“There’s a tree that grows in Brooklyn. Some people call it the Tree of Heaven. No matter where its seed falls, it makes a tree which struggles to reach the sky. It grows in boarded-up lots and out of neglected rubbish heaps. It grows up out of cellar gratings. It is the only tree that grows out of cement. It grows lushly . . . survives without sun, water, and seemingly without earth. It would be considered beautiful except that there are too many of it.”
This quote about the tree of heaven, Ailanthus altissma, which serves as the central metaphor for the story A Tree Grows in Brooklyn by Betty Smith, provides a glimpse into the reasons behind the species success outside its home range and what makes it such a powerful threat to our native habitat. While it serves as a great analogy in the popular story for the main character’s ability to thrive, this tree is an unwelcome guest in many places and has often been given the more appropriate moniker, the Tree of Hell.
The tree of heaven has a long history in the United States, first being brought over from Europe by a Philadelphia gardener in 1784 as an ornamental specimen. Once used in its native range of China and Taiwan for medicinal purposes and as a host plant to feed silkworms, the tree quickly became a favorite in urban landscapes in the United States due to its fast growth and resistance to air pollution. However, it wasn’t long before the tree’s invasive tendencies had worn the bloom off of the tree of heaven’s allure.
Tree of heaven has all the makings of a successful invader: prolific seed production, fast growth and ability to grow even in the most unfavorable conditions. Ailanthus can reproduce both sexually, from seed starting from seedling age as well as asexually from the roots and stump. The tree is dioecious, meaning it has male and female flowers on separate trees – but the female specimens have been known to produce as many as 325,000 seeds per tree! The tree easily can grow between 3 and 14 feet during its seedling years, reaching a maximum height of 65 to 100 feet tall when mature. Not only do young Ailanthus seedlings outcompete native species by their growth rate but also with a natural herbicide called ailanthone. This compound found in the tree’s bark, leaves, and other tissues is toxic to many other plants and can inhibit any neighboring plants from establishing nearby. Tree of heaven can also thrive where almost no plant species can, growing out of a crack in the sidewalk or in areas with high levels of sulfur and mercury (making it suitable for the hardships of an urban environment and leading to its prevalence in the cityscape).
Perhaps if Ailanthus wasn’t so invasive and there weren’t so many around, maybe we could find beauty in its tropical foliage and abundant winged seeds. With its large compound leaves the tree of heaven is often mistaken for our native black walnut or sumac species but there are some distinct differences: the tree of heaven has alternatively arranged compounded leaves made up of 11-25 leaflets with smooth, not serrated, margins and a glandular tooth located near the leaf base. And unlike the distinct leaf scar of the black walnut, the tree of heaven has large heart shaped leaf scars that are difficult to miss when the tree is without leaves. Its smooth bark conveys its impressive growth rate with tan fissures that look like stretch marks that appear as it matures. The flowers that occur in early summer are large clusters of yellow above the leaves with male flowers being more conspicuous and abundant than their female counterparts. In female trees these flowers gave way to winged papery seeds with twisted tips that turn reddish over the course of the season. Almost every part of the tree has lived up to Ailanthus’ Chinese translation of a “foul smelling tree,” with the male flowers and the bark and leaves (when crushed) smelling of burnt or rancid peanut butter
While undesirable in most places in the United States, the tree of heaven is notoriously hard to remove, especially once it has established a taproot. Specimens have been known to survive cutting, burning and having herbicide applied and come back all the more vigorous. Therefore, seedlings should be removed as early as possible to prevent establishment. Once the trees become larger repeated cutting and possibly even repeated herbicide applications to the cut stump or the leaves is required. The tree of heaven also lacks any major pests or pathogens that could potentially keep its population in check – however, prevention is always the preferred method of control.
For D.C. residents, since the tree of heaven poses such an environmental threat, its removal does not require replacement under the District Urban Forest Preservation Act and the Tree of heaven is not eligible for any of our residential planting programs. For more information about invasive species that threaten the District’s tree canopy and how to control these species, consider attending one of our non-native invasive removal workshops held in the spring or summer.