4. Rhamnus japonica Maxim. (Japanese buckthorn)

Map 2399

Plants shrubs or occasionally small trees, 1–5 m tall, dioecious, often incompletely so. Main stems 1 to several, the main branches ascending to loosely ascending and elongate, also with shorter, spreading to loosely ascending, relatively straight branches, these mostly thorn-tipped. Bark gray to reddish brown, sometimes with lighter blotches, relatively smooth, but with relatively prominent, raised lenticels in cross lines and branch scars, somewhat peeling on older, larger stems. Twigs relatively stout, slightly flattened, gray to yellowish brown, with small, dark lenticels, initially often minutely hairy, but soon glabrous or nearly so, the winter buds narrowly ovoid, with several overlapping scales, these reddish brown, glabrous except for marginal hairs. Leaves mostly opposite (occasionally subopposite on new growth, the petioles 5–18 mm long. Leaf blades 2–8 cm long, 1.5–3.0 times as long as wide, obovate to elliptic-oblanceolate or broadly oblanceolate, mostly widest at or below the midpoint, angled at the base, tapered to a sharply pointed tip or the smaller leaves sometimes merely angled or rounded, the upper surface green to dark green, minutely hairy, at least toward the tip and/or along the main veins, slightly shiny, the undersurface light green, glabrous, the lateral veins mostly 4 or 5 pairs, these strongly arched toward the blade tip. Inflorescences axillary, small clusters of 2–6 staminate or pistillate flowers or reduced to solitary flowers, the clusters sessile, the individual flower stalks 3–9 mm long. Flowers imperfect. Sepals 4, 1.3–2.0 mm long. Petals 4, 1.0–1.3 mm long in staminate flowers, 0.6–0.9 mm long in pistillate flowers, lanceolate, entire at the tips. Style noticeably 2-branched toward the tip. Fruits 5–7 mm long, more or less globose, with 2 stones, black at maturity. May–June.

Introduced, uncommon, known thus far only from single specimens from Boone and Jackson Counties (native of Japan; introduced sporadically in Illinois, Missouri). Disturbed, mesic upland forests.

The first collection of this species to date (Smith & McKenzie 3569 on 19 May 2000), which is in the Missouri Botanical Garden herbarium, originally was misdetermined as the superficially very similar R. cathartica. It was redetermined during studies by Guy Nesom toward a forthcoming treatment of the genus in the Flora of North America series. Elsewhere in the United States, this taxon is still known thus far only from the Chicago metropolitan area (Swink and Wilhelm, 1994), where it has become increasingly frequent during the past few decades.