The Tree of Heaven (Ailanthus altissima) is an exotic deciduous tree native to China that has become a highly invasive species across North America. Introduced in the late 1700s, it spread quickly due to its rapid growth rate and tolerance for poor soil and pollution. Identification is important because its aggressive root system can damage pavement and foundations. The species reproduces aggressively via root suckers and prolific seed production, forming dense colonies that displace native vegetation. It also serves as the preferred host for the invasive spotted lanternfly.
Identifying Features of the Leaves and Bark
The leaves of Ailanthus altissima are pinnately compound and large, often reaching lengths between one and four feet. Each main leaf stem holds numerous individual leaflets, typically ranging from 10 to 41. The most reliable visual identifier is the leaflet margin, which is smooth (entire) along its length. This smooth margin is broken only at the very base of each leaflet, where one or two prominent, rounded protrusions known as glandular teeth are located. These basal glands help distinguish the Tree of Heaven from many native look-alikes. Young trees have smooth, pale gray or brownish-green bark. As the tree matures, the bark develops a light tan or gray-brown color with vertical fissures. This mature bark texture is often described as resembling the mottled skin of a cantaloupe. The twigs are stout and easily broken.
The Unique Odor and Seed Structure
The Tree of Heaven is often called the “stink tree” because it emits a foul odor when any part of the plant is crushed or damaged. This sensory cue is a definitive method for positive identification. The smell is often compared to rancid peanut butter, stale gym socks, or burnt oil. The odor is released from the leaves, broken twigs, and even the bark when scratched. Female trees produce distinct fruit in late summer and fall. These seeds are enclosed in a samara, a dry, winged fruit that is twisted and flattened. Samaras are typically one to two inches long, with the single seed located in the center of the wing. They are borne in large, dense, hanging clusters that transition from green to dull orange or reddish-brown as they mature. The clusters often remain on the female tree long into the winter.
Distinguishing Tree of Heaven from Look-Alikes
Misidentification is common because the Tree of Heaven shares the compound leaf structure with several native species, most notably Sumac (Rhus species) and Black Walnut (Juglans nigra). The most reliable point of differentiation lies in the margins of the leaflets. Both Sumac and Black Walnut have serrated, or finely toothed, margins along the entire edge of their leaflets. In contrast, the Tree of Heaven leaflet is smooth (entire) except for the one or two glandular teeth at its base. Another distinction is found by examining a broken twig: the Tree of Heaven has a large, spongy, uniform brown pith (the soft tissue at the center of the stem). Black Walnut, however, possesses a dark, chambered pith that is partitioned into small, hollow sections. Differentiation can also be made by observing the fruit and growth habits. Black Walnut produces large, round nuts, and Sumac produces dense, upright clusters of fuzzy, red berries. Sumac species also tend to be shorter, more shrub-like trees. The large, twisted samaras and the basal glands remain the most practical methods for distinguishing it from its native counterparts.