While They Are Sleeping: Dormant Tree Care

But after their annual, multihued climax, trees are typically forgotten until the spring – left to battle the cold months ahead alone. Many people don’t understand that winter’s high winds, heavy snow and ice, frozen soil and temperature fluctuations can damage trees, particularly those that stand isolated on residential and commercial landscapes without protection from other forest trees.

While landscape tasks become a fraction of what they are during the peak summer months, there is still work to do in the winter. Contractors can lessen the adverse effects of wintry weather on trees with preventative maintenance.

THE BASICS. Some tree care fundamentals like proper irrigation and adequate mulch application shouldn’t be forgotten as temperatures dip and clients switch from T-shirts and shorts to sweaters and long pants.

Winter winds cause evergreens – and even some deciduous trees – to suffer from winter drying. If water is not available as moisture is drawn from living tree cells, permanent damage known as winter burn can result, warned Robert McMullin, founder and president, Keystone Tree Experts, Doylestown, Pa. "The best prevention against winter burn consists of planting only hardy species in areas of prolonged exposure, watering plants adequately in the fall and mulching to insulate the soil and roots from severe cold," he offered.

Since evergreens continue transpiring in the winter more than their deciduous counterparts, watering them two to three times throughout the winter is plenty, said Bruce Phillips, local manager, Bartlett Tree Experts, Fulton, Md.

There’s even an old wives’ tale that marks Thanksgiving as the time to water evergreens. "The theory behind the tale is that if you have evergreens, they will actively transpire in winter and if you don’t have active moisture in the soil, they will become water stressed, especially if they get afternoon sun or wind," explained Roger Funk, vice president of the Davey Institute, Kent, Ohio. "So, you should water around Thanksgiving and then again if there is thawing in January. But if there’s plenty of snow cover in January, don’t water."

Contractors also can spray evergreen trees with an anti-desiccant or wax-like substance in November or December for extra protection against the wind’s drying effects, McMullin pointed out.

Concerning deciduous trees, Funk recommended contractors be cautious when deciding whether or not to water. "You don’t want to waterlog them," he said. "The water from snow may be enough. Most deciduous trees die from overwatering rather than underwatering."

Renewing mulch layers in the fall also holds water and moisture in the soil and insulates tree roots from severe cold. Since the same active, absorbing tree roots can be killed in the heat of summer as well as winter, Funk said mulch offers essential root protection. "If mulch is put down in the fall before the soil temperature cools, then the soil will retain a warmer temperature, keeping it stable," he explained. "This can also extend the growing season slightly and help to prevent weeds."

Contractors should never apply mulch more than 4 inches high, McMullin warned. "Unfortunately, the practice of applying excessive, sometimes enormous, amounts of mulch around trunks just seems to be getting more common," he lamented.

Funk said contractors should imagine the forest floor’s natural mulch that protects trees when convincing their clients of mulching benefits. "Trees are used to a lot of organic matter in the woods," Phillips added. "Trees don’t compete with turf in the woods. They have the extra organic matter from the natural mulch, and they have better root growth as a result."

Mulch should be applied 2 to 4 inches deep over relatively clean, weed-free soil, McMullin stated. "Depth should be no more than 2 inches if soil is not well drained and up to 4 inches if drainage is good," he said, adding that mulch should not touch tree trunks, should be 3 to 5 inches away from the trunks of young trees and 8 to 12 inches away from older tree trunks. "More finely textured mulches should be no thicker than 1 or 2 inches because they allow less oxygen penetration than coarser materials, such as nuggets," he said.

NO BETTER TIME. Many contractors claim that winter is the best time to examine and treat trees for structural weaknesses. Dormancy is a particularly good time to prune deciduous trees because arborists can more easily pinpoint a tree’s superstructure without leaf cover, Funk pointed out.

"It is much easier to spot defects such as cracked or overextended limbs when leaves do not restrict visibility," McMullin explained, pointing out that proper pruning encourages growth, improves plant health, repairs damage and adds aesthetic appeal. "A skilled arborist has no trouble telling the dead limbs from the live ones when there are no leaves."

Plus, winter is the only time to prune trees susceptible to boring insects, Funk stressed. "When you wound a tree through pruning, the tree gives off substances that these boring insects zero in on," he said. "Vascular wilts are spread by insects that are attracted to these wounds. But during the winter, the beetles aren’t active, so this is a good time to prune. You wouldn’t want to wound trees that are susceptible to this insect when they are active."

Just as orchard owners prune their trees heavily in the winter for increased fruit production, Phillips said arborists perform heavy crown reductions on clients’ dormant trees. Since trees store energy for the next growing season instead of using it up, pruning tips to reduce overall size in winter helps contractors avoid removing leaves that are providing the tree with food or energy for growth, Phillips said.

But dormant deciduous trees aren’t the only ones that need a little attention in the winter. Evergreens also can be pruned in the fall and winter, particularly to cut back long ends that can break from heavy snow and ice build-up. "Pruning trees before storms occur makes them better able to carry the extra weight," McMullin commented. "Branches can be thinned to reduce their surface area and wind resistance. This encourages the formation of the strongest possible limbs as well as the strongest branch attachments."

According to Phillips, as little as ½ inch of ice or more on branches can cause severe damage. To prune evergreens correctly and help prevent breakage, cut back long ends to lateral limbs going in the opposite direction, he explained. "The piece you take off should not be less than one-third the size of the lateral you cut back to," he said. "For example, when you cut back a 6-inch limb, you would not take off more than 2 inches in diameter of the branch."

EXTRA SUPPORT. When pruning alone isn’t enough, properly installed cables, braces, stakes or guy wires can add support to a weakened part of a tree before heavy winter storms and winds hit. But this needs to be done on a case-by-case basis because tree trunks only grow stronger without stakes, Funk warned.

Contractors should keep an eye on exposed trees’ reactions to winter storms as a first step in deducing whether or not a tree needs extra support. For instance, Funk remembers when a storm started to heave the wind side of one exposed 20- to 30-foot-tall tree last year. Even though he doesn’t recommend staking trees smaller than 20-feet tall, this particular tree needed support. "It didn’t come out of the ground, but the wind loosened it," he said. "So this year we will probably guy wire it because it will be even more sensitive to wind damage."

While staking and guying are traditionally for overall trunk and tree support, cabling and bracing can support weak branches that are higher in the tree. Phillips said cabling can be put on the top one-third of a tree for support if it has a weak fork so pressure is kept off of the tree base. A long branch with cavities or holes also can be cabled back to keep wind from whipping the branch and breaking it.

When a tree forms a wishbone where it can be pulled apart easily, bracing the two branches together with a rod also can add winter support.

In addition to wind and storm stress, trees must battle winter sun. On sunny winter days, a tree’s trunk and main limbs can warm up to 15 degrees higher than the air temperature, McMullin said. As soon as the sun’s rays stop reaching the stem, its temperature goes down fast, causing injury or permanent damage to the bark in one of two main forms – sunscald or frost cracking. McMullin said contractors can protect the trunks of young, susceptible trees with a suitable tree wrap.

Extra protection also comes in the form of nutrients. The root systems of dormant trees never go dormant. When temperatures are at or below freezing, a tree’s xylem flow slows down and cannot absorb water, but the minute conditions allow for growth, absorption becomes possible, Funk said. Consequently, soluble fertilizer can be injected into trees before temperatures plummet below 50 degrees Fahrenheit.

"The only problem is that if you put soluble nitrogen down and the root system can’t take it up then the product is susceptible to leeching," Funk explained. "You need to do this early enough in the fall."

For this reason, using slow-release fertilizers during dormancy is typically a better bet, Phillips advised, recommending the injection of liquid, slow-release fertilizer with a long needle about 1 foot into the tree’s root system.

Despite all the protection contractors provide trees via mulch, pruning, irrigation and fertilization, sometimes Mother Nature is unmerciful and storm damage is inevitable.

After a severe storm, contractors should first remove broken limbs from clients’ trees, McMullin said, advising contractors not to wait too long to perform this task. "Pruning to remove broken stubs and restore the balance of the crown can be put off a little while, but it shouldn’t be delayed more than one growing season," he said, adding that damaged trees should be watched more carefully the following year.

Fully restoring trees to their former health and beauty may take some time, McMullin said, but most trees generally can make a full recovery.