Synonym(s): Dioscorea batatas
Family: Dioscoreaceae (Yam Family)
Duration and Habit: Perennial Vine
Herbaceous, high climbing vines to 65 feet (20 m) long, infestations covering shrubs and trees. Twining and sprawling stems with long-petioled heart-shaped leaves. Spreading by dangling potato-like tubers (bulbils) at leaf axils and underground tubers. Monocots.
Native Lookalikes: Currently no information available here yet, or there are no native Texas species that could be confused with Chinese yam.
Ecological Threat: D. oppositifolia is a fast growing, twining vine that has escaped from cultivation and has the ability to rapidly invade pristine habitats, especially riparian corridors. It has a swift rate of vegetative growth and a prolific rate of asexual reproduction via bulbils. In North American infested areas, it lowers native species richness and abundance by outcompeting and eliminating native plant species. It does this by quickly outgrowing the native herbs and seedlings, thickly blanketing all adjacent vegetation, and competitively excluding light. It may also weight-down and break branches of large trees and shrubs (similar to kudzu, Pueraria montana). An entire stand of native shrubs may become covered by D. oppositifolia, and it shades and eventually kills the stand. It is also able to completely cover the ground so that all native herbaceous ground cover is excluded.
Biology & Spread: Tu (2002) states that D. oppositifolia can reproduce both sexually and asexually. Although it is capable of sexual reproduction, it has not been documented to reproduce sexually in North America. This could be because it is a dioecious species, and female (pistillate) plants have not been observed in the wild. It does, however, reproduce vigorously asexually, via the production of small potato-like axillary propagules, called bulbils. These bulbils exhibit a relatively low rate of survival in the field (versus in the greenhouse), but plants apparently produce adequate numbers of bulbils to more than compensate for their low rate of survival (Beyerl 2001). Each vine is capable of producing an average 20 bulbils per year (Tu, 2002).
History: Introduced from Asia as possible food sources in the 1800s. Ornamentals often spread by unsuspecting gardeners intrigued by the dangling yams. Presently cultivated for medicinal use.
U.S. Habitat: D. oppositifolia can survive in a number of different habitats and environmental conditions, but is most commonly found at the edges of rich, mesic bottomland forests, along stream banks and drainageways, and near fencerows (Yayskievych 1999, in Tu, 2002). Tu (2002) states that initial infestations are generally associated with human-caused disturbances, such as near old home sites and along roadways. From these areas, it can easily spread into nearby riparian swaths and undisturbed habitats. D. oppositifolia can tolerate light levels ranging from full sun to full shade, but mostly grows at intermediate light levels along forest edges. Since it is often associated with riparian habitats, it is typically found in silty loam soils, which are typical of alluvial habitats (Beyerl 2001).
U.S. Nativity: Introduced to U.S.
Native Origin: Temperate eastern Asia (Bailey, L.H. and E.Z. Bailey, Hortus Third: A Concise Dictionary of Plants Cultivated in the United States and Canada, MacMillan Publishing Co., Inc., New York , (1977).); NatureServe Explorer
U.S. Present: AL, AR, CT, GA, IL, IN, KS, KY, LA, MD, MO, MS, NC, NJ, NY, OH, OK, PA, SC, TN, VA, VT, WV
Distribution in Texas: Eastern and midwestern United States.
Ipomoea spp. - Morning glory (Ipomoea spp.) has a cordate leaf shape but lacks the aerial tubers (SEPPC, Undated).
Smilax spp. - Smilax has a similar leaf shape to D. oppositifolia but lacks the bulbils, has thorns (on some but not all species), and has blue to purple berries (SEPPC, Undated).
Convolvulus arvensis - Bindweed (Convolvulus arvensis) has a cordate leaf shape but lacks the aerial tubers (SEPPC, Undated).
Dioscorea villosa - Main distinguishing characteristics of wild yam include vines that twine right to left, pubescence present on the upper leaf surfaces, and the absence of aerial tubers (SEPPC, Undated).
Preventative measures: Tu (2002) states that as with all prolific invaders, the key to the successful control of D. oppositifolia is to prevent new infestations or to control them as soon as possible. In North America, it has a wide range of environmental adaptability and few pests and predators. It has a high degree of asexual reproductive vigor, and is difficult to manage once firmly established. The use of manual and mechanical methods followed by another control technique (for example, periodic herbicide sprays to control for new bulbil recruitment and root sprouts) for several years should be accompanied by active restoration efforts to obtain desired results.
Physical: Manual and/or mechanical methods of plant removal can effectively control small isolated patches. These methods, however, are extremely time and labour-intensive, as the large, deep tuber makes manual removal very difficult. All pieces of the tuber must carefully be removed or resprouting may occur. The removal of aboveground biomass appears to eventually exhaust the tuber, and indicates that perhaps a management regime of repeated grazing or burning may also work to kill the plant. These other methods, however, have not been tried. Manually picking the aerial bulbils off the vines will not kill the plant, but will prevent the further spread of D. oppositifolia for a growing season. Once the bulbils have dispersed, hand-pulling the young germinating bulbils from soil can be an effective control measure if the entire bulbil is removed (K. Johnson, pers. comm., in Tu, 2002). Although there are no conclusive results reported from long-term fire effects on D. oppositifolia yet, Kristine Johnson of the Great Smoky Mountains National Park has noted that sites burned in a wildfire from the previous fall had reduced amounts the following year.
Chemical: Herbicide application appears to be the most effective means to control large infestations. One application of some herbicides can effectively kill all new germinating bulbils, but repeat treatments are probably necessary to completely kill large underground tubers that originally supported large mature vines. The herbicides glyphosate or triclopyr have been the most successful at killing the weed. Glyphosate also significantly lowered rates of plant growth from germinated bulbils as measured by stem length and numbers of leaves.
Biological: There are currently no available biocontrol agents for D. oppositifolia. Snails and caterpillars have been observed browsing on leaves of this species, but do not appear to damage the plants significantly. Rodents and other small mammals also consume the bulbils, but the degree of consumption and damage to the plants has not been quantified (Beyerl 2001). The exact species of these consumers have not been determined, nor has it been elucidated if they are specifically feeding on D. oppositifolia or are only generalist feeders.
Tu M. 2002. Element Stewardship Abstract for Dioscorea oppositifolia L.. The Nature Conservancy.
SEPPC (Southeast Exotic Pest Plant Council). Undated. Exotic Plant Management Plan
Miller, J.H. 2003. Nonnative invasive plants of southern forests: a field guide for identification and control. Gen. Tech. Rep. SRS-62. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 93 pp (USDA SRS).
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