Updated: Apr 28
The views and natural surroundings that are the essence of Creston are important and precious to all owners. They are, however, under constant siege from two foreign adversaries that seek to displace our native trees and shrubs (e.g. maples, oaks, poplars, sourwoods) in favor of noxious and somewhat Jurassic-looking tree species.
Yes, I’m talking about Tree of Heaven (Ailanthus) and Princess Tree (Paulownia). These trees were imported to the U.S. from China between 1780 and 1833 as ornamentals in and around the Philadelphia and New Jersey areas, but have evolved into an occupying force. You should know them when you see them. Tree of Heaven has large compound leaves which can easily be mistaken for native sumac or black walnut. To distinguish them, the Tree of Heaven leaves have notches at the base of each leaf pinna and are smooth, not serrated, along their edges. They also smell like peanut butter when crushed. Heart-shaped Paulownia leaves are among the largest on the mountain, only rivaled by perhaps sycamores.
Both trees grow in otherwise inhospitable soil but shun shade preferring open sunny spaces to populate. That is why you will see relatively more of each on the south side of our mountain and along roadways where sun is abundant.
The Creston Community has been at war with these invasive species, but especially Tree of Heaven (ToH), for over 8 years. The POA has hired professional crews to cut down trees in large stands, and the Maintenance Committee along with other volunteers have likewise cut trees with the aim of reducing the population. So, how are we doing?
Well, our approach has clearly reduced ToH numbers in the short term, improving opportunities for reforestation by native trees. However, the “cut ‘em down” strategy has likely awakened the invasives, like zombies, to come back in even greater numbers. Perhaps no example of this awakening is clearer than the stretch of south facing bank along Mount Hebron Road between Creston and Catawba Falls entrances. The simple, but extensive removal of vegetation by Duke Energy to install new power poles only a year ago has resulted in an explosion of both invasives at the exclusion of native trees. Why is this happening?
Well, both invasives have biological features that give them unfair population advantages over our native tree species. The first is their ability to propagate not only by seeds, but by “cloning” via shallow root suckers that can emerge as new trees up to 50 ft away from the parent trunk. The second is that they are allelopathic, meaning they release biochemicals from their roots that retard the health and growth of other plants in the vicinity, helping them crowd out the natives. And to make matters worse, cutting the tree actually signals the root systems to expand and sprout, exacerbating the invasion. Even poisoning cut stumps is of limited value only in the immediate area of the stump.
So, (with apologies to Rogers and Hammerstein) how do you solve a problem like Paulownia? Several agricultural research efforts (see references below) have suggested an approach that takes advantage of the biology of the trees. The goal is to deliver a strong herbicide targeted to the entire root system at a time when transport of photosynthetic nutrients from the leaves downward to the roots is highest. This is usually between mid-July and October.
The first method (basal bark treatment) involves spraying the preferred herbicide (Trichlopyr ester, tradename Garlon 4) diluted 1:4 in an oil base liquid (diesel fuel works well) on all of the bark of the tree from ground level up to about 16 inches. The oilbased vehicle drags the herbicide through the bark to the cambium transport layer of the tree to be sent to the roots. This method works for smaller trees with base diameters less than 6 inches.
For larger TOH, the ”hack-and-squirt” method is preferred by which a hand axe is used to cut into the bark at about a 45° angle at regular intervals (~ every 3 inches) and the herbicide (diluted 1:1 in water) is squirted into the resulting crevice. It is important not to girdle the tree, but to leave some of the transport system intact to carry the poison. Coloring the herbicide mixtures with a dye (see photo) helps keep track of treated trees. Continues on next page… Continues from the previous page… Hack-and-Squirt
Do these methods work? I have effectively killed roughly 60 TOH in a large stand on our property and along Warbler using a combination of both methods as appropriate. I plan a return treatment for remaining survivors, but indications suggest root sprouting has been dramatically reduced. These methods also work on Paulownia (see photos below). Triclopyr 4 (1 gal. for $68) can be purchased from Amazon along with blue lazer dye, surfactant, and squirt bottles. I’m happy to discuss these methods with anyone interested and help get you going.
So, it is advised not to simply cut down these trees as they will soon return with a vengeance. Rather use their unique biology against them with a well-timed application of herbicide that is less labor intensive than cutting. For more details on the rationale and methods, you can refer to these excellent websites from Rutgers and Penn State University.
Rutgers - Tree of Heaven Resource Site
Penn State University - Tree of Heaven Resource Site