Celastrus orbiculatus Thunb.

Oriental bittersweet infests forest edges, woodlands, fields, hedgerows, coastal areas and salt marsh edges, particularly those suffering some form of land disturbance. While often found in more open, sunny sites, its tolerance for shade allows oriental bittersweet to invade forested areas.

More info for the terms: competition, cover, density, fire management, frequency, fresh, hardwood, introgression, invasive species, liana, litter, natural, nonnative species, prescribed fire, presence, root crown, shrubs, succession, tree

Impacts: Oriental bittersweet is considered a "severe" pest plant in the Northeast and Southeast [152]. In the Northeast, it is listed as a high threat in deciduous, coniferous, and mixed conifer-deciduous forests, old fields, grasslands, riparian areas, and fresh wetlands; and an unknown threat in tidal wetlands [21]. On the west end of Long Island, for example, Oriental bittersweet was the most abundant nonnative species on Jamaica Bay Wildlife Refuge, where it invaded old fields, thickets, and woodlands [146]. Invasion traits of Oriental bittersweet include:

Reports of Oriental bittersweet's invasiveness vary. Some classify it as invasive [38] to highly invasive [24]. Voss [166] describes it as "sometimes aggressive" when escaped from cultivation in Michigan. On the Energy Oak Ridge National Environmental Research Park, Tennessee, Oriental bittersweet was ranked the 5th most invasive nonnative dicot and the 9th most invasive nonnative plant species overall [22]. In Farmington, Maine, surveyors of invasive nonnatives in mixed hardwood-spruce (Picea spp.) forests ranked Oriental bittersweet as intermediate in abundance, behind Japanese knotweed (Polygonum cuspidatum), Morrow's honeysuckle, Tatarian honeysuckle (Lonicera tatarica), and hybrids of the 2 honeysuckles [6].

The ease of seed dispersal and horticultural interest in Oriental bittersweet in the East and elsewhere in the United States creates a potentially large area for Oriental bittersweet invasion [24]. The southern Appalachians are particularly affected by new invasions [24,81]. In southwestern North Carolina, for example, Oriental bittersweet was one of the most commonly encountered nonnative species, occurring at 53% mean frequency on all of 25 watersheds sampled [80]. Available data and climate models suggest that Oriental bittersweet is likely to benefit from warming temperatures and increasing precipitation in the Northeast, where it is likely to increase and spread northward (review by [27]).

Oriental bittersweet presence may alter soil chemistry, plant succession, and stand structure; threaten native plant diversity; and reduce productivity in silvicultural and agricultural systems.

• Effects on soils
• Effects on succession and stand structure
• Effects on diversity
• Effects on silvicultural and agricultural systems

Effects on soils: Oriental bittersweet may alter soil pH and nutrient levels [88], and Oriental bittersweet's relatively deep roots may allow it to outcompete surrounding vegetation for soil resources [139]. In hardwood forests of Connecticut, Oriental bittersweet litter contained 3 times more calcium than red oak litter, and its leaf pH was higher. The authors concluded that these factors led to the higher soil pH on plots with than on plots without Oriental bittersweet [88]. See Soils for further information.

Effects on succession and stand structure: Oriental bittersweet may outcompete native vegetation for light and modify stand structure, altering historic patterns of plant succession ([9,24,164], review by [152]). Photosynthesis of host and understory plants can be reduced or prevented by Oriental bittersweet. Patterson [120] noted the scarcity of other plant species beneath Oriental bittersweet canopies in Pennsylvania and attributed it to shading by Oriental bittersweet. Oriental bittersweet's growth habit (blanketing and shading out support species) negatively affects the health of host plants and increases continuity of vegetation among forest strata [26,34,48,93] (see Fuels). Twining Oriental bittersweet stems may girdle support vegetation, restricting sap and water flow and damaging or killing host plants. Damaged hosts are at risk for stem breakage and uprooting from ice- and windstorms [24,25,26,93,104,152]. Oriental bittersweet may overtop other plant species in all strata. It may also inhibit or facilitate growth of other lianas. In Connecticut, Oriental bittersweet altered hardwood forest succession by inhibiting reproduction and growth of native shrubs and trees and facilitating growth of fox grape, a late-successional native liana, into the canopy (see Old fields) [34]. Conversely, Oriental bittersweet interfered with growth of native grapes (Vitis spp.) on the Pisgah National Forest, North Carolina [104]. See Successional Status and Stand structure for more information.

Effects on diversity: Oriental bittersweet can displace native species. Its thickets [104] and climbing stems cast too much shade for many native plant species to establish and grow. For example, Oriental bittersweet canopies inhibited establishment of understory spring ephemerals in Illinois [65]. Along the Blue Ridge Parkway in North Carolina, Oriental bittersweet cover was negatively associated with native plant diversity [48]. On Plummer's Island in the Potomac River of Maryland, a 1912 survey documented presence of American bittersweet and common hop (Humulus lupulus), but in 1980, surveyors concluded that those species "appear to have been replaced entirely by the aliens" Oriental bittersweet and Japanese hop (H. japonicus) [141].

Oriental bittersweet is apparently expanding its range at the expense of American bittersweet [127,148,152]. Partially as a consequence of Oriental bittersweet competition, American bittersweet has protection status is several areas [161]. For example, Connecticut lists American bittersweet as a "species of special concern", and Great Smoky Mountains National Park lists American bittersweet as a "nonreproducing rare plant" (Langdon 1993 cited in [24]). The same characteristics that make Oriental bittersweet often preferred over American bittersweet as an ornamental: faster growth, greater fecundity, and a higher tolerance to varying environmental conditions, are the same characteristics that have enabled Oriental bittersweet to become a successful invader [81]. A field study showed Oriental bittersweet increased its photosynthetic rate with increasing light intensity, while American bittersweet's photosynthetic rate tended to saturate under low light conditions [15]. In a common garden study, Oriental bittersweet showed significantly higher photosynthetic rates and faster growth, aboveground biomass gain, and survivorship than American bittersweet in both sun and shade. Oriental bittersweet showed a positive growth response to presence of neighbors, while American bittersweet's response to neighbors was neutral (P=0.05 for all variables) [87]. In the greenhouse, Oriental bittersweet showed increased height, aboveground biomass, and total leaf mass compared to American bittersweet when both species were grown under reduced red:far red light conditions. The authors concluded that Oriental bittersweet's superior ability to grow under these conditions allows it to persist in the understory and "forage" for gaps and sunflecks, whereas American bittersweet's relative inability to gain height and biomass growth under a canopy ensures its decline unless a canopy gap occurs. Further, Oriental bittersweet's ability to detect far red light, which is transmitted and reflected by neighboring plants, may confer ability to "detect" and grow toward neighboring plants that could potentially provide support for Oriental bittersweet's stems [86].

Surveys generally show that Oriental bittersweet is more adaptable and prolific than its native congener. In Connecticut, very wet sites were the only sites where transplanted American bittersweet seedlings outperformed transplanted Oriental bittersweet seedlings (58% vs. 18% mortality for Oriental and American bittersweet, respectively). Oriental bittersweet averaged higher survival (90% vs. 68%) and about 3 times more aboveground biomass (1.93 g vs. 0.67 g) than American bittersweet in low light (≤6.4% transmittance) [90]. A New Jersey study showed a 90% germination rate for first-year, soil-stored Oriental bittersweet seed compared to a 65% germination rate for first-year, soil-stored American bittersweet seed [162]. In a Connecticut study, Dreyer and others [26] found Oriental bittersweet showed significantly higher pollen and seed viability than American bittersweet (P<0.001). While recognizing that many environmental and genetic factors affect seedling establishment, the authors stated that such viability could favor Oriental bittersweet over American bittersweet [26].

American bittersweet is further threatened by potential hybridization and introgression with Oriental bittersweet. Greenhouse studies confirm that the 2 bittersweets are cross-fertile [127,172,176] (see Taxonomy). In a preliminary greenhouse study, interspecific pollination between the bittersweets was more successful than intraspecific pollination, with 1 seedling/flower resulting from hybridization and 0.6 seedling/flower resulting from intraspecific crosses. Although not statistically significant due to small sample sizes [127], these results show the need for field studies documenting the extent of Oriental bittersweet × American bittersweet hybridization.

Effects on silvicultural and agricultural systems: Oriental bittersweet may smother or kill timber trees and understory vegetation [125]. Girdling and stem damage from Oriental bittersweet stems lowers the value of timber trees that host Oriental bittersweet. Where it was present before tree harvest, Oriental bittersweet can rapidly overtake a site after harvest. Its sprouts may overtop understory species and overstory trees. On the Pisgah National Forest, Oriental bittersweet covered sapling-sized hardwood and eastern white pine (Pinus strobus) regeneration on small clearcuts [104]. In a Massachusetts clipping experiment, Oriental bittersweet growth ranged from 6.9 to 15 feet (2.1-4.7 m) in 1 year. In contrast, bigtooth aspen (Populus grandidentata) sprouts grew from 3.0 to 5.9 feet (0.9-1.8 m) in 1 year, and yellow-poplar sprouts averaged 4.6 feet (1.4 m) in 1 year (review by [31]). On the Bent Creek Experimental Forest, a high-quality stand of upland oaks was clearcut in the summer of 1977. Oak site index before harvest was above 80, with a basal overstory area of 120 feet² (11 m²). Preharvest Oriental bittersweet density was 830 seedlings/acre and 27 saplings/acre (seedlings were <0.6 inch (2.0 cm) DBH; saplings were >0.6 inch DBH). Seven years after tree harvest, the canopy was nearly 100% Oriental bittersweet [104].

Oriental bittersweet is an alternate host for Xylella fastidiosa. This bacterium vectors several crop diseases including Pierce's grapevine (Vitis) disease, periwinkle (Vinca) wilt, plum leaf scorch and phony peach (Prunus) disease, and variegated chlorosis (affects several genera including oaks, elms, sycamores, citrus (Citrus), and mulberries (Morus)) [101].

Control: The Southeast Exotic Pest Plant Council [145] recommends that Rank 1 and Rank 2 species such as Oriental bittersweet be controlled and managed in the early stages of infestation whenever possible. Because Oriental bittersweet appears to build only a short-term seed bank [30], there are better opportunities for control and a higher probability of success than if seeds were longer-lived. If on-site plants and nearby seed sources are killed before arils mature, subsequent seedling establishment may primarily come from off-site seed sources, with little seedling emergence from the seed bank [24,30]. Monitoring and early control of new outbreaks can then help control Oriental bittersweet [24,24]. Greenberg and others [48] recommend preventing seed dispersal. This implies treating on- and off-site plants before fruiting, whatever control method is used.

Since Oriental bittersweet resembles the native and rare American bittersweet, it is important to correctly identify Oriental bittersweet before control measures begin [152]. See General Botanical Characteristics for information on identification keys.

Prevention: The most efficient and effective method of managing invasive species such as Oriental bittersweet is to prevent their invasion and spread [140]. Preventing the establishment of nonnative invasive plants in wildlands is achieved by maintaining native communities and surveying, monitoring, and controlling new infestations. Dreyer [24] recommends preventing the introduction of Oriental bittersweet into uninfested areas and making early control of small infestations a priority. Inventories to establish Oriental bittersweet presence and densities are needed before control programs, or any silvicultural treatment that opens the canopy, begin. McNab [102] cautions that if Oriental bittersweet is present in the understory, canopy disturbance will probably stimulate its growth.

Monitoring is an important part of an integrated program for Oriental bittersweet control [143]. Monitoring efforts are best concentrated on the most likely sites of Oriental bittersweet invasion: disturbed soil, roadsides, old fields, woodlands, and waterways. Survey uninvaded sites periodically to detect new invasions [24]. Because Oriental bittersweet retains its leaves longer than most associated native species, its yellow leaves are easy to spot in late fall, even from a distance. Consistent fall monitoring can identify new infestations, allowing managers to implement control programs and prevent new infestations from spreading. Managers in Great Smoky Mountains National Park recommend scouting for infestations every 2 weeks after most native species have dropped their leaves, which is approximately November 10th in the Park [24]. Discouraging nurseries from stocking Oriental bittersweet [55] and encouraging plantings of alternative native ornamentals (see Ornamental and rehabilitation use) can reduce new introductions [55]. The Center for Invasive Plant Management provides an online guide to noxious weed prevention practices.

Monitoring may be more efficient if areas at high risk for Oriental bittersweet invasion are identified. See Site Characteristics for further information on Oriental bittersweet site preferences. For mountainous terrain in the southern the Appalachians, McNab and Loftis [103] describe a rapid survey technique for Oriental bittersweet hazard rating, and they provide a model for estimating probability of Oriental bittersweet occurrence for similar environments based on environmental, competitive, and disturbance factors.

Fire: See Fire Management Considerations for information on using prescribed fire to control Oriental bittersweet.

Cultural control: No information is available on this topic.

Physical or mechanical control: Frequent cutting, mowing, or grubbing helps control Oriental bittersweet. Any portion of stems or roots left on site may sprout [94]; grubbed roots usually sprout unless they are completely removed [23,24,103,152]. Small plants can by hand-pulled, but they need to be moved off site to prevent rooting [168]. Climbing or trailing stems must be cut as close to the root crown as possible (fact sheet by [24]). When grubbing, roots need to be bagged and either removed from the site or allowed to sit in the sun until the bagged plants and seeds have died ([152], fact sheet by [24]). To prevent posttreatment seedling establishment, mechanical treatments are best implemented before Oriental bittersweet is in fruit [152]. Occasional mowing, cutting, or grubbing only encourages root sprouting and is not recommended. Unless the entire root system is completely removed, treatments must be frequent enough to eventually exhaust the underground carbohydrate supply. That may be accomplished by cutting or mowing every 2 weeks [23,24,103,152]; however, that is usually not practical in wildlands.

Biological control: As of early 2011, pathogens from Oriental bittersweet's native range had not been approved for use in the United States [24,94,152]. A leaf spot fungus (Marssonina celastri) causes defoliation of Oriental bittersweet in Korea, where Oriental bittersweet is native [142]. Oriental bittersweet has no known pathogens in North America [152]. This may be a factor in Oriental bittersweet's invasiveness in the United States [117].

Chemical control: Oriental bittersweet can be controlled with herbicides, using either cut-stem or foliar applications. Systemic herbicides (for example, triclopyr or glyphosate) are recommended [24,152].

Effective use of herbicides requires appropriate herbicide concentration, application technique, and timing. For cut-stem treatments, best Oriental bittersweet control occurs when the herbicide is applied soon after stems are cut or mowed [24,152]. Cut the stems about 2 inches (5 cm) above the root crown. A second treatment may be needed to control sprouts [152]. Polatin [126] found mid-October application of triclopyr gave better control of Oriental bittersweet than spring application. Herbicide applications in early spring, before native herbs have emerged, or in late fall when natives are dormant but Oriental bittersweet is still green, can minimize effects to nontarget plants [152].

In red pine (Pinus resinosa) forests in Connecticut, late-summer (17-18 September) herbicide treatments gave fair to good control of Oriental bittersweet. Fourteen treatments involving four different herbicides, used alone or in combination, were used, with various application rates. One year after treatments, imazapyr and triclopyr gave best results with low concentrations. See Ahrens [3] for details of herbicide combinations, concentrations, and other treatment results.

See these sources: [12,17,24,32,94,126,152] for more information on chemical control of Oriental bittersweet.

Herbicides may provide initial control of a new invasion or a severe infestation, but used alone, they are rarely a complete or long-term solution to invasive species management [12]. Herbicides are most effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change the conditions that allowed the invasion to occur (for example, [177]). See The Nature Conservancy's Weed control methods handbook [159] for considerations on the use of herbicides in Natural Areas and detailed information on specific chemicals.

Integrated management: No single treatment provides effective, long-term control of Oriental bittersweet. Integrated management includes early detection, assessment, and containment of infestations before they spread. Factors to be addressed before a management decision is made also include assessment of nontarget vegetation, soil types, climatic conditions, important water resources, and an evaluation of the benefits and limitations of all control methods [112]. Hobbs and Humphries [57] advocate an integrated approach to the management of plant invasions that includes "a focus on the invaded system and its management, rather than on the invader" and "identification of the causal factors enhancing ecosystem invasibility" as an effective approach to controlling invasive species. This emphasizes removing the ecological stressors that may be underlying the causes of invasion rather than focusing on direct control of invasive species [57].

Few studies to date (2011) investigated using multiple control methods for managing Oriental bittersweet. Compared with mowing alone or mowing and prescribed fire combined, Polatin [126] found a combination of mowing and triclopyr was "by far the most effective treatment for controlling bittersweet and allowing for grass establishment". See Plant response to fire for details of this study. Hutchison [65] recommends either grubbing or a combination of cutting and herbicide treatment. When practical, he recommends uprooting and removing individual Oriental bittersweet stems from infested sites. In other situations he recommends hand cutting after the first killing frost, then spot-treating cut stems with glyphosate. To maintain control, he advocates immediately pulling and removing invading individuals off site [65].

Mixed forests, forest margins, thickets on grassy slopes; 400-2200 m.

Do not buy, plant, transplant Oriental bittersweet or dispose of live or dead seed-containing material. Manual, mechanical and chemical methods can be employed to control it. Vines can be pulled out by the roots, cut repeatedly or treated with systemic herbicides. No biological controls are currently available for this plant.