A hemlock tree, belonging to the Tsuga genus, is a graceful, slow-growing conifer valued for its dense, fine-needled foliage and pyramidal form. The practice of “topping,” which involves cutting the top off a tree to reduce its height, is strongly discouraged by certified arborists. This method causes severe biological and structural damage rather than maintaining the tree’s health or structure. This article explains why topping a hemlock is harmful, detailing the tree’s physiological reaction and resulting long-term hazards.
The Biological Response to Removing the Leader
A hemlock tree’s characteristic conical shape is determined by a natural process known as apical dominance. The central leader, or topmost growing point, produces a growth hormone called auxin, which travels downward through the tree’s vascular system. This hormone suppresses the growth of lateral buds lower down the trunk, ensuring the tree prioritizes vertical growth and maintains its single, strong central stem.
When the central leader is removed through topping, this hormonal control is instantly lost, confusing the tree’s growth regulation system. The sudden loss of the auxin signal causes the dormant buds just below the cut to activate in a stress response. This results in the rapid, chaotic production of multiple new shoots, often referred to as water sprouts or epicormic growth.
Topping also creates a wide, open wound on the main trunk, which the hemlock struggles to seal off effectively. Conifers like hemlocks are not strong compartmentalizers, meaning they are less efficient at forming protective barriers to isolate the injury. This large, exposed surface area becomes a significant drain on the tree’s energy reserves as it attempts to heal.
Long-Term Structural Risks and Tree Health
The rapid-growth water sprouts produced after topping are structurally weak and poorly attached to the main trunk. These shoots lack the strong wood tissue and natural branch collar that anchor a typical, healthy branch. Consequently, the new canopy that forms is dense, bushy, and highly prone to breakage during high winds, heavy ice, or snow loads.
The large, unsealed cuts left by topping provide a direct entry point for decay fungi and various pathogens, which begin to rot the interior wood of the main trunk. This internal decay compromises the tree’s structural integrity, transforming the tree into a long-term hazard that is more likely to fail. The decay process can be slow, but it is often irreversible once established deep within the trunk.
The physiological stress of topping makes the tree more vulnerable to pest infestations and disease. For hemlocks, this vulnerability is concerning in areas affected by the Hemlock Woolly Adelgid (HWA), an invasive, sap-sucking insect. A stressed and weakened tree, unable to allocate energy to its natural defenses, is more likely to suffer severe damage or mortality from this pest.
Proper Techniques for Reducing Tree Height
If a hemlock’s height must be reduced, the correct technique involves using “reduction cuts” rather than topping. This method requires cutting a long branch back to a smaller, healthy lateral branch, which then takes over as the new terminal point. This lateral branch must be at least one-third the diameter of the removed limb to ensure sufficient structural support and nutrient flow.
This selective pruning respects the tree’s natural growth structure, allowing for proper compartmentalization of the wound and minimizing the chaotic epicormic growth response. Reduction cuts reduce the canopy size while maintaining the tree’s natural form and structural integrity. This approach results in a healthier, more stable tree that is less prone to future failure.
While reduction cutting is the proper technique, major height reduction should not be undertaken as a do-it-yourself project. If the desired height reduction is substantial, or if the tree is near structures or power lines, consulting a certified arborist is necessary. Arborists are trained to assess the tree’s health and perform complex pruning according to industry standards, ensuring the tree’s safety and longevity.