null

WINTER PRUNING

by Katie Himanga, Heartwood Forestry

Winter is such a great time of year to prune most trees. It has many advantages over summer pruning.

  • Defective tree parts such as dead, cracked or broken branches, weak forks and branch unions and other structural defects are easy to see when trees are without leaves.
  • Trees can adapt to the loss of branches by adjusting the size or number of leaves the following season.
  • Trees rapidly develop callus tissue around pruning cuts during the following summer, especially on cuts made in late winter.
  • Tree pathogens are dormant so there is little risk of spreading infectious diseases.
  • Frozen soil supports heavy equipment such as bucket trucks with little or no lawn damage.
  • High-quality mulch can be processed from leafless debris.
  • Qualified arborists and tree workers are more readily available.

WHY PRUNE?

Most of the economic and risk management benefits of winter pruning come from removing defective tree branches and dealing with defective forks on tree trunks: dead, cracked or broken branches; branch union with included bark and other structural problems. Too often, tree owners delay pruning until a tree looks unsightly. Then they prune for appearance. The result may be a tree that looks okay, but is full of defects that set the stage for storm damage and premature death. Even worse is the old technique of topping trees, which leads to trees full of defects and decay. Prune trees for safety and tree health first. Consider appearance after that.

Prune dead areasPRUNING PRIORITIES:

FOR SAFETY

  • Remove dead, damaged or broken branches, or branches that are weakly attached.
  • Train a young tree to have one main trunk by pruning off branches(double leaders) that turn up and compete with it.
  • Train a young tree to have well-spaced, well-attached side branches.
  • Remove branches that interfere with the sidewalk, the street or other human needs.
  • Let a professional with special training and special equipment remove branches that grow toward electrical power lines.

FOR HEALTH

  • If two branches cross or rub, remove the least desirable branch. Remove diseased and low-vigor branches, suckers and water sprouts.
  • Prune low, temporary branches so they stay smaller than half the diameter of the trunk.
  • If a branch rubs on a sign, wire or other object that might damage the bark, remove it or prune it back to a side branch that is growing in a different direction.
  • Thin branches for good structure, air movement, light penetration and/or weight reduction.
  • Look for girdling roots and cut them before they strangle the tree.

FOR APPEARANCE

Before you prune for appearance, consider:

  • What is the natural shape and character of this tree? If a tree has the natural pyramidal form of a littleleaf linden, do not try to change it into the rounded form of an Ohio buckeye. If wide, gnarled limbs are part of the character of an old oak, don't try to transform it to the tidy, rounded from of a young maple.
  • What is the function of this tree? If a tree is supposed to slow the winter winds or block an unsightly view, don't remove the lower branches that do that job. If a tree is supposed to frame a view, not hide it, remove branches that are in the way. (A word of caution: Be patient and let your young tree get tall enough before you remove lower branches.) You can remove selected branches from the crown of the tree to allow a view of something as long as you do not remove more than one-fourth of the foliage of the tree.

How to PruneHOW TO PRUNE

Before you prune, inspect your tree from the top down and plan the work. Remember tree safety and health come before appearance. Make pruning cuts with respect for the natural defense system of the tree. Make cuts at branch unions. Leave branch collars. Use sharp tools and make smooth cuts. There should not be any loose bark around the cut. If a branch is too big to hold in your hand, use three cuts to remove it. When you must cut the main part of a branch back to a side branch, the side branch should be large enough to become the new leader. Select a side branch that is one third or more the diameter of the branch that you must cut off.

Learn to tell the difference between a strong branch union and one that is weak. A strong union is U- shaped and has a ridge of branch bark running through it. A weak branch attachment has a narrow, V-shaped union and bark turns into the union and is trapped between the branches. Never damage a branch collar with a pruning cut.

Plan which branches to remove so your tree develops well-spaced side branches. For a strong tree, the side branches should be less than one half the width of the main trunk.

WHEN TO PRUNE

Start winter pruning in November and finish by April 15. Avoid pruning live wood from trees when leaves are forming or falling. During these times, the tree is busy either putting on spring wood and new leaves or storing starch and putting on new roots. Less energy is available to respond to pruning wounds than at other times of year. There is no need to cover tree wounds with wound dressing during winter pruning operations. In the spring, sap will flow from late-winter pruning wounds on a maple or a birch. It is not harmful to the tree.

Begin pruning a young tree the first winter after planting. Prune sparingly if last season's twig growth was less than 4" in length. The tree needs more time to recover from the shock of transplanting. Once the tree recovers vigor, focus on removing defective tree parts so little problems don't grow into big problems. Removing one limb of a defective fork on the tree trunk may mean the difference between a tree that splits in two the first time the wind gusts more than 60 m.p.h. and a tree that stands up to the elements for decades.

Removing dead and damaged branches are all that is usually needed for a mature tree that has had regular care throughout its life.

PRUNING A YOUNG TREE

  • Prune every one to two years.
  • Limit pruning to one fourth of the live branches per year.
  • Look specifically for branches that turn up and compete with the leader (trunk) and remove them completely or head them back to slow down their growth.
  • Leave lower branches on the tree for several years, but keep them small relative to the trunk until it's time for removal.
  • Select the main side (scaffold) branches that are well spaced along the tree trunk; about 18 inches apart on large species trees; 6-8 inches apart on small species trees.
  • Remove broken, split or rubbing branches and those that interfere with the tree trunk or the main branches.

Strong union on left, weak union on rightPRUNING A MIDDLE-AGED TREE

  • Prune every two to four years.
  • If you need a ladder or a chain saw, hire a professional arborist.
  • Remove fewer live branches than you would on a young tree.
  • Continue to prune as for a young tree to develop a strong trunk and well-spaced, well-attached branches.
  • Remove problem branches such as those blocking site lines or rubbing on buildings.
  • As the height of the tree increases, remove temporary lower branches in the bottom one-third of the tree. PRUNING A MATURE TREE
  • Hire a professional arborist to prune the tree every five years.
  • Remove dead, cracked and broken branches and only a few live ones if necessary.
  • Never remove more than one-fourth of the live foliage in a single season.
  • When old, dead branches have collars grown out along the branch, remove the branch and leave the collar uninjured.

PRUNING A STORM- DAMAGED TREE

  • Never top a storm-damaged tree!
  • A seriously damaged tree may need replacement.
  • Properly prune dead, dying, broken or cracked branches.
  • If you must leave a branch whose end is broken, remove only the broken part without cutting into the undamaged part.
  • Remove loose bark, but don't disturb live bark that is still attached to wood. Wait one growing season to prune for appearance.
  • Hire a professional arborist to prune the tree if there are broken or cracked branches higher than you can reach from the ground or if you need a chain saw.

PRUNING MANAGEMENT

To estimate the number of trees you need to prune each year, take the total number of trees under your jurisdiction that are more than five years old and divide by five. Add to that the number of trees that are up to five years old. If well-maintained trees is your goal, plan to prune at least that many trees each year.

As is true in other professions, the field of arboriculture has industry standards or standards of practice. Standards help clients and arborists communicate on what work is to be accomplished during a tree-care operation. Anyone who writes tree maintenance specifications should be familiar with Standard Practices for Trees, Shrub and Other Woody Plant Maintenance, ANSI A300. It is available from the American National Standards Institute or from the National Arborist Association. ANSI A300 defines general categories, or types, of pruning. If you hire a tree service company, you can specify which type of pruning best fits your budget and your needs. In addition to the type of pruning, you must specify the minimum and maximum size branches to remove. The ISA booklet Tree Pruning Guidelines complements ANSI A300. The standards tell what to do; the guidelines explain how to do it. For people who don't work with trees every day, the guidelines are easier to understand.

Ask any arborist you plan to hire if he or she is "ISA Certified." Certified Arborists carry an International Society of Arboriculture (ISA) Certification Identification Card. To become certified, an individual with at least three years of tree-care experience must pass an examination. To stay certified, the arborist completes 30 hours of continuing education every three years. ISA certification is a good indication of knowledge about tree care, but it is not a guarantee of quality work.

Katie Himanga is a consulting forester and owner of Heartwood Forestry, Lake City, MN. Among other things, she teaches tree pruning and line clearing in Minnesota, Wisconsin and North Dakota and is an I.S.A. Certified Arborist.

Parts of this article are extracted from Tree Trust Community Outreach Manual, 1996. Used with permission. To purchase this manual, contact Tree Trust at 612-920-9326.