1. Celastrus orbiculatus Thunb. (Oriental bittersweet)

C. articulatus Thunb.

Pl. 350 d, e; Map 1502

Bark dark brown, not exfoliating. Leaves with petioles 1–2 cm long. Leaf blades 3–8 cm long, 2–5 cm wide, obovate to ovate, broadly elliptic, or nearly circular, those of the flowering branches mostly broadly elliptic to nearly circular, the margins bluntly toothed, rounded, angled, or short-tapered at the base, rounded to strongly tapered at the tip, the upper surface glabrous, green to dark green, the undersurface glabrous or rarely sparsely hairy along the veins, light green to pale green. Flowers in clusters of 2–5(–7), axillary, the staminate flowers also in terminal clusters. Sepals 1.0–1.5 mm long. Petals 3–5 mm long, 1.0–1.5 mm wide, narrowly oblong, rounded at the tip, greenish white. Fruits 6–8 mm long, 8–9 mm in diameter, the valves with the outer surface yellow at maturity, the inner surface orangish yellow. 2n=46. May–June.

Introduced, uncommon and sporadic but often locally abundant (native of eastern Asia; introduced in the eastern U.S. west to Wisconsin, Nebraska, and Arkansas; Canada). Bottomland forests and mesic upland forests; also old fields.

Celastrus orbiculatus was first reported for Missouri from Boone County by Yatskievych and Summers (1993). It is distinguished from the very similar C. scandens by having flowers produced in small axillary clusters, in contrast to the terminal clusters in C. scandens. The leaves of C. orbiculatus tend to be more rounded on the flowering branches, but both species can vary greatly in leaf shape on a given individual. The two species have the same chromosome number and can be artificially crossed, but the offspring are not vigorous and only sparingly fertile (White and Bowden, 1947).

Celastrus orbiculatus is fast becoming a serious weed in the eastern United States. It is reported to be more vigorous than the native C. scandens, with higher seed viability (Dreyer et al., 1987). The fruits and seeds are attractive to birds, which disperse the seeds widely. Celastrus orbiculatus proliferates by root suckers, grows fast, and can form a smothering blanket over whole plant communities. The tightly twining stems can constrict the phloem of other plants on which they twine and can kill young trees (Lutz, 1943). These plants are invasive and highly tolerant of low light conditions. In some areas, C. orbiculatus appears to be replacing C. scandens, although the native taxon appears to tolerate somewhat drier conditions than the invader.