The term “locust tree” commonly refers to two species: the Black Locust (Robinia pseudoacacia) and the Honey Locust (Gleditsia triacanthos). Both are known for their hardiness and fast growth, but they can develop troublesome root systems, with specific problems varying between the two. Understanding these root issues is the first step toward effective management, which involves cultural practices, strategic planting, and targeted intervention. This guide addresses the most common root-related complaints and provides actionable solutions.
Surface Root Hazards and Infrastructure Protection
Mature locust trees often develop shallow, wide-spreading lateral root systems that cause physical hazards and infrastructure damage. This tendency is exacerbated in poor growing conditions, such as compacted clay soil, where roots grow close to the surface seeking adequate oxygen, water, and nutrients. Exposed lateral roots create tripping hazards in lawns and interfere with routine mowing operations.
The expansion of these surface roots exerts immense pressure on nearby hardscapes, leading to cracked sidewalks, lifted driveways, and compromised patio foundations. Aggressive locust roots, especially those of the Black Locust, are notorious for invading and choking underground drainage pipes and sewer lines, seeking moisture from small leaks. Cutting these surface roots is ill-advised, as they are crucial for anchoring the tree and transporting water, and their removal can severely injure the tree.
Mitigation strategies focus on encouraging deeper root growth and physically redirecting or blocking root expansion near vulnerable structures. Applying a deep layer of organic mulch, two to four inches deep, over the root zone helps retain moisture and insulate the soil, discouraging roots from growing directly on the surface. For protection against infrastructure damage, a professional-grade physical root barrier can be placed between the tree and the vulnerable structure. This sturdy plastic panel must be installed vertically to a sufficient depth, typically several feet, to redirect the lateral roots downward.
Controlling Aggressive Root Suckering
Root suckering is the aggressive growth of new shoots directly from the tree’s root system, often appearing many feet away from the main trunk. This phenomenon is primarily associated with the Black Locust (Robinia pseudoacacia), which has an extensive network of lateral roots that readily sprout, forming dense clonal colonies. While the Honey Locust is less prone to this, suckering can still occur in older or stressed specimens.
Suckering is often intensified by damage to the root system, such as from construction activities, tilling, or frequent mowing over the root zone. When the main trunk is cut down, the remaining root system releases a surge of growth hormones, causing a flush of new suckers as the tree attempts to regenerate. Simply cutting these sprouts back is ineffective and counterproductive, as the injury stimulates the root to produce even more growth.
Effective control requires a systemic approach using herbicides. Systemic herbicides containing triclopyr or glyphosate are absorbed by the plant tissue and transported throughout the entire root system. For removing a mature tree, the most effective method is a cut-stump application. A concentrated herbicide solution is applied directly to the freshly cut surface, targeting the cambium layer just inside the bark, within minutes of cutting. Alternatively, young suckers can be treated with a foliar spray or a basal bark application of a triclopyr-based product.
Identifying and Treating Root Pests and Diseases
Locust trees are susceptible to several biological threats that compromise root health, with fungal root rot being a common and serious issue. Root rot is frequently caused by poor soil drainage or excessive irrigation, which creates an anaerobic environment where water molds and fungi thrive. The compromised roots cannot effectively take up water and nutrients, leading to above-ground symptoms such as general yellowing of the leaves, canopy dieback, and premature leaf drop.
Another threat is the wood-boring insect, such as the Honeylocust Borer, which targets trees already stressed by environmental factors or disease. These borers typically attack the root crown, the area where the trunk meets the soil, tunneling under the bark and disrupting the tree’s vascular system. External signs of a borer infestation include small, circular exit holes on the trunk and oozing sap at the wound sites.
Preventative measures are the most reliable defense against these biological threats. Ensuring proper soil drainage is paramount, and avoiding overwatering, especially in heavy clay soils, prevents the conditions that lead to root rot. Maintaining the tree’s vigor through appropriate fertilization and avoiding soil compaction around the root zone helps the tree resist pests. If root rot is severe, indicated by extensive death and discoloration of bark and wood at the base, the tree may require removal, but for less advanced cases, crown reduction pruning can sometimes help the tree recover.