Ash trees, belonging to the genus Fraxinus, are deciduous trees commonly found across North America. These trees grow to medium or large sizes, prominent in many landscapes. Understanding their characteristics, including advantages and disadvantages, is important for property owners.
Benefits of Ash Trees
Ash trees offer several positive attributes to landscapes and natural environments. Their aesthetic qualities include a majestic presence, often featuring arching canopies that provide substantial shade. Many ash species display brilliant fall foliage, with colors ranging from golden yellow to deep brick-red and vibrant purple hues.
Ecologically, ash trees are important to North American woodlands and diverse ecosystems. They serve as a food source and provide habitat for a variety of wildlife, including birds, insects, and squirrels. White ash (Fraxinus americana) is considered a keystone species, supporting a wide array of organisms from pollinators to various microorganisms.
Beyond their environmental contributions, ash trees have practical uses due to their wood quality. Ash wood is known for its strength and flexibility, making it a favored material for products such as baseball bats, tool handles, and furniture components. This wood is also utilized in flooring and cabinetry.
Ash trees also demonstrate adaptability to various growing conditions. Many varieties thrive in moist, rich soils, requiring full sun exposure and ample space. Green ash (Fraxinus pennsylvanica) shows tolerance to different soil types, urban pollutants, and road salt. Blue ash (Fraxinus quadrangulata) performs well in both dry and moderately wet sites.
Challenges of Ash Trees
Despite their benefits, ash trees face significant challenges, primarily from invasive pests and diseases. The Emerald Ash Borer (EAB), Agrilus planipennis, is a wood-boring beetle originating from Asia, causing widespread death. This metallic green beetle has caused the death of tens to hundreds of millions of ash trees across North America.
EAB larvae tunnel through the wood directly beneath the bark, disrupting the tree’s ability to transport water and nutrients. This damage can lead to the death of an infested tree within two to six years. First detected in Michigan in 2002, EAB has spread across North America, often through the movement of infested firewood. Infestation symptoms include canopy thinning, branch dieback, increased woodpecker activity, D-shaped exit holes, and S-shaped larval galleries under the bark. Infested trees can become brittle.
Other pests also affect ash trees, such as the ash leaf curl aphid (Prociphilus fraxinifolii). These aphids cause leaves to curl, distort, and sometimes drop prematurely. While their feeding is primarily aesthetic, they excrete a sticky substance called honeydew, which can lead to the growth of sooty mold on affected surfaces. This aphid damage does not compromise the tree’s overall health.
Ash trees are also susceptible to several diseases, including Ash Yellows and Anthracnose. Ash Yellows, caused by Candidatus Phytoplasma fraxini, is a chronic disease affecting the tree’s vascular system. Symptoms include reduced growth, smaller and light green leaves, premature fall coloration, and progressive dieback of branches. Distinctive “witches’ brooms,” dense clusters of shoots, may also form, particularly at the base of the trunk. White ash is especially vulnerable to this disease, for which there is no known cure.
Anthracnose, caused by the fungus Gnomoniella fraxini, is another common disease affecting ash leaves and twigs. This fungal infection manifests as irregular brown areas along leaf veins and water-soaked spots, often leading to premature leaf drop. Young leaves may become deformed, and prolonged cool, wet weather exacerbates the disease. While not fatal, severe infections can cause significant defoliation and branch dieback. Furthermore, female ash trees produce winged seeds, known as samaras, which can create a significant litter problem.
Managing Ash Tree Concerns
Effective management of ash trees involves proactive strategies, especially concerning the Emerald Ash Borer. Regular monitoring for signs of EAB, such as canopy thinning or D-shaped exit holes, is important. For high-value ash trees, preventative systemic insecticide treatments can provide protection against EAB. These treatments, which include trunk injections of emamectin benzoate or soil drenches with imidacloprid, must be reapplied every two to three years for continued effectiveness. Soil drench applications are suitable for trees with a trunk diameter less than 15 to 20 inches.
When an ash tree is heavily infested or in severe decline, removal becomes a necessary consideration. Dead or dying trees pose significant safety hazards and can be more costly to remove than healthy ones. It is important to avoid moving firewood from infested trees to prevent further spread of EAB. If more than 50% of the tree’s canopy is thinning, treatment is not effective, and removal is recommended.
Beyond pest and disease specific measures, general tree health practices support ash tree resilience. Proper watering, especially during dry spells, involves deep soaks to encourage robust root growth. Fertilization with a balanced product in spring, or organic-based macro and micronutrients for mature trees, can provide essential nutrition.
Pruning ash trees is best performed during their dormant season. This practice involves removing dead, damaged, or crossing branches to improve the tree’s structure and air circulation within its canopy. Due to their opposite branching pattern, regular pruning helps develop a strong framework. Applying a three to four-inch layer of mulch around the base of the tree, keeping it a few inches away from the trunk, helps retain soil moisture and suppress weeds. For complex issues or larger trees, consulting a professional arborist is advisable for diagnosis, treatment, or removal.
Deciding on Ash Trees for Your Landscape
When considering ash trees for a landscape, evaluating local Emerald Ash Borer prevalence and the associated risk is important. Some ash species, such as blue ash, demonstrate slightly more resistance to EAB compared to others. The capacity for ongoing maintenance, including the financial commitment for preventative treatments, should also be assessed.
Responsible planning involves diversifying tree plantings rather than relying heavily on a single species. This approach helps mitigate the impact of widespread pest or disease outbreaks. For those in EAB-infested areas, or seeking alternatives, several tree species offer similar benefits without the same level of risk. Examples include Ginkgo, London Planetree, Hackberry, Kentucky Coffeetree, Tulip Poplar, American Elm, and various Oak species.