Maintaining a healthy looking, yet safe tree is an important plant health care challenge for homeowners.

Although trees are strong and durable, they are susceptible to physical injury such as that caused by ice and windstorms, as well as lawnmower and other mechanical wounds. When trees are wounded, the newly uncovered tissue is exposed to fungal pathogens often called cankers and other decay microorganisms. The microorganisms digest tissues that are responsible for nutrient and water translocation throughout the vascular system and the structural support (heartwood). When this occurs it often results in unhealthy, unsightly, or unsafe trees

Despite these limitations, at times homeowners need to prune their trees to maintain aesthetic characteristics, remove diseased or dead limbs, or improve structural stability. Proper approaches to pruning can go a long way to minimize the issues mentioned above. These approaches are based on an understanding of tree biology and the wound healing process.

Trees attempt to close wounds naturally by forming callus tissue. Callus formation rates vary for different tree species and are affected by environmental conditions such as nutrient and water availability. Some trees may never completely close their wounds, depending on these and other factors, like wound size. However, numerous studies demonstrate that favoring callus formation can significantly reduce infection and colonization by decay organisms and other canker pathogens.

There are several ways that callus formation rates can be enhanced, or at least not inhibited. First, it is essential to avoid limiting oxygen availability to the wounded tissues. Oxygen is necessary for the proper healing process to take place. Therefore, painting a wound be it from natural causes or from pruning with any kind of material that interferes or impedes oxygen access will delay or even prevent wound closure by slowing or stopping callus formation.

Trees are pruned for a variety of reasons, for example, to develop proper canopy shape or remove potentially hazardous limbs. Although the optimal time for pruning varies by region and tree species, the best time to prune many trees is when rates of growth, and therefore callus formation, are the greatest. In Ohio, this means that pruning should take place in the spring to early summer., although hazardous tree limbs should be removed regardless of the time of year to minimize the risks of injury and damage.

While pruning trees in spring and summer is the ideal time to prune trees theirs always exceptions. The exception to this rule is with oak (Quercus spp.) trees. Oak trees in Ohio recommended pruning time is from Oct. 15 through April 15. The reason is to reduce the risk to your oak trees from contracting oak wilt.

Oak wilt is a fungus, Bretziella fagacearum. This lethal fungus invades the vascular system and can kill a mature Oak in less than a months’ time. The fungus is spread by sap beetles, also referred to as Picnic Beetles (Coleoptera: Nitidulidae).

The Picnic Beetles feed on freshly pruned or damaged Oaks during the summer months. Pruning actively growing Oaks causes them to bleed which can attract picnic beetles. If the Picnic Beetle has fed on an Oak tree that was infected with the Oak Wilt Fungus, it will carry the fungus to the freshly pruned Oak. If an oak tree becomes infected, it can transmit the fungus to other nearby oaks via roots grafting with adjacent oaks.

While all oaks are susceptible to this fungus, those in the red group; pin, scarlet, shingle and Shumard oak are extremely susceptible and can die within a few weeks of infection. Oaks in the white group that include bur, chinquapin, swamp white and white oak are more tolerant of the disease and can even survive infection for one or more years though will display declining symptoms.

Since we are in winter oak trees are dormant and it is a perfect time to prune. In addition, many certified arborists tend to have more availability at this time of year. A certified arborist is a professional tree care practitioner that has the required knowledge, training, and expertise to perform proper tree care safely. After passing the International Society of Arboriculture exams the Arborist becomes certified through their organization.

The ISA has a locator tool on their website. Put in the zip code to search for local arborists. https://www.treesaregood.org/findanarborist/findanarborist.

Oak wilt information is at https://ohioline.osu.edu/factsheet/plpath-tree-02.