Elm trees, like many deciduous species, undergo significant changes as winter approaches, entering a period of dormancy. While appearing inactive, these trees are engaged in a complex biological process that allows them to endure cold temperatures and prepare for spring growth. Understanding these winter adaptations and providing appropriate care can help ensure the long-term health of your elm.
Elm Tree Winter Dormancy
As daylight hours shorten and temperatures decrease, elm trees begin their natural progression into dormancy. Leaves change color and eventually fall, a process where the tree reabsorbs valuable nutrients before shedding them to conserve resources.
During this period, the tree’s metabolic activity significantly slows. Tissues harden, developing increased cold resistance to protect against freezing damage. This “winter rest” is a controlled state where internal processes continue at a reduced rate. Dormancy is primarily triggered by changes in day length, not temperature alone. This preparation allows elms to survive harsh winter conditions and resume vigorous spring growth.
Winter Preparation and Care for Elm Trees
Watering
Proper hydration is important for elm trees heading into winter. Thoroughly water the tree in late fall before the ground freezes solid. This ensures roots have sufficient moisture reserves for dry winter spells, when frozen soil limits water uptake. Young trees, in particular, benefit from deep watering as their root systems are less established.
Mulching
Applying a layer of organic mulch around the base of your elm tree offers several benefits during winter. Mulch helps insulate the soil, moderating temperature fluctuations and protecting roots from extreme cold. It also aids in retaining soil moisture, especially important if winter conditions are dry. A layer of mulch between 2 to 4 inches deep, spread in a donut shape around the tree, is effective, but avoid piling it directly against the trunk to prevent disease and decay.
Pruning
Winter, when elms are dormant and leafless, is ideal for pruning. This allows clear visibility of the tree’s structure, making it easier to remove dead, diseased, or damaged branches. Dormant pruning minimizes tree stress and reduces disease transmission, particularly for pathogens like Dutch elm disease spread by beetles active in warmer months. Use the three-cut method for larger branches to prevent bark tearing. Avoid removing over 25% of the canopy or “topping” the tree, which severely damages its health.
Protection from Pests and Diseases
Winter vigilance plays a role in managing potential pest and disease issues. While many pests are inactive, the dormant season is a good time to assess the tree for signs of overwintering insects or fungal spores. Removing dead or diseased branches during winter pruning can help reduce sites where pests or diseases might harbor. For specific concerns like Dutch elm disease, pruning during winter minimizes the attraction of elm bark beetles, which spread the fungus. Some preventative treatments, like fungicide injections for Dutch elm disease, are performed by professionals and offer protection for multiple years.
Protection from Physical Damage
Protecting elm trees from physical damage during winter can prevent long-term issues. Young, thin-barked trees are susceptible to sunscald, a condition where rapid temperature fluctuations on sunny winter days can injure the bark, particularly on the south or southwest side of the trunk. Applying a light-colored tree wrap from November to April can help maintain a consistent bark temperature and reflect sunlight.
Heavy snow and ice accumulation can lead to limb breakage, especially on weak-wooded species like Siberian elm. Gently removing excess snow from branches with a broom can alleviate stress, but avoid knocking off ice, which can cause more damage. Installing hardware cloth around the trunk of young trees can deter rabbits and mice from gnawing on the bark, which can cause significant injury.
Common Winter-Related Issues for Elm Trees
Even with careful preparation, elms can face winter-related problems. Winter burn, or desiccation, occurs when cold winds and low humidity cause excessive water loss from the tree’s tissues, especially when the ground is frozen and roots cannot absorb moisture. This often manifests as browning or scorching of leaves or branch tips, particularly on the windward side of the tree.
Frost cracking, also known as sunscald, affects tree bark. It happens when the sun warms the bark during a cold day, causing it to expand, followed by a rapid temperature drop at night that contracts the outer bark faster than the inner wood. This differential contraction results in vertical cracks, sometimes with a loud sound. Younger elms with thinner bark are particularly susceptible. While frost cracks may heal, they can reopen in subsequent winters, providing entry points for insects and diseases.
Limb breakage from heavy snow and ice loads is a concern. While all trees are susceptible, those with weak wood or a poor branch structure are more prone to damage. Siberian elm, for instance, is noted for being a weak-wooded tree that can experience limb breakage. Additionally, winter stress can predispose elms to certain diseases, such as Nectria canker and Cytospora canker. These fungal diseases often colonize trees weakened by environmental stressors like freezing temperatures or drought, leading to cankers, dieback, and discolored bark.