Can You Trim Ash Trees in the Summer? - Plant Care Guide

Yes, you can trim ash trees in the summer, and for specific purposes, it can actually be an advantageous time to prune. However, summer pruning is fundamentally different from dormant (winter) pruning and should be approached with caution, focusing on light maintenance rather than heavy structural cuts. It's especially useful for reducing vigor or addressing immediate health concerns.

Why is Timing Important for Trimming Ash Trees?

Timing is crucial for trimming ash trees (or any tree) because it directly impacts the tree's health, growth response, wound healing, and susceptibility to pests and diseases. Pruning at the wrong time can stress the tree, stunt its growth, or even expose it to serious threats.

Here's why timing is so important:

1. Impact on Growth and Vigor:
   • Dormant Pruning (Winter): Pruning when the tree is dormant (no leaves) stimulates a strong flush of new, vigorous growth in spring. This is the best time for structural pruning, size reduction, and invigorating growth.
   • Summer Pruning (Late Spring/Early Summer): Pruning when the tree is actively growing (with leaves) tends to slow down vegetative growth (new shoots) and redirects the tree's energy towards healing wounds and strengthening existing structures. This is beneficial for growth control or for certain issues.
2. Wound Healing:
   • Trees compartmentalize (seal off) pruning wounds to prevent decay. They do this most effectively when actively growing and healthy.
   • Summer Advantage: During summer, the tree's vigorous growth allows it to rapidly produce callus tissue and seal wounds quickly, which can reduce the window for disease entry.
   • Fall/Early Winter Risk: Pruning late in the season, just before or during dormancy, can leave open wounds exposed to pathogens and cold temperatures for extended periods, making healing less efficient.
3. Disease Susceptibility:
   • Specific Diseases: Certain tree diseases are more prevalent or spread more easily during specific times of the year. For ash trees, the primary concern is the Emerald Ash Borer (EAB), though its threat dictates avoiding pruning during its flight season, not necessarily restricting summer pruning otherwise. Other diseases like Oak Wilt (not for ash, but an example) are known to be spread via summer pruning wounds.
   • Summer Pruning Advantage: For some trees, summer pruning can remove diseased parts quickly before they spread, or reduce dense canopy, improving airflow and preventing fungal issues.
4. Aesthetics and Clearance:
   • Summer pruning allows you to clearly see the tree in its full foliage, making it easier to prune for specific aesthetic goals, to improve views, or to achieve clearance around structures.
5. Flower/Fruit Production (Less relevant for Ash, but a general tree pruning factor):
   • For flowering/fruiting trees, timing is crucial to avoid removing flower buds (dormant pruning for spring bloomers) or new growth that will flower (summer pruning for summer bloomers). Ash trees are not typically grown for ornamental flowers or fruit.

Understanding these impacts means that choosing the correct season for trimming ash trees is a thoughtful decision based on the desired outcome and health considerations.

What Are the Benefits of Trimming Ash Trees in the Summer?

Trimming ash trees in the summer, specifically during their active growth phase (late spring to mid-summer), offers several distinct benefits, particularly when the goal is to manage growth, improve tree health, or address specific issues. Summer pruning is a tool with precise applications.

Here are the benefits of trimming ash trees in the summer:

1. Reduces Overall Tree Size and Vigor:
   • Mechanism: When you remove branches with leaves during the summer, the tree loses a portion of its photosynthetic capacity. This signals the tree to divert energy to healing wounds and maintaining existing growth, rather than producing new shoots.
   • Benefit: Excellent for controlling the overall size of an ash tree or managing aggressive growth without stimulating a flush of new, weaker shoots, as dormant pruning often does. It can reduce the amount of energy available for future growth.
2. Improves Light Penetration and Air Circulation:
   • Mechanism: Thinning out dense parts of the canopy in summer allows more sunlight to reach interior branches and improves airflow throughout the tree.
   • Benefit: Enhanced light can improve the health of inner foliage. Improved air circulation reduces humidity, which can help prevent some fungal diseases.
3. Corrects Unsightly Growth:
   • Mechanism: With the tree in full leaf, it's easier to identify and remove specific branches that detract from the tree's aesthetic appeal, obscure views, or interfere with structures.
   • Benefit: Allows for very targeted aesthetic pruning or clearance pruning to achieve immediate results.
4. Removes Damaged or Diseased Wood (Immediate Action):
   • Mechanism: If an ash tree sustains storm damage (broken branches) or develops signs of disease (e.g., specific cankers or leaf spots), summer is the time to remove it immediately. For diseases, early removal prevents spread.
   • Benefit: Prevents further damage, disease progression, and improves tree safety.
5. Reduces Watersprouts and Suckers:
   • Mechanism: Removing these vigorous, unproductive growths during summer diverts the tree's energy away from them.
   • Benefit: Helps maintain a cleaner, healthier tree form and reduces competition for resources from the main canopy.
6. Better Wound Healing:
   • Mechanism: A healthy, actively growing tree has more energy and resources to quickly form callus tissue over pruning wounds.
   • Benefit: Faster wound closure reduces the window for disease entry and minimizes decay.

While summer pruning is beneficial, it should always be light and targeted, with heavy structural pruning reserved for dormancy.

What Are the Risks of Trimming Ash Trees in the Summer?

While summer trimming of ash trees offers certain benefits, it also carries several significant risks if not done correctly or at the wrong time. These risks primarily relate to tree stress, pest attraction, and the timing of certain ash-specific threats.

Here are the main risks of trimming ash trees in the summer:

1. Emerald Ash Borer (EAB) Attraction (Major Risk):
   • Mechanism: This is the most critical concern for ash trees. The Emerald Ash Borer (Agrilus planipennis) is highly active and in its flight season (laying eggs) during late spring and summer (typically May through August). Fresh pruning wounds release volatile chemicals that are incredibly attractive to female EAB beetles.
   • Impact: Pruning wounds act as entry points or attractants for EAB, increasing the risk of infestation, which is usually fatal for ash trees.
   • Recommendation: Many arborists and forestry services strongly recommend avoiding any pruning of ash trees during the EAB flight season in infested areas, unless absolutely necessary for safety reasons.
2. Tree Stress from Energy Loss:
   • Mechanism: In summer, the tree's leaves are actively photosynthesizing, producing the sugars (energy) it needs for growth, maintenance, and defense. Removing a significant portion of leaves removes the "food factory," forcing the tree to expend energy on healing wounds and regenerating foliage.
   • Impact: Excessive summer pruning can stress the tree, potentially reducing its vigor, making it more susceptible to other pests and diseases, and possibly leading to sunburn on previously shaded bark.
3. Sun Scald:
   • Mechanism: If dense foliage is removed from the sunny side of the tree in summer, the previously shaded bark on the trunk and large branches can suddenly be exposed to intense sunlight.
   • Impact: This can lead to sun scald, where the bark is damaged or killed, creating wounds vulnerable to pests and disease.
4. Delayed Dormancy and Frost Damage:
   • Mechanism: Late summer pruning (especially late August/September) can stimulate a flush of new growth that won't have time to harden off before winter.
   • Impact: This tender new growth is extremely vulnerable to frost damage, weakening the tree.
5. Less Clear Structural Vision:
   • While you can see the overall canopy, the density of leaves can obscure finer structural details, making it harder to identify crossing branches or weak crotches compared to dormant pruning.

Given the significant threat of EAB, the primary risk for ash trees in summer, avoiding pruning during peak EAB activity is often the most important consideration, overriding other potential benefits of summer trimming.

What is the Emerald Ash Borer and How Does it Spread?

The Emerald Ash Borer (EAB), Agrilus planipennis, is a highly destructive, invasive, metallic green beetle that is responsible for the widespread mortality of ash trees across North America. It is arguably the most devastating tree pest currently impacting North American forests and urban landscapes.

Here's what the Emerald Ash Borer is and how it spreads:

• The Pest:
  • Adult Beetles: Small, about 1/2 inch long, with a distinct metallic green color. They are active from late spring to late summer (typically May to August).
  • Larvae: The most damaging stage. Cream-colored, segmented, worm-like larvae that feed just beneath the bark.
• Origin: Native to Asia, where ash trees have developed resistance. It was accidentally introduced to North America, likely in solid wood packing material, and first detected in Michigan in 2002.
• Mode of Attack:
  1. Egg Laying: Adult female EAB beetles lay eggs in bark crevices on ash trees.
  2. Larval Feeding: Once hatched, the larvae bore into the tree and feed on the phloem (the tissue that transports sugars from the leaves to the roots) and the cambium (the growth tissue) in characteristic S-shaped tunnels.
  3. Girdling: This larval feeding essentially girdles the tree, cutting off the flow of nutrients and water between the canopy and the roots.
• Impact on Ash Trees:
  • Ash trees typically die within 2-4 years of becoming infested.
  • Symptoms include canopy dieback, D-shaped exit holes (where adults emerge), epicormic sprouts (suckers) from the trunk/roots, and vertical bark splits.
• How it Spreads:
  1. Adult Flight: Adult EAB beetles can fly and infest nearby ash trees. They can fly several miles, but most spread is more localized.
  2. Movement of Infested Wood (Primary Cause of Long-Distance Spread): This is the most significant mechanism for long-distance spread. When people unknowingly move infested ash firewood, nursery stock, logs, or other ash wood products to new, uninfested areas, they transport the larvae inside the wood.
  3. Pruning Wounds: Fresh pruning wounds release volatile chemicals that are highly attractive to female EAB, increasing the risk of infestation in that tree.
  4. Natural Range Expansion: Over time, the beetles naturally expand their range from infested areas into new ones.

Due to the EAB's devastating impact and efficient spread through human activities, strict quarantines and efforts to control its movement (e.g., "Don't Move Firewood" campaigns) are in place. For home gardeners, the EAB threat heavily influences decisions about planting, maintaining, and removing ash trees.

What are the Recommendations for Pruning Ash Trees in EAB Infested Areas?

In areas infested with the Emerald Ash Borer (EAB), recommendations for pruning ash trees become extremely strict and often involve avoiding pruning altogether during the beetle's flight season unless absolutely necessary for safety. The goal is to minimize attracting the pest.

Here are the recommendations for pruning ash trees in EAB infested areas:

1. Avoid Pruning During EAB Flight Season (Critical!):
   • Timing: The EAB flight season is typically from May through August (it can vary slightly by region and year, but this is the general window).
   • Recommendation: Do NOT prune ash trees during these months if EAB is known to be in your area or nearby.
   • Why: Fresh pruning wounds release volatile chemicals that are highly attractive to egg-laying female EAB beetles, making the tree a target for infestation.
2. Best Time to Prune (Dormant Season):
   • Recommendation: Perform any necessary pruning (structural, deadwood removal, size reduction) during the dormant season, specifically from October 1st to April 30th (or before May 1st).
   • Why: During these months, adult EAB are not active and not laying eggs, significantly reducing the risk of attracting them to fresh wounds.
3. Only Prune When Absolutely Necessary:
   • Recommendation: Even during the dormant season, practice minimal pruning. Focus only on removing dead, diseased, or hazardous branches. Avoid excessive aesthetic pruning.
   • Why: Reducing the number of wounds on the tree is always beneficial.
4. Proper Disposal of Ash Wood:
   • Recommendation: Any ash wood removed (pruned branches, fallen limbs) must be processed or disposed of properly to prevent EAB larvae from emerging.
   • Methods: Chip the wood to less than 1 inch in two dimensions, burn it immediately (where permitted), or transport it directly to a designated disposal facility within the quarantined area. Do NOT move ash firewood or logs outside the regulated zone.
5. Monitor for EAB Infestation:
   • Recommendation: Regularly inspect your ash trees for signs of EAB, even if you are treating them. Look for canopy dieback, D-shaped exit holes, bark splits, and epicormic sprouts.
   • Why: If a tree is already heavily infested, pruning might be a futile effort, and removal might be the best option.
6. Consider Prophylactic Treatment:
   • Recommendation: If you have high-value ash trees you wish to save, consider insecticide treatments applied by a certified arborist to protect them from EAB. These treatments are typically applied in spring.
   • Why: Treatment can protect trees, but it's an ongoing commitment.

By adhering to these strict guidelines, especially avoiding summer pruning, you can help manage the EAB threat and give your ash trees the best possible chance of survival in infested areas.

What is Compartmentalization and How Does it Relate to Pruning?

Compartmentalization is a vital natural defense mechanism that trees use to seal off or isolate wounds, preventing the spread of decay and disease within their wood. Understanding compartmentalization (often referred to as CODIT: Compartmentalization of Decay in Trees) is fundamental to proper pruning techniques, as it guides how to make cuts that promote rapid wound closure and tree health.

Here's what compartmentalization is and how it relates to pruning:

• The Tree's Defense Strategy: When a tree is wounded (e.g., from a broken branch, animal damage, or a pruning cut), it doesn't "heal" in the same way an animal does by regenerating lost tissue. Instead, it forms physical and chemical barriers around the wound to contain decay-causing fungi and bacteria.
• The "Walls" of CODIT: Trees form four "walls" to compartmentalize the wound:
  1. Wall 1: Plugs xylem vessels above and below the wound, limiting vertical spread.
  2. Wall 2: Forms a growth ring boundary, limiting inward spread.
  3. Wall 3: Creates a barrier against lateral spread, often the weakest wall.
  4. Wall 4: Forms new wood (cambium) that grows over the wound, completely sealing it off. This is the callus tissue you see.
• How Proper Pruning Promotes Compartmentalization:
  • Clean Cuts: Make clean, smooth cuts with sharp tools. Jagged or torn cuts are harder for the tree to seal.
  • Correct Placement: Pruning cuts should be made at specific locations on a branch, typically just outside the branch collar (the swollen area where the branch meets the trunk or a larger limb) and at a slight angle.
  • Avoid Damaging the Branch Collar: The branch collar contains specialized cells that are crucial for initiating the growth of Wall 4 (the callus tissue). Cutting into or leaving a stub that is too long (cutting inside the branch collar) will damage these cells, inhibiting proper wound closure and increasing decay.
  • Avoid Wound Dressings: Generally, wound dressings (paints, tars) are not recommended for pruning cuts. They do not prevent decay and can actually trap moisture, hindering the tree's natural compartmentalization process.
• Impact of Incorrect Pruning:
  • Delayed Healing: Improper cuts (stubs, flush cuts) are difficult for the tree to compartmentalize, leaving the interior wood exposed to decay-causing organisms for longer.
  • Decay and Disease: This allows fungi and bacteria to penetrate deeper into the tree, leading to internal rot, structural weakness, and increased susceptibility to disease.

By understanding and respecting the principle of compartmentalization, arborists make precise cuts that align with the tree's natural defense mechanisms, ensuring rapid wound healing and promoting the long-term health and structural integrity of ash trees and all other tree species.

When Should You Hire a Professional Arborist for Trimming Ash Trees?

You should definitely hire a professional arborist for trimming ash trees in most situations, especially for mature trees, large cuts, or if you suspect disease. While light pruning of small branches may be a DIY task for young trees, ash trees present specific challenges and risks that often warrant expert intervention.

Here's when to call a professional arborist:

1. Any Branches Near Power Lines or Structures:
   • Why: This is a major safety hazard. Contacting power lines can be lethal. Working near buildings requires expertise to prevent property damage.
   • Action: Always call a professional, ideally one trained in utility line clearance if near power lines.
2. Large Branches (Over 2 Inches in Diameter):
   • Why: Larger cuts require specific techniques to avoid tearing bark (branch collars), which damages the tree. Heavy branches are also difficult and dangerous to remove without proper rigging.
   • Action: Professional arborists have the tools, training, and experience to make clean, safe cuts.
3. Trees That Require Climbing or Use of a Ladder/Lift:
   • Why: Working at height is inherently dangerous. Ladders can be unstable, and climbing trees without proper safety gear and techniques is extremely risky.
   • Action: Arborists have appropriate safety equipment, climbing gear, and sometimes bucket lifts.
4. Suspected Disease or Pest Infestation (Especially EAB):
   • Why: Proper diagnosis and treatment (or management) of tree diseases (like Mimosa Wilt) or pests (like EAB for ash trees) require expert knowledge. Incorrect pruning can spread disease.
   • Action: A certified arborist can accurately diagnose the problem and recommend the most effective (and safest) course of action, which might include removal or specific pruning during dormant season.
5. Structural Issues or Tree Health Concerns:
   • Why: If your ash tree has weak crotches, cracks, internal decay, dead limbs, or generally looks unhealthy, a professional can assess its structural integrity and overall vigor.
   • Action: They can recommend corrective pruning for structural defects or advise on whether the tree needs removal for safety.
6. Lack of Experience or Proper Tools:
   • Why: Making correct pruning cuts requires knowledge of tree biology. Using dull or incorrect tools can damage the tree and make the job unsafe.
   • Action: Arborists have professional-grade, sharp, and sterilized tools.
7. Valuable or Historically Significant Trees:
   • Why: For cherished trees, expert care ensures their longevity and aesthetic appeal.
   • Action: A professional arborist can develop a long-term pruning plan.

A certified arborist is trained in tree biology, health, and safe removal/pruning practices. They are often insured, which protects you from liability if an accident occurs. While there's a cost, the safety and long-term health of your ash tree are invaluable.