Ash trees are common in many North American landscapes. Homeowners frequently question the proper timing for pruning these trees. While trimming maintains tree health and structure, performing it during the warmer months often causes concern. The decision to prune ash trees during the active growing season, typically June through August, depends heavily on the specific goals of the pruning and the regional environmental threats present.
The Standard Timing for Deciduous Tree Pruning
For most deciduous species, arborists advise scheduling significant pruning during the dormant season. This period extends from late fall after leaf drop through late winter or early spring before bud break. Pruning during this window minimizes stress because the tree’s metabolic processes have slowed considerably. The lack of leaves also provides an unobstructed view of the tree’s structure, allowing for more precise removal of crossing or poorly formed branches.
Winter pruning also reduces the presence of active insects and pathogens that can enter fresh wounds. When a cut is made in late winter, the tree is poised to begin its rapid growth phase in the spring, which facilitates faster wound closure. This quick sealing limits the time open tissue is exposed to decay-causing organisms. Some trees, like maple or birch, may exhibit heavy sap flow if pruned in late winter, but this phenomenon does not harm the tree’s health.
Physiological Effects of Pruning Ash Trees in Summer
Ash trees can tolerate light pruning during the summer months without serious long-term damage. The tree responds to injury through compartmentalization of decay in trees (CODIT), sealing the wound with new wood rather than healing it. During the growing season, the tree can actively invest energy into forming these protective boundaries, which is a key part of its defense mechanism against decay.
However, summer pruning occurs when the tree is fully leafed out and actively photosynthesizing, which diverts energy reserves away from growth. The tree must expend resources to seal the wound instead of focusing on carbohydrate production and storage. Removing a large portion of the canopy (heavy thinning) during summer can significantly reduce the tree’s total leaf surface area. This reduction in photosynthetic capacity can stunt growth and deplete the energy stores needed for the following year.
Removing dense outer branches can also expose the inner bark and previously shaded limbs to sudden, intense sunlight. This can lead to sunscald, where the bark tissue is damaged by the heat, creating an entry point for insects and disease. While light corrective pruning, such as removing dead, diseased, or broken wood, is acceptable at any time, major structural cuts should be avoided during the peak summer months to mitigate these physiological stressors.
Minimizing Emerald Ash Borer Attraction
The most significant factor prohibiting summer pruning of ash trees is the presence of the invasive pest, the Emerald Ash Borer (EAB). This metallic green beetle is highly active during the warmer months, and fresh wounds on ash wood are a major attractant. Adult EAB beetles typically emerge from May through September, with peak emergence and mating often occurring in June and July.
EAB females are drawn to volatile organic compounds (VOCs) released by newly cut wood, which signals a suitable host tree for egg-laying. A fresh pruning cut acts as a beacon, potentially attracting EAB females to an otherwise healthy tree. The female beetles then lay their eggs in the bark crevices, and the resulting larvae tunnel beneath the bark, cutting off the tree’s water and nutrient transport systems.
In regions where EAB is established or high risk, arborists advise against pruning live ash wood during the EAB flight season (May 1st through September 30th). To minimize infestation risk, restrict ash tree pruning to the EAB dormant period (October 1st to April 30th). Before performing any summer pruning, consult with local extension offices or certified arborists to determine the specific EAB risk level in your immediate area.