about whether or not to trim a limb, leave it until the next year," he said. "I'd wait until the tree greens up in the spring then tie some ribbon around the dead limbs so I know what to cut off next spring."

Be careful with trees that have not be kept up with regular pruning, Hybner said. They can have hidden weaknesses from rotted or poorly formed limbs or trunks. Branches being held down by a crossing limb can pop free once the ill-growing limb is cut off. And weak roots can give way.

When trimming nonhedge shrubs, especially fruit bearing shrubs, he said, any main trunk that grows to more than 1" diameter should be cut down at the base of the shrub and younger growth allowed to freshen the plant.

For people who want to plant trees, whether as a yard decoration or a windbreak, he recommended searching out native species but to stay away from "trashy" species, such as willow, which lose a lot of limbs every year.

If the trees are to be planted in a shelter belt, and the landowner wants to cut down on the amount of pruning needed, Hybner recommended planting in tree clusters that include fruit-bearing shrubs. This creates a micro-habitat for birds, deer and other wildlife, that is left to grow as in nature. Only the ground around the clusters is kept grass-free.

While this publication is liekly appearing after deciduous trees have begun budding leaves, summer is still a good time to remove dead growth and stubs, as well as suckers and diseased limbs.

This is also a good time to examine the plants' health and formulate a plan for future pruning of live limbs.

Evergreens and flowering shrubs are likely reaching time for pruning.

Some of Hybner's recommended sources include:

• "Pruning Made Easy," by Lewis Hill.

• MSU Cooperative Extension bulletins: "Pruning Deciduous Trees," "Pruning Fruit Trees in Montana" and "What's Wrong with This Tree?"

• Arbor Day Foundation's "Right Tree for the Right Place."

• Internet search of "Alex Shigo Tree Pruning" for links to a variety of articles, educational materials and videos.

• USDA Natural Resources Conservation Service, http://www.nrcs.usda.gov.

(This story originally ran in the April edition of the Farm and Ranch tab.)

Feeding the Young Tree

People need to resist putting a lot of nutrients in the hole with the newly planted tree, Hybner said. This will discourage the tree from spreading out its roots in search of more nutrients. Trees need the roots to grow out to make the tree more stable. The soil should be mixed with 1/3 compost to 2/3 soil from the hole.

Another recommendation from Hybner for developing strong roots is to avoid drip irrigation, which encourages roots to grow on the surface. It is better, he said, to give the tree a good soaking on occasion.

And fertilizer spikes don't work, he said. People will do more for their tree spending money on ground fabric around the base of the tree to keep grass and weeds away. These smaller plants will use up the tree's nutrients and invite harm from weed-eating and mowing around the base of the tree.

Which Limbs to Cut

Hybner recommended pruning in this order:

• Dead limbs

• Suckers - unless you are trimming overly mature growth from a shrub and suckers are needed to refill the plant's shape.

• Limbs growing straight up, straight down, toward the trunk, or across another limb.

• Unnecessary limbs and growth - including too many limbs in one area, an over-long limb or a double leader.

• Diseased vegetation - which is trimmed back 6 inches from the last sign of disease. Cuttings with disease should be burned right away and all pruning tools cleaned with hydrogen peroxide before being used on another plant.