Controlling Non-Native Invasive Plants in Ohio Forests: Ailanthus (Tree-of-Heaven)

Ailanthus (Ailanthus altissima), also known as tree-of-heaven, is a moderate-sized (60- to 80-foot-tall) deciduous tree that can grow up to 6 feet in diameter. The tree was first introduced into the United States from Asia in the late 1700s for use as an urban landscape tree (Kasson, et al., 2013) and in strip mine reclamation in the Eastern United States. Ailanthus has numerous invasive characteristics:

• It grows rapidly with sprouts attaining a height of 6–12 feet the first year and growing 3 or more feet per subsequent year.
• It is a prolific seeder that may begin to bear fruit at age 2–3.
• It is a persistent stump and root sprouter.
• It is an aggressive competitor that thrives in full sunlight and flourishes with disturbance.
• It produces an allelopathic compound that suppresses seed germination and the growth of many native woody and herbaceous species.
• It grows in relatively infertile, shallow soils of varying pH, and is highly tolerant of poor air quality.

Identification

Ailanthus may be commonly mistaken for black walnut, sumac, or butternut (white walnut). A number of factors can be used to correctly identify a tree as ailanthus:

• Leaves are pinnately compound, 12–36 or more inches long, with 11–27 leaflets that are green on the upper side and grayish green on the lower side. Leaflets are long and tapered with lobed bases and smooth edges. Each leaflet has one to several glandular teeth near the base (Figure 1).
• Twigs are very stout but easily broken. They are light chestnut brown; are smooth to velvety with large tan bumps (lenticels); have a spongy, salmon-colored pith; and have heart-shaped leaf scars.
• Bark is pale gray and smooth with vertical streaking that develops into light-tan fissures with age.
• Flowers grow in large terminal clusters (up to 20 inches) of small, yellowish-green flowers, appearing from April to June. Male and female flowers grow on separate trees.
• Fruit grows as a samara or winged fruit. The color ranges from yellow-green to reddish-brown, contains a single seed, forms in clusters (panicles) visible from mid- to late-summer, and often persists through late winter or spring (Figure 2).

Note: All parts of ailanthus have a strong offensive odor (particularly the male flowers), often described as smelling like rotting peanuts or cat urine.

Control Methods

Environmental note: Many of the following herbicides are labeled to be mixed with a penetrating basal oil, diesel fuel, or kerosene as their carrier agent. The choice to utilize basal oil (particularly a methylated seed oil) instead of diesel fuel or kerosene is a more environmentally-friendly practice.

Controlling ailanthus depends on plant size, the size of the infestation, potential non-target impacts, and a landowner's comfort level with specific control methods. While not always easy to control, ailanthus less than 8 inches in diameter have proven considerably easier to control than larger trees. Small ailanthus trees up to ½ inch in diameter can be controlled by physical means or with foliar herbicides. Larger trees, up to about 8 inches in diameter, can be effectively controlled using basal herbicide application, with the need for occasional follow-up to control sprouting. Trees larger than 8 inches are commonly more difficult to control. Basal spraying, cut-stump herbicide application, or herbicide injection control methods may result in a need for follow-up treatments to control sprouting. Frilling or girdling is not recommended for killing ailanthus, as it almost always results in prolific sprouting. Whichever method is used requires monitoring of treated areas to make sure that 100% control is achieved. Any trees that survive initial treatment should be re-treated along with any new sprouts or seedlings. For a more detailed description of control methods, see Controlling Undesirable Trees, Shrubs, and Vines in Your Woodland (ohioline.osu.edu/factsheet/f-45). Figure 2 provides information on ideal and acceptable times of year to conduct herbicide treatments for ailanthus along with select tree physiology information.

MECHANICAL CONTROL

Small infestations of young plants can be pulled, dug, cut, or mowed. Because none of these methods remove the entire tree (including roots) they must be repeated multiple times until food reserves are exhausted, and the tree dies. Cutting and mowing will likely cause an abundance of resprouting from both the existing stump as well as the underground root system. This can create a situation where control becomes more difficult. Cutting and mowing is most effective when initiated in early summer when food reserves are at their lowest. Again, the key to any mechanical control is to repeat frequently until the food reserves are exhausted. Pulling or digging out small plants is most effective if completed when the ground is moist. However, the entirety of the root system must be removed to avoid resprouting.

Mechanical control is generally not an effective choice when dealing with larger trees. Cutting large stems often stimulates the production of hundreds of sprouts. The following control methods are more effective for larger trees.

FOLIAR HERBICIDE

Foliar spraying is a method of control in which a dilute herbicide is sprayed directly on the leaves (Figure 3). Herbicides need to be applied sometime after the plant is in full leaf and before the onset of fall color to maximize effectiveness. Generally, foliar herbicides are applied to thoroughly wet the leaves but not to the point of runoff.

Exercise caution when applying foliar herbicide. Only use herbicides when the target plants are within easy reach of the sprayer. Spray directed at taller or less-accessible plants can damage or kill valuable non-target plants through herbicide drift or overspray. Herbicides recommended for foliar spraying of ailanthus in a forest setting are listed in Table 1.

BASAL BARK APPLICATION OF HERBICIDE

Basal bark applications are commonly used to control small- to medium-sized ailanthus by spraying the circumference of the lower 12–18 inches of the trunk with an herbicide mixed in an oil carrier (Figure 4). This method is very effective in controlling trees up to 8 inches in diameter. Larger trees can be controlled, but research and experience is limited regarding the success rate; therefore, follow-up treatments will be necessary if sprouting occurs.

Current research and experience suggest that basal bark treatments can be applied effectively from May through October and may be most effective in the later part of the season. Herbicides recommended for basal spraying are listed in Table 2. To avoid the unintentional killing of desirable plants when basal spraying, special care should be taken to avoid over-spraying (spraying that does not strike or stay on the targeted trunk) or excessive spraying that causes runoff. Basal bark treatments should only be applied when the trunk is dry.

CUT-STUMP HERBICIDE TREATMENT

Cut-stump treatments for tree-of-heaven are generally not very effective (Figure 5). While cut-stump treatments will keep the stump from resprouting, they cause the root system to send up multitudes of sprouts. This will require multiple follow-up treatments to control the sprouting to eliminate the plant.

Table 3 contains water-carried and oil-carried herbicides for cut-stump application. Herbicides carried in water should be applied to the outer ¹/₃ of the top of the stump within minutes of making the cut. Oil-soluble herbicides should be applied to the entire surface and sides of the stump. When using an herbicide carried in oil, immediate application is not as crucial as when using an herbicide carried in water. These herbicides have proven to be most effective on ailanthus when applied from late May through November.

HACK and SQUIRT (HERBICIDE INJECTION)

Hack and squirt, or herbicide injection, involves introducing herbicide into an ailanthus tree through spaced cuts made around the trunk of the tree with an ax, hatchet, or tree injector (Figure 6). When using an ax or hatchet, small amounts of herbicide are applied to each cut using a pint or quart spray bottle, such as those available at garden stores. When using an injector, the herbicide is automatically applied to the cut when the cut is made. It is important to ensure gaps of living tissue are between the cuts in the tree as this living tissue helps translocate the herbicide into the root system.

Several types of injector systems are available including one that utilizes an herbicide shell. The choices between using an ax or hatchet versus an injector, and the type of injector, depend on individual preference, the herbicide to be used, and the size of the job. Because the herbicide is applied to only a small area, this method minimizes the potential for non-target effects through overspray, drift, or runoff. However, non-target species can still be impacted if they are connected to the targeted plant through root grafts. This method can result in extensive sprouting and require follow-up treatments during periods of heavy sap flow or severe drought. Herbicides recommended for hack and squirt or injection systems are listed in Table 4.

Future Control Options

Researchers are experimenting with a more natural and perhaps more effective way to control ailanthus. A naturally occurring, soil-borne fungus, Verticillium nonalfalfae, causes the vascular tissue of ailanthus trees to shut down, effectively killing the tree. This fungus has been found to kill stands of ailanthus trees in Pennsylvania, Virginia, and Ohio. Early results show the fungus spreads rapidly throughout an ailanthus stand after inoculating a few trees within the stand. Tests have shown that Verticillium nonalfalfae severely impacts ailanthus with little impact on any other plant species.

Several companies are trying to replicate this natural fungus into a commercially viable biological control for dealing with ailanthus. Research is underway in the United States and Europe to create this product and make it available for purchase. Currently, a timeline is unavailable for a commercial version of this type of product.

Summary

Label recommendations should be followed to maximize the potential for successful control. At a minimum, monitor treated ailanthus trees for two years to determine if complete control is achieved. Trees that resprout or are not completely killed by the first treatment will require a follow-up treatment. Ailanthus seeds may remain viable in the soil for longer than five years (Rebbeck & Jolliff, 2018). New seedlings can develop in treated and untreated woodlands.

Herbicides, like all pesticides, are approved (labeled) for specific uses by the Environmental Protection Agency. Approved uses and application methods are listed and described on each herbicide's label. The herbicides listed in this fact sheet were appropriately labeled at the time of publication. Because pesticide labeling may change at any time, you should verify that a particular herbicide is still labeled for your intended use. At the time of this writing, copies of most herbicide labels and Safety Data Sheets could be obtained online at the Crop Data Management System website (cdms.net). Others are available through the websites of the individual manufacturers.

Ohio State University Extension does not endorse any of the products mentioned in this fact sheet and assumes no liability resulting from the implementation of these recommendations.

Additional Resources

Download a PDF of this fact sheet.

References

Kasson, M. T., Davis, M. D., & Davis, D. D. (2013). The invasive Ailanthus altissima in Pennsylvania: A case study elucidating species introduction, migration, invasion, and growth patterns in the northeastern U.S. Northeastern Naturalist 20(10), 1–60.
DOI:10.1656/045.020.m101

Rebbeck, J. & Jolliff, J. (2018). How long do seeds of the invasive tree, Ailanthus altissima remain viable? Forest Ecology and Management 429, 175–179.
DOI:10.1016/j.foreco.2018.07.001

Originally written in 2009 by Kathy Smith, Program Director–Forestry, School of Environment and Natural Resources, Ohio State University Extension; and Annemarie Smith, Invasive Species Forester, Ohio Department of Natural Resources, Division of Forestry.