Dioscorea bulbifera

Dioscorea bulbifera (Dioscoreaceae) is characterized by its aggressively high-climbing annual twining stems, large heart-shaped leaves with prominent veins, and potato-like aerial tubers in the leaf axils (Morton 1974, Long & Lakela 1976, Morton 1982.) An excellent line drawing is in Correll and Correll (1982) and a color photograph is in Bell and Taylor (1982).

D. bulbifera may be confused by D. alata in the field. Although the latter species has opposite leaves in contrast to the former's alternate leaves, D. alata often aborts one leaf and may appear to be alternate. The remaining leaf scar is evident and may be used to distinguish the species. Additionally, D. alata often has winged stems and the bulbils may have emergent "roots" (Gann-Matzen, pers. comm. 1992).

Dioscorea bulbifera has been widely distributed through human activities and has become naturalized in many tropical and sub-tropical regions of the world (Martin 1974). In 1905, the U.S. Bureau of Plant Industry sent samples of air potato to horticulturalist Henry Nehrling for experimental cultivation in Gotha, Florida (Nehrling 1944). [This is the earliest record of its introduction to the state that can be substantiated by this author]. The species may now be found from Key West (Doria Gordon, pers. comm. 1992) to at least as far north as Alachua County (Dan Ward, pers. comm. 1992). We have found no written reference of this species invading other states within the continental U.S. However, the species appears to be present in southern Illinois (Randall, pers. comm. 1992 as reported to him by Vicki Nuzzo).

In peninsular Florida, air potato is an aggressive weedy invader in many different mesic habitats including thickets, disturbed areas, fence rows (Bell & Taylor 1982) and hardwood hammocks (Wunderlin 1982). Martin (1974) pointed out that an accumulation of leaves on the ground may be necessary to protect sprouting aerial tubers from dessication. In South Florida, the species is almost always found in tropical hammocks, and invades habitats ranging from xeric uplands to deep solution holes. It is rarely found along disturbed edges of pinelands. Air potato is not salt tolerant and thus is not invasive in marine areas (Rob Line, pers. comm. 1992). Air potato is particularly abundant in Dade County where it is growing in almost every county park with hammock vegetation (Carol Lippincott, pers. comm. 1992). (Some records may confuse presence of D. bulbifera with that of D. alata. In Central and North Florida, D. bulbifera is most common in mesic hammocks and urban lots (Gordon, pers. comm. 1992) and alluvial flood plain forests (Francis E. Putz, pers. comm. 1992). Air potato growth appears most vigorous on hammock edges or where canopy gaps occur (Gordon, pers. comm. 1992, Lippincott, pers. comm. 1992).

The yam group (Dioscorea spp.) is one of the most important root or tuber crops grown in tropical areas (Martin 1974). Dioscorea bulbifera is one of the most common and widespread food yams and can be found in every hot humid tropical region of the world. Although not one of the best quality yams, some varieties of D. bulbifera produce large numbers of edible aerial tubers over long periods of time. The species is believed to be native to both Asia and Africa. The varieties from these 2 regions are quite distinct (see differences in bulbil morphology, discussed above). D. bulbifera is not native to the Western Hemisphere but it is so widespread that it is noted in most floras of tropical Central and South America. Wild forms, usually bitter and often poisonous, are the most common while the more desirable cultivars do not survive in the wild for long periods (Martin 1974).

U.S. Government chemists who received tubers back from Nehrling found them worthless for their intended use - medicinal research. Nerhling (1944) wrote: "It grew like a weed and there were thousands of air potatoes along the stems. If not gathered, they all sprouted and soon formed impenetrable masses. With the exception of the kudzu vine, I have never seen a more aggressive and dangerous weed in Florida." Despite his terrible review of D. bulbifera, this species has long been recommended for ornamental planting. Hume (1929) stated its "leaves are very attractive and form a dense shade. It is valuable for covering tree trunks and for screening purposes." Dickey (1968), in IFAS Bulletin 172B, wrote that Dioscorea "yams, although not recommended for permanent plantings, are very satisfactory for use as quick temporary vines." Unfortunately, he did not tell how to insure their temporary status. The IFAS Circular 860, Vines for Florida (Black 1990), included air potato with the warning "should be planted in confined area as apt to spread rapidly." Conversely, D. bulbifera has been listed as an invasive pest species by many authors (Long & Lakela 1976, Morton 1976 & 1982, Bell 1982). It was listed among the top 15 invasive species on the Exotic Pest Plant Council's "1991 List of Florida's Most Invasive Species".

Coursey (1967) described the life of a typical Dioscorea plant as an annually repeated cycle of growth and dormancy. During the active phase, the majority of the plant consists of above ground stems and leaves; the dormant phase has the subterranean tuber only. Growth of the stems at the beginning of the active phase can be very rapid, up to 15 cm/day (Coursey 1967). In relatively frost-free Dade County, die- back of the stems and leaves usually occurs in October and growth resumes in March (Brinkley, pers. comm. 1992, Line, pers. comm. 1992), but can be variable into July (Gann-Matzen, pers. comm. 1992).

Hortus Third (1976) describes the subterranean tubers of D. bulbifera as small or not present. Martin (1974) reported that these tubers vary from none to very large depending on the variety. He added the Asian races produce spherical or ellipsoidal tubers, whereas those of African races are much more branched and irregular. In Florida, these tubers are usually less that 10 cm in diameter but can be up to 20 cm in sandy soil (Gann-Matzen, pers. comm. 1992, Line, pers. comm. 1992). The author (pers. obs. 1992) unearthed a tuber in Alachua County measuring 25 cm in diameter and weighing about 5 kg.

In Florida, bulbils begin to form in June or July and may begin to drop to the ground by late August (Gann-Matzen, pers. comm. 1992). They usually are from 1-5 cm in diameter but can grow up to 10 cm or more and are usually nearly round in outline (Ward 1977, Gann-Matzen, pers. comm. 1992). These tubers can last a year or more on the ground or on old vines before sprouting, and can produce stems without soil contact (Brinkley, Gann-Matzen, Line pers. comms. 1992). There seems to be little or no predation on these tubers (Brinkley, pers. comm. 1992, Putz, pers. comm. 1992). Air potato may have some impact on surrounding vegetation when it is growing in shaded areas (Gann-Matzen, pers. comm. 1992), but has greater impact when the foliage reaches direct sunlight (e.g. where a tree falls down to cause a light gap in the canopy). In these situations, its rapid growth and dense foliage can smother tree seedlings and other undergrowth near the ground (Line, pers. comm. 1992). Vines reaching the canopy can completely cover the trees present (Brinkley, pers. comm. 1992; Morton 1974).

Almost all of the people contacted for this ESA had never seen D. bulbifera flower or fruit in Florida (Brinkley, Gann-Matzen, Gordon, Line, Lippencott, Maguire, pers. comm. 1992). Long and Lakela (1976) and Bell & Taylor (1982) also stated that flowers are not produced in our area. Putz (pers. comm. 1992) has observed flowering air potato in Alachua County. The author located three flowering speciments at the University of Florida Herbarium (FLAS): Dade County (10/03/72), Lake County (09/23/68) and Marion County (10/02/40). George N. Avery, who made the Dade County collection, noted on the specimen label that the fragrant flowers were 3-5 m above the ground. Martin (1974) confirmed that the species is dioecious and observed that its flowers are small and often inconspicuous.

Dioscora bulbifera insures its propagation by vegetative means, producing an abundance of bulbils (aerial tubers) in the leaf axils. Bulbils tend to form once stem growth is well under way and act as the main storage organ. These bulbils have the appearance and morphology of condensed stems. It has been noted that mature bulbils float in water suggesting that they may be dispersed by flood waters (Coursey 1967). Even small bulbils (1 cm diameter) can sprout and grow into new vines (Line, pers. comm. 1992).