1. Koelreuteria paniculata Laxm. (golden rain tree, star of India)

Map 2589

Plants trees to 12 m tall, lacking tendrils. Bark gray to grayish brown, with relatively thick ridges and plates and often reddish brown, longitudinal furrows. Twigs light reddish brown, somewhat angular or with irregular lines of low corky ridges, glabrous or sometimes minutely hairy near the tips of fertile branchlets, with inconspicuous pale lenticels and relatively prominent leaf scars, the winter buds oblong-obovoid, with usually 1 pair of exposed, overlapping scales, these usually glabrous. Leaves alternate, short-petiolate. Stipules lacking. Leaf blades elliptic to narrowly elliptic in overall outline, pinnately compound with 7–17 leaflets (terminal leaflet present), these mostly subopposite along the rachis, 1–10 cm long, sessile or short-stalked, ovate to broadly ovate, elliptic, or obovate, usually not noticeably curved, rounded or abruptly tapered at the often slightly asymmetric base, tapered to a sharply pointed tip, the margins coarsely toothed and/or pinnately lobed, sometimes some of the largest leaflets more or less fully compound, also usually minutely hairy, the upper surface glabrous or more commonly minutely hairy along the midvein, the undersurface usually moderately pubescent with fine, curved hairs along the relatively prominent, main veins, sometimes also somewhat glandular. Inflorescences terminal on the branches, broadly ovoid panicles, well-branched and with numerous flowers, the branches and stalks densely and minutely hairy. Flowers zygomorphic (asymmetric), hypogynous, short-stalked. Calyces deeply 5-lobed, more or less zygomorphic (2 of the lobes slightly smaller than the other 3), yellowish green, the lobes 1.5–2.5 mm long, ovate, finely irregular and glandular-hairy along the margins, glabrous or sparsely glandular-hairy on the outer surface, usually persistent but inconspicuous and withered at fruiting. Corollas asymmetric (the 4 free petals all oriented more or less toward the top of the flower), each 5–7 mm long, bright yellow, the blade narrowly oblong, flat, usually bent backward, glabrous, orange basally, narrowed abruptly to a slender, hairy, stalklike base, with a small, lobed, orange appendage on the upper surface where the blade meets the stalk. Stamens 8(–10) (appearing short but fully formed in pistillate flowers), the filaments long-hairy, angled downward, the anthers brownish purple. Pistil of 3 fused carpels (except in staminate flowers), 3-locular, with 2 ovules per locule. Style 1, exserted at flowering, elongate, unbranched, the stigma deeply 3-lobed. Fruits capsular, 45–70 mm long, oblong-ovate in profile, triangular in cross-section, truncate to slightly concave at the base, tapered to a sharply pointed tip, the outer wall inflated and papery, appearing veiny, dark brown to straw-colored at maturity, but then sometimes pinkish-tinged, incompletely 3-locular (only toward the base), 3–6-seeded (seeds 1 or 2 per locule, attached at the tip of the 3-locular portion). Seeds 6–8 mm long, globose or nearly so, glabrous, the surface appearing smooth to finely granular, often slightly shiny, black, sometimes with a white coating around the base. May–July, occasionally reblooming August–September.

Introduced, uncommon and widely scattered, mostly in and around urban areas (native of Asia, introduced widely but sporadically in the eastern U.S. west to Kansas and Texas, rarely farther west). Disturbed mesic upland forests and ledges and tops of bluffs; also fencerows, railroads, roadsides, and open, disturbed areas.

Long cultivated as a shade tree and ornamental in Asia, the species was brought into European horticulture in the late 1700s and was introduced in the United States as early as 1809, when Thomas Jefferson received seeds from France (Meyer, 1976). The plants are fast-growing, disease resistant, and tolerate a variety of growing conditions. The attractive, broad-crowned trees produce wind-dispersed fruits prolifically and anywhere that the species is successful numerous seedlings begin to grow in the vicinity. Thus, it is not surprising that K. paniculata has escaped to disturbed areas adjacent to plantings. In recent years, conservationists in the Midwest have begun to notice populations developing in more natural forest and bluff habitats, which has led to predictions that the species will become a problem invasive exotic in the future.