Tree of Heaven

Overall Appearance

Tree-of-heaven grows rapidly, up to 30 metres in height and 2 metres in diameter. It can be confused with tree species that are native to Canada, such as black walnut, staghorn sumac, and ash. When the leaves or twigs are crushed, they release a foul odor that can help distinguish tree-of-heaven from similar native species.

Leaves

Tree-of-heaven leaves are large (30-120 cm) and compound, meaning they are comprised of multiple leaflets, which are arranged alternately along a central stem. Examining certain features of their leaflets closely will aid in distinguishing tree-of-heaven from other similar-looking species. Tree-of-heaven leaflets have smooth (as opposed to toothed) margins, except for one or two lobes called glandular teeth at their base. There is a visually noticeable gland on the underside of these teeth.

Twigs and Bark

The twigs on a tree-of-heaven are hairless and can appear greenish, pink, reddish, or brown. Their heart-shaped leaf scars and spongy brown centers when broken are distinguishing features. The bark of young trees is thin, smooth and brownish-green, becoming thicker, rougher, and light brown to grey in colour when mature.

Flowers and Fruit

Tree-of-heaven flowers are small and pale yellow to green, growing on the tree in large clusters. Female trees produce clusters of samara fruits: single seeds encased in a papery wing. The samaras are 1-2 inches long. Note that black walnut and staghorn sumac do not have samaras.

Tree-of-heaven reproduces primarily through wind dispersal of its samaras. It has a high seed production (over 300,000 seeds annually) and the seeds have a high germination rate. The tree also reproduces vegetatively by sprouting shoots from its roots, and can even reproduce from root and stem fragments. Individuals produced in this way are clones of the parent plant and can grow in large clusters. Tree-of-heaven is a dioecious plant, meaning individual trees are either male or female.

Tree-of-heaven has been transferred and planted intentionally around the world as an ornamental plant. Seeds and roots are easily transferred unintentionally by vehicles and machinery.

In North America tree-of-heaven is generally found in urban areas where it can tolerate the poor soil and air conditions, but it can grow in a wide variety of habitats (Saldonja et al., 2015). Being intolerant of shade, it will quickly colonize disturbed areas like forest openings and edges. It has become a common (and sometimes even dominant) component of forest vegetation in parts of the U.S., having been found in hemlock, oak-hickory, and maple-birch forests (Saldonja et al., 2015). Corridors, like roadsides, railways, and riverbanks, can help facilitate tree-of-heaven’s spread. This tree’s ability to establish under poor environmental conditions (like drought and pollution) that many native plants would not tolerate contributes to its invasiveness.

Native to northeast and central China and Taiwan, tree-of-heaven was introduced to North America in 1784 near Philadelphia (Jackson and Wurzbacher, 2020). It was introduced again to New York in 1820 and to California in the 1850s (Fryer, 2010). Tree-of-heaven became available in nurseries by 1840 and was planted in urban areas, having been considered desirable as a hardy ornamental plant (Jackson and Wurzbacher, 2020). It is now widespread in the United States. In Canada, tree-of-heaven has been found in British Columbia, Ontario and Quebec (Canadian Food Inspection Agency, 2021). Although its distribution is limited by cold temperatures (occurring from 22^o to 34^o N in latitude), it has been documented on every continent except Antarctica (Udvardy, 1998, as cited in Cornell Cooperative Extension, 2019).

Ecological Impacts

Once tree-of-heaven has colonized a natural area, it can displace native plants, altering the surrounding plant community (Saldonja et al., 2015). Tree-of-heaven does this through outcompeting surrounding plants for water, sunlight, space for roots, and nutrients, and through allelopathy, meaning it releases toxins into the soil that inhibit other plants’ growth. A shade-intolerant plant, tree-of-heaven particularly impacts early-successional environments, where there is full sunlight. As a result, it can completely alter the trajectory of ecological succession (the shifting species composition of plant communities, as soil chemistry, amount of sunlight, and other conditions change over time after a disturbance such as logging) in that area.

Tree-of-heaven is also the preferred plant host for spotted lanternfly, a destructive invasive pest that will cause significant harm to grape vines, tender fruit trees, and forests if it establishes in Canada. Spotted lanternfly is a nuisance in urban areas as well, as it swarms trees in large numbers and secretes a sticky substance.

Economic and Social Impacts

In urban areas where tree-of-heaven has often been planted, it can be detrimental to the urban landscape and to human health. The tree’s roots break up asphalt and crack through walls and building foundations, and it can also enter sewers (Saldonja et al., 2015). It is costly and difficult to eradicate.

Tree-of-heaven’s pollen causes allergies (Kowarik and Saumel, 2007, as cited in Saldonja et al., 2015) and its sap causes dermatitis and very rarely, myocarditis if it enters the bloodstream (Bisognano et al., 2005, as cited in Saldonja et al., 2015). The tree also has an unpleasant odour.

In its native range in Asia, different components of the tree-of-heaven have been historically used in folk medicine. With antibacterial, antioxidant, and anti-inflammatory properties, tree-of-heaven may have modern medicinal potential as well (Fryer, 2010). In China the tree is important for timber and fuelwood (Fryer, 2010).

Tree-of-heaven has been used in its invasive range for restoration of disturbed, toxic sites, since it has much higher tolerance for pollutants like mercury and sulfur, saline soils, and low pH than most native species (Fryer, 2010). However, given its extremely invasive properties it is usually best avoided even for ecological restoration purposes.