Oriental bittersweet (Celastrus orbiculatus) was introduced to the United States in the 1860s from east Asia. This woody, deciduous, perennial vine has since naturalized and become an extremely aggressive and damaging invader of natural areas. Oriental bittersweet chokes out desirable native plants by smothering them with its dense foliage and strangling stems and trunks. In some areas, it forms nearly continuous blankets along entire stretches of woodlands. Despite its aggressive nature and capacity to replace native plant communities, it is still sold and planted as an ornamental.
Single vines can reach 60 feet in length, though it will only grow as high as the vegetation it is climbing. As a perennial vine, it puts on yearly growth and can reach diameters of over 10 inches.
Distinctly round with toothed edges, the leaves are alternately arranged along the stem and between 3 and 4 inches in length. In late summer the leaves turn vivid yellow, usually before native plants gain their fall color, making this vine easy to spot from a distance.
Oriental bittersweet is dioecious; pollen and fruit are borne on separate male and female plants. In late spring, the female yellow-green flowers, each less than ½ inch long, grow from the leaf axils all along the stem in clusters of two or three. The male flowers are not distinct.
Yellow-skinned fruit first appear on female plants in late summer. In fall the yellow skin splits to reveal a bright red center. The fruit is retained on the stem through winter. The conspicuous combination of yellow and red make Oriental bittersweet simple to identify even after leaf drop.
Young growth is bright green; larger stems have red-brown bark that has a cracked, fish-netted texture. The smooth stems do not have tendrils, barbs, or aerial rootlets since Oriental bittersweet climbs by twining or winding itself around host plants.
American bittersweet (Celastrus scandens) is a similar but far less common native species that is listed as rare or vulnerable in several states. American bittersweet leaves are more football shaped than rounded. Their flowers and fruit also emerge only from the ends of the stems, rather than at each leaf axil, as with Oriental bittersweet. The fruit of American bittersweet also has a bright red covering instead of yellow. While the two species do hybridize where they co-occur, American bittersweet is rare enough that the likelihood of an individual being the nonnative invasive species is high.
Oriental bittersweet reproduces by seed and vegetatively by sprouting from an extensive root system. Its conspicuous fruit is spread primarily by birds and persists from late summer through winter. A significant vector of this vine is its continued use as a component of decorative wreaths—its seeds remain viable even after drying and can germinate once the wreath is discarded. Once an individual is established, it spreads by sending up sprouts from its roots. Following cutting, Oriental bittersweet resprouts vigorously from cut stems and roots.
While Oriental bittersweet prefers full sun, it tolerates dense shade while young. Sprouts growing in shade seek out full sun by climbing nearby vegetation and forming a blanket over the forest canopy. It thrives especially well in moist areas and areas with exposed mineral soil, such as disturbed sites, but it grows in many soil conditions, including sand dunes and bogs.
Though attacking the root system is the only way to kill the vine, freeing surrounding trees and other vegetation from the weight of the aerial stems by cutting them at ground level is typically the first step in controlling the vine. When mature, one root system may support dozens of stems, many of which may be very small or wrapped around desirable trees, making them impractical to treat with herbicides. Often, the most feasible approach is to cut the existing stems, forcing the roots and stumps to send up new shoots, and then treat the regrowth with foliar-applied herbicides.
On well-developed vines, most of the leaf area is in the upper canopy of the host tree, out of reach for foliar herbicide applications. Cutting the vines kills the aerial portion and forces the roots to generate new growth. Cutting can be done any time of year. The “window-cut” method is recommended, where each vine is cut in two places, at the ground and again at eye level. This ensures all vines are located and cut and clears the site at ground level to facilitate follow-up spraying. Do not pull the cut vines from trees; this can further damage host plants and pose safety risks. The dead vines will shed their leaves, dry, and decompose over time, so the weight will no longer be an issue.
Cutting alone is only effective at controlling the vines when resprouts are repeatedly cut until the root system is exhausted. This will take multiple cuttings annually over several growing seasons. Mowing has been shown to encourage root sprouting and may not control the plant even when repeated periodically. Missing even one cutting during this regimen is likely to give the vine a chance to recover and reestablish.
The most practical method to injure the root system of Oriental bittersweet is to treat the regrowth following cutting with a foliar herbicide application. Resprouts provide a smaller and more practical target for follow-up herbicide applications. Ideally, this should be done after the regrowth has had at least eight weeks to sprout. If treated too soon, the new foliage will still be growing aggressively, and the herbicide will not move into the root system. All herbicide treatments to vines should be made late in the growing season, no earlier than July 1, to enhance translocation to roots.
Treating stumps after cutting will reduce the amount of regrowth but not eliminate all root sprouts in most instances. Often, the best option is to simply cut all the vines and wait to foliar spray the regrowth. The challenge will be treating the new vines before they get a chance to intermingle with foliage of desirable plants. When spraying foliage, use a mixture of glyphosate and water-based formulations of triclopyr with a surfactant added. This mixture will not only control vine regrowth but can also be used to treat other invasive plants encountered during the operation.
Directly treating all vines on a well-developed infestation with stem treatments (e.g., hack and squirt or basal bark) is challenging and often impractical if the vines are tightly wrapped around desirable trees, as accidental application to the host tree is possible. Basal bark treatments are effective on stems under 6 inches in diameter. When making basal bark applications, use an oil-soluble triclopyr ester product and avoid getting spray solution on the bark of desirable trees and shrubs. Applying large amounts of concentrated triclopyr ester solutions to vines near the base of desirable trees poses a potential risk of injury if picked up through their roots and should also be avoided.
Stems at least 1 inch in diameter and larger that aren’t tightly twined around desirable trees can be treated using the hack-and-squirt method. This method is a highly targeted approach that uses a minimal amount of herbicide. A hatchet is used to make downward-angled cuts in the stem at a convenient height. Using a handheld sprayer, apply the water-based herbicide solution, saturating the cuts but avoiding runoff. To facilitate translocation to roots, space the cuts no more than 1 inch apart and do not girdle the stem.
Established root systems can be parent to many stems that can blanket trees with their rapid growth. Gaps created by broken limbs or downed trees open the canopy, releasing sunlight to the forest floor and providing favorable habitat for Oriental bittersweet to thrive. As described in prescriptions to address other invasive plant invasions, the best approach to combat this habit is to “save the best.” In other words, plan to work from the least to the most invaded areas or in areas where there is desirable native vegetation. This will maximize uninvaded acreage, which is not only of higher ecological value but also creates a much greater sense of accomplishment. Because Oriental bittersweet seeds are dispersed by birds, new invasions can and will occur. Spot removal of isolated individuals must be a part of any long-term invasive plant control program.