In many areas, when winter sets in, temperatures drop and trees go dormant, it can be the best time for pruning. Some tree care customers have the misconception that with leaf drop, production tree work comes to a halt, as well. The exact opposite is true. Winter can be the perfect time to prune and maintain trees and shrubs. A little bit of planning and foresight can go a long way to help trees and shrubs survive the dormant season and use it as a springboard to vital, flourishing warm-season growth.
Benefits of winter pruning
In winter, when some trees lack foliage, structure is much easier to see and select for. Good examples of this are small ornamentals, such as the Malus family, commonly know as crab apples. These types of trees and many others can add winter interest to a landscape, even without leaves, through bark coloration, fruit and a well-formed structure. The ability too see the branching structure in winter allows an arborist to identify which branches to preserve.
Mature trees can also be more easily evaluated for structure in winter months. A well-structured tree, large or small, looks good year-round and is healthier and more resistant to possible storm damage.
Another aspect of winter pruning is the lack of insect and disease pathogens in the landscape. The best example of this is Ulmus americana or American elm. These trees are prone to Dutch elm disease (DED) of which the native elm bark beetle Hylurgopinus rufipes is one of a few vectors. When pruned in the growing season, the wounds leak sap that attracts the bark beetle, and thus, DED. Winter means no bark beetles and less sap. Therefore, winter is the prime time to prune elms. Many tree species experience similar benefits of winter pruning.
Recent scientific studies have called into question this particular benefit of winter pruning. While fewer pathogens are present in colder weather, the dormant tree is also slower to start sealing off pruning cuts. Whether or not this slow compartmentalization reaction with fewer pathogens is worse than quicker compartmentalization in warmer weather with pathogens present is something the arborist must consider to make the best possible decision.
Not only can trees benefit from winter pruning, but the sites they live in can benefit, as well. Gardens chock full of annuals and perennials in the spring and summer are rarely a concern in the winter. Frozen turf is much more resistant to equipment and heavy foot traffic and limb dragging than in warmer months. Many thin-barked trees have loosely attached bark during the growing season and tend to peel easily, often with minimal pressure or minor cuts. The colder temperatures and reduced sap flow of winter greatly reduce this tendency.
Winter pruning concerns
While it is certainly handy to have leaves on a deciduous tree to make deadwood easy to spot, it is not necessary. Twig color, the presence of viable buds and bark condition can all be used as accurate markers to judge whether a branch is alive or not. However, it requires keen observation skills and good arboricultural knowledge to spot deadwood in the winter months.
Most shrub work should be avoided in the winter months. Certainly things like deadwood and structure can be addressed. However, most shrubs will have an initial flush of growth in early spring. Attempts to crown reduce or shape shrubs are often futile due to this rapid spring growth. Large and/or dense shrubs can be thinned to help them handle heavy snow loads, but this should be done early in the winter, well before the threat of snow is a concern. Wrapping smaller shrubs in burlap and/or tying them up with twine may also be an effective method for reducing snow damage. Care should be taken not to pull the plant so tight as to damage limbs. All twine and burlap should be removed in a timely manner so as not to inhibit the spring flush of growth or deform limbs and branches.
Trees can also benefit from early winter crown reduction and thinning to help long, heavily foliaged limbs handle snow loads. Pinus strobus, white pines, are a good example of a tree that tends to form overextended branches that may break with a heavy snow load. Some judicious crown reduction can go a long way to helping trees resist winter damage. As always, care should be taken to prune with the tree’s natural structure in mind and to not cause excessive wounding and over thinning.
Other winter strategies
Many evergreen plants, such as those in the Ilex, or holly, family suffer in colder climates from excessive drying out. Cold temperatures and harsh winds literally suck moisture from the leaves, and sodium-based ice and snow melting products suck moisture from the roots.
There are commercial anti-desiccant products available to protect foliage. The timing of application is crucial to ensure proper coverage and enhance longevity. Check with local suppliers and companies that may use these types of products in your area. They are the best source of information and advice.
Winter conditions often demand the use of snow and ice melt. Some of these products can have detrimental effects on tree and shrub health. Avoiding use is best, but if they must be used there are some strategies to help lessen the damage. Use only in critical areas, such as shaded sidewalks, intersections, steps and the like. Mixing the melting agent with abrasives, such as sand or ash, will lessen the amount needed to maintain traction. Well-drained areas will allow salts to leach out of plant beds more easily. Use calcium chloride or calcium magnesium-based snow and ice melt when possible.
As stated, the best choice is to avoid chemicals in areas where plants are the priority. Conversely, not planting close to areas like shaded parking lots and sidewalks where chemicals must be used will have a double effect: Remove the plants and reduce the shade; and no plants in the area means no concern about excessive sodium in the soil.
Burlap can be used as a barrier to keep salt spray from damaging exposed foliage. Crown reduction and raising may also be options to include in your winter pruning plan. Crown raising trees over drives and parking lots will give snow removal and other winter maintenance equipment room to work and pile accumulated snow. Crown reducing trees and shrubs from walkways will also keep fragile branches from becoming broken during snow and ice removal.
Clearly marking roadways and walkways will also keep snow removal personnel literally “on the right track” when plowing or shoveling. Also, in tight areas, be sure to mark appropriate areas where snow and ice can safely be piled. Many a shrub or small tree has met the fate of being buried in snow and ice never to awaken come spring.
A drop in temperature does not necessarily mean a drop in tree and shrub care. Winter provides some unique opportunities and conditions to complete any number of important and often overlooked pruning, tree health and site improvement projects. Proper planning, timing and astute judgement can help protect and preserve trees and shrubs through winter’s long, dark days. Tree and shrub winterization will prepare your client’s property to look its best come spring.
Tony Tresselt, a writer, ISA certified arborist, TCIA certified tree care safety professional and instructor for North American Training Solutions, works for Arborist Enterprises in Lancaster, Pa.
Tresselt , Tony. "winterizing trees." Tree Services Magazine 2010: 1. Web. 8 Dec 2010. <http://www.treeservicesmagazine.com/article.php?id=5997