Another fall chore is to get out the chainsaw and tackle those trees vulnerable to toppling or broken branches this winter – especially the ones threatening roofs. UW-Madison horticulturist Laura Jull shares a few tips on pruning deciduous trees.
Most deciduous trees should be pruned in late fall to winter. At this time of year, you can see the overall branch structure easily, and most insects and disease causing organisms are no longer active. Late fall/winter pruning is especially important for oak trees to help prevent spread of the fungus that causes oak wilt. Late spring and summer are usually not good times of year to prune because disease pathogens are present and wound closure is slower. If you prune in late winter, some trees may bleed or ooze sap excessively in the early spring. Such bleeding may be unsightly, but does not harm the tree, notes Jull. Examples of trees that bleed excessively are maple, willow, birch, walnut, beech, hornbeam, elm and yellowwood.
She says after a young tree is established two to five years, it can be pruned to encourage a well-branched canopy. Lower branches can be removed to raise the canopy, too. Scaffold branches to be maintained in the tree should be selected so they’re 12 to 18 inches apart, evenly distributed around the trunk and have wide crotch angles. Remove no more than a third of the total crown of a tree at one time. Young trees also need corrective pruning to remove crossing branches, double leaders, watersprouts and root suckers.
Jull defines a few tree-trimming terms:
Branch collar – The ring of trunk tissue that surrounds a lateral branch at the point of attachment to the stem.
Double leaders – Two major, terminal growing points located at the top of the tree.
Root suckers – Vigorous upright, adventitious shoots that arise from latent buds below the graft union or at the base of the tree.
Scaffold branches – The large branches that form the main structure of the crown of a tree.
Topping – an improper pruning technique that reduces the height of a tree by removal of large branches back to larger primary branches. This technique is not recommended.
Watersprouts – Vigorous, vertical, adventitious shoots that arise from latent buds above the ground or graft union on older wood.
Older, established trees, if properly trained when young, require little pruning. Jull says older trees should never be topped as that leads to poor branch structure and increased limb breakage. “Use the three-point method of limb removal for pruning large branches (See diagram and description below). This method ensures proper pruning and closure of wounds.
Before you prune, disinfect the pruning tools with alcohol or a 10 percent bleach solution before pruning each tree to avoid spreading diseases. On large limbs, the first cut should be 12 to 18 inches form the limb’s point of attachment. The pruning cut should be an undercut made halfway through the branch (see diagram). This cut is very important because it relieves weight from the branch collar and prevents accidental tearing of bark from the tree’s trunk when the limb is removed.
The second pruning cut should be made on the outside of the first cut (i.e. farther from the trunk). Cut all the way through the limb from the top down, thus removing the weight of the branch.
The final cut should be made next to the tree’s trunk outside of the branch collar. Cut from the top down and cut all the way through the remaining branch stub. The branch collar should be left intact. Jull stresses not to cut the branch flush with the tree’s trunk. A proper cut avoids large wounds, and allows the tree’s wound to close quickly.
In general, wound treatments, such as tree paint or wound dressing, aren’t recommended. These compounds slow down wound closure and promote decay, according to Jull. One exception is oaks pruned during the growing season; using wound treatments on oaks is important to keep out insects that transmit the oak wilt fungus.
Bert Swanson at the University of Minnesota says mice, rabbits and deer can all damage trees in the winter by feeding on the tender twigs, bark and foliage. They can girdle trees and shrubs and eat shrubs to the ground line. Deer can also cause significant injury and breakage by rubbing their antlers on trees during the fall.
Trees can be protected from rodent damage by placing a cylinder of quarter-inch mesh hardware cloth around the trunk. The cylinder should extend 2 to 3 inches below the ground line for mice and 18 to 24 inches above the anticipated snow line for rabbit protection (See the diagram). Hardware cloth can be left on year-round, but it must be larger than the trunk to allow for growth. For small trees, plastic tree guards are also effective.
Sun scald is characterized by elongated, sunken, dried or cracked areas of dead bark, usually on the south or southwest side of a tree. On cold winter days, the sun can heat up bark to the point where cambial activity is stimulated. When the sun is blocked by a cloud, hill, or building, bark temperature drops rapidly, killing the active tissue.
Young trees, newly planted trees, and thin-barked trees (such as cherry, crabapple, honey locust, linden, maple, mountain ash and plum) are most susceptible to sun scald. Trees that have been pruned to raise the lower branches are also sensitive because the lower trunk is no longer shaded. Older trees are less subject to sun scald because the thicker bark can insulate dormant tissue from the sun’s heat ensuring the tissue will remain dormant and cold hardy.
Sun scald can be prevented by wrapping the trunk with a commercial tree wrap, plastic tree guards, or any other light-colored material. The wrap will reflect the sun and keep the bark at a more constant temperature. Put the wrap on in the fall and remove it in the spring after the last frost. Newly planted trees should be wrapped for at least two winters and thin-barked species up to five winters or more.
To repair sun scald damage, cut the dead bark back to live tissue with a sharp knife, following the general shape of the wound, rounding off any sharp corners to facilitate healing. Wrap the trunk in subsequent winters to prevent further damage. Do not use a wound dressing. Spraying the area with a fungicide may help prevent fungal infection of the wound.
Roots don’t become dormant in the winter as quickly as stems, branches and buds, and roots are less hardy than stems. Roots of most trees and shrubs that grow in the Upper Midwest are killed at temperatures at or below zero to 10 degrees. These plants survive because soil temperatures normally are much higher than air temperatures and because soil cools down much more slowly than air temperature.
Many factors influence soil temperature. Moist soil holds more heat than dry soil, so frost penetration will be deeper and soil temperatures colder for droughty soils. With newly planted trees, cracks in the planting hole backfill will allow cold air to penetrate into the root zone, reducing fall root growth or killing newly formed roots.
To encourage fall root growth and to reduce root injury, mulch new trees and shrubs with 6 to 8 inches of wood chips or straw. If the fall has been dry, water heavily before the ground freezes to reduce frost penetration. Check new plantings for cracks in the soil and fill them with soil.