UNIVERSITY PARK, Pa. -- Horticulture researchers in Penn State's College of Agricultural Sciences are developing a program to rid the state's highways of tree of heaven, an invasive plant species that grows so fast and so prolifically that it has been nicknamed "the tree from hell."
"Tree of heaven can grow anywhere -- between cracks in pavement, in drainage inlets and in poor, rocky soils along roadsides," says Larry Kuhns, professor of ornamental horticulture. "It's becoming a plague on Pennsylvania roadways because other control programs, such as mowing or cutting, actually cause the tree to grow more prolifically due to its ability to produce new stems from its extensive root system."
An invasive plant is defined as a plant that grows aggressively, spreads and displaces other plants in an ecosystem. Tree of heaven fits all three criteria. The trees can grow several feet in a single year, and up to 15 feet from stumps if established trees are cut to the ground. It will reach heights of 40 to 60 feet very quickly, which can obscure road signs and sight lines along rights of way.
Kuhns and two project associates in horticulture, Art Gover and Jon Johnson, have been refining a four-step process that will significantly reduce tree of heaven infestations. The project is funded by the Pennsylvania Department of Transportation.
The program calls for an initial treatment to control the existing stems; a follow-up herbicide treatment to the foliage of stems that have resprouted; reseeding the infested area to provide competition from other plants; and establishing an annual preventive maintenance program to treat any resprouted stems while they are small.
The initial treatment can involve cutting and treating the stump with herbicide, or applying an herbicide mixed in a penetrating oil to the lower 18 inches of the stems. Because new sprouts are inevitable after either treatment, it is essential to apply an herbicide to new shoots during the summer or early fall. The researchers developed a grass seed mixture, called Formula L, which provides competition, stabilization and erosion control for soils.
"Eliminating a well-established infestation is a long-term process that requires the integration of many different techniques," Kuhns says.
Kuhns says the aggressive growth of tree of heaven will require PennDOT to adopt a different strategy in its brush control program. "It's the only way to eradicate this highly adaptive plant from roadsides," Kuhns adds.
Tree of heaven (Ailanthus altissima) came to the United States from Asia in the 18th century. Originally an ornamental, the fast-growing tree became featured in many gardens. Tree of heaven also was prominently featured in literary circles as the central metaphor for the resiliency of an impoverished family in the novel "A Tree Grows in Brooklyn," by Betty Smith.
The tree's resiliency and amazing capacity to take root anywhere is the primary reason the plant has become an invasive tree species. "The seeds produced by tree of heaven are shaped like a wing with the ends twisted like a propeller," Kuhns explains. "The wind can carry the seeds quite a distance. The tree has infested roadways so easily because major roads are like wind tunnels."
Kuhns says the tree also establishes forest stands quickly because each tree produces root sprouts that grow in concentric rings around the "mother" plant.
"Tree of heaven grows fast, and its wood is weak, which is a very bad combination," Kuhns says. "It can fall across roadways in snow, ice or windstorms."
Tree of heaven also spreads easily from rights of way to adjacent fields, where the hardy tree can wreak havoc with farm equipment. "A cornpicker can cut a small tree of heaven, but cutting does not kill the roots," Kuhns explains. "Even after successive cuttings over several seasons, the growth energy stored in the roots can produce -- in one growing season -- a plant 10 feet tall that can break blades and damage farm equipment."
Kuhns also points out that cutting larger trees causes the roots to produce many root sprouts. "If you cut one tree, 20 may come up to replace it," he says.
Like many invasive weeds, Kuhns says tree of heaven reproduces geometrically. "If you have one tree of heaven one year, the next year you may have dozens instead of four."
Kuhns says tree of heaven has some weaknesses. It doesn't tolerate shade well, so heavily wooded areas have little problem with it. It also does not transplant well, which prevents well-meaning gardeners from using it in landscapes.
EDITORS: For more information, contact Larry Kuhns at 814-863-2197.
Contacts: John Wall email@example.com 814-863-2719 814-865-1068 fax