The Tree of Heaven (Ailanthus altissima) is a highly invasive species in North America, establishing itself aggressively in disturbed areas, roadsides, and urban environments. This fast-growing tree poses a serious threat to native ecosystems by rapidly displacing indigenous flora and causing damage to infrastructure like pavement and sewer systems. Eradication is difficult due to its unique biology, including the production of allelopathic compounds that suppress the growth of nearby plants. Successfully killing a Tree of Heaven requires employing specific, targeted techniques designed to destroy the extensive root system.
Identifying the Target: The Biology of Tree of Heaven
The primary reason Tree of Heaven is challenging to kill lies in its robust capacity for vegetative reproduction. Unlike many trees, this species responds to damage by aggressively sending up numerous new shoots, known as root suckers, from its widespread lateral root system. Simple cutting or mowing a mature tree is ineffective and usually worsens the infestation by stimulating this prolific suckering response.
The tree’s ability to reproduce is also enhanced by its high output of seeds; a single mature female tree can produce hundreds of thousands of wind-dispersed, winged seeds each year. Furthermore, the plant releases allelopathic chemicals, particularly the compound ailanthone, which accumulate in the surrounding soil and inhibit the growth of competing native plant species. This combination ensures the tree quickly forms dense, single-species thickets, making eradication a complex, multi-year process.
Chemical Eradication Techniques
The most dependable method for achieving permanent eradication of Tree of Heaven is the use of systemic herbicides. These must be applied in a way that ensures the chemical translocates effectively to the root system. Proper timing is paramount: herbicides should be applied in mid-to-late summer, typically from July until the onset of fall color. This timing coincides with the period when the tree is actively moving carbohydrates down to its roots for winter storage, maximizing the delivery of the herbicide to the target.
Hack-and-Squirt Treatment
The hack-and-squirt method is one of the most effective and selective techniques for treating mature trees with a trunk diameter of one inch or larger. This process involves using a hatchet to make downward-angled cuts, or “hacks,” that penetrate the bark and cambium layer into the sapwood. A concentrated herbicide, such as glyphosate (50-100% concentration) or triclopyr amine (100% concentration), is immediately applied into the pocket created by the cut, typically 1 to 2 milliliters per cut.
It is crucial not to completely girdle the trunk; a minimum of two cuts should be made, leaving one to two inches of uncut bark between them. This allows the herbicide to be carried to the roots through the vascular system. If the tree must be removed, apply the herbicide first and wait at least 30 days before cutting it down to ensure the chemical moves into the root network. Treating a newly cut stump is largely ineffective for this species.
Basal Bark Treatment
For smaller trees and saplings, generally those less than six inches in basal diameter, the basal bark method offers a highly targeted approach. This technique uses an oil-soluble formulation of a systemic herbicide, such as a triclopyr ester, mixed with a penetrating oil. The mixture is applied to the entire circumference of the lower trunk, from the ground line up to a height of 12 to 18 inches. Ensure the bark is completely saturated but avoid excessive runoff. This oil-based formulation allows the herbicide to be absorbed directly through the bark without making cuts.
Mechanical and Biological Control Options
While chemical methods are the most reliable for mature trees, non-chemical approaches can be used in specific situations, such as near waterways or for very young plants. Manually pulling or digging is only effective for small seedlings before the taproot becomes established, which can happen in as little as three months. Because broken root fragments can sprout, mechanical removal of mature trees is not recommended without chemical follow-up.
Girdling involves removing a continuous ring of bark and cambium completely around the trunk, which kills the above-ground portion of the tree. However, girdling alone is not a successful eradication method because it almost always triggers an aggressive production of root suckers. To improve success, an herbicide should be applied immediately to the cut surface of the girdle to prevent this suckering response.
A promising long-term solution lies in biological control using the native soil-borne fungus, Verticillium nonalfalfae. This fungus causes a vascular wilt disease that is highly virulent and specific to Tree of Heaven, often killing the tree within two to four months after infection. While this biological agent is not commercially available for general use, its natural occurrence and research into its application represent a significant step toward large-scale management.
Post-Treatment Monitoring and Management
Successfully killing Tree of Heaven requires a long-term commitment to monitoring and follow-up treatment, as the extensive root systems retain energy reserves for a significant time. Even with effective systemic herbicide applications, new sprouts, or suckers, are likely to emerge in the years following the initial treatment. These suckers must be addressed immediately to prevent the root system from recovering and rebuilding its carbohydrate reserves.
Instead of cutting the suckers, which stimulates more growth, they should be treated with a foliar application of an appropriate herbicide, typically a low-concentration glyphosate or triclopyr mixture. This ensures the chemical is carried back to the root system. Once the tree is confirmed dead, which may take up to a year or two, the site should be immediately planted with native species. Establishing a thick cover of native plants helps occupy the vacant niche, shading out new seedlings and discouraging re-establishment from the persistent seed bank.