Tree-of-heaven and the Spotted Lanternfly: Two Invasive Species to Watch

Increasingly, Pennsylvania's forests are experiencing threats from invasive insect and plant species. Right now, one insect and one tree species, which maybe inextricably linked, are in the news. They are the spotted lanternfly, which was accidentally introduced to the United States in Pennsylvania in 2014, and the tree-of-heaven (Ailanthus altissima), purposely introduced to the United States and Pennsylvania in 1784. How do these two species relate?

First, if you have not heard of the spotted lanternfly, you will. In just four years, this insect from China has rapidly spread, and is found in 13 southeastern Pennsylvania counties, which are now under quarantine. The spotted lanternfly is threatening fruit and grape production (think about Pennsylvania's burgeoning wine industry) and at least 25 forest tree species. To learn more about the spotted lanternfly, start your web search at Penn State Extension's website. This insect is a major threat and worthy of everyone's attention.

Tree-of-heaven is also native to China. For years, foresters and woodland owners across Pennsylvania and the east have worried about how this invasive tree species can and will affect forests. It is a rather aggressive invasive species that shows up in unexpected places. It is especially well suited to urban environments and does well on disturbed sites such as those found along transportation rights of ways. It can even take advantages of cracks in sidewalks. You may have heard of it as "A Tree Grows in Brooklyn." This is the same tree.

So, both the spotted lanternfly and tree-of-heaven are from China. Why is this important? The science is still unfolding. While it seems that the spotted lanternfly can reproduce on many trees and shrubs, it does seem to show a distinct preference for tree-of-heaven. However, researchers are working on this question. Importantly, though, controlling tree-of-heaven could play a role in slowing the lanternfly's advance. Therefore, learning to identify tree-of-heaven is the first step in managing the pest's spread.

In late summer and early fall you might easily recognize tree-of-heaven as the female trees have showy clusters of orangey-red maturing seeds across the upper crowns, which look a bit like uniquely colored hydrangea flowers at a distance. Later, the color will fade, becoming off-white and remaining until spring in the leafless trees. The compound leaves look very similar to black walnut and sumac; however, on closer examination, they have many more leaflets (from 15 to 41) and approach 3-feet in total length. Each of the individual leaflets has a small gland at the base of the blade near the petiole. The real telling difference is the offensive smell when leaves or stems are crushed, which is sometimes described as "spoiled peanut butter."

Tree-of-heaven grows rapidly and can become relatively large. Last measured in 2004, Pennsylvania's largest tree-of-heaven was 80 feet tall, with a crown spread of 50 feet, and 4 feet, 9 inches in diameter. When suppressed under a forest canopy or pruned in urban settings, tree-of-heaven may be shrubby. It has smooth, thin bark and a straight stem.

A challenge to controlling tree-of-heaven is its rooting characteristics and its response to the cutting of its stems. The tree develops large thick roots near the trunk, which extend into a network of finer shallow roots. These wide spreading roots readily sprout, often resulting in the appearance of dense clumps of smaller trees near larger stems. When individual stems are cut, the root system prolifically sprouts. One study found that two years after a tree-of-heaven harvest in Pennsylvania, there were on average 17,860 two-year old sprouts per acre averaging 9 feet in height and an additional 10,019 sprouts one-year old and averaging 2 feet in height. Spouts will rapidly self-thin, but can form a competitive canopy. While the tree is generally considered short-lived (30 to 70 years), sprouts from the first tree planted in 1784 were still growing in Philadelphia's Bartram Botanical Garden at the turn of the 21st century. Tree-of-heaven is allelopathic, meaning it exudes chemicals that may suppress other tree species, which further benefits is ability to colonize areas.

While root sprouting is a major way by which the tree colonizes, reproduction also occurs from seed. Individual flowers can contain hundreds of seeds. Estimates are that individual trees may produce upwards of 350,000 seeds annually. In a 1928 publication on tree-of-heaven, it was reported that a 12-inch diameter tree in Pennsylvania in one year produced more than one million seeds. Most seeds are viable, even those that overwinter on the tree and disperse in spring. Reportedly even small breezes can lead to wide dispersal. To successfully seed into an area, though, the seeds need sufficient light.

Limiting the spread and occurrence of tree-of-heaven might slow spotted lanternfly spread and subsequently protect other plants and trees injured by its feeding. With all invasive species, the first response is to recognize the problem and control it early. Early efforts can involve less heroic measures and simply pulling young shoots and avoiding breaking roots may find some success. Unfortunately, tree-of-heaven's response to cutting is prolific sprouting. Therefore it is necessary that all control treatments have a direct effect on root sprouting capacity, which immediately suggests the need to include targeted herbicides to control sprouting. Foliar and basal herbicide applications are effective on smaller trees. Larger trees are best controlled with stem injections followed by cutting. To learn more about controlling tree-of-heaven, visit Penn State Extension's website.

To learn more about Ailanthus altissima visit the US Forest Service's tree database.