Amorpha fruticosa L. (false indigo)

A. fruticosa var. angustifolia Pursh

A. fruticosa var. croceolanata (P.W. Watson) Mouill.

A. fruticosa var. emarginata Pursh

A. fruticosa var. oblongifolia E.J. Palmer

A. fruticosa var. tennesseensis (Shuttlew. ex Kunze) E.J. Palmer

Pl. 388 d, e; Map 1709

Plants shrubs. Stems 1–2(–4) m tall, several-branched, glabrous or sparsely to moderately pubescent with minute curved and/or short straight hairs, not glandular or sparsely and inconspicuously gland-dotted. Leaves with 9–25 (often 17–19) leaflets, the petiole 10–30 mm long, longer than the width of the lowermost leaflet, the rachis 9–19 cm long, the petiole and rachis moderately short-hairy, often becoming nearly glabrous with age, occasionally sparsely pustular gland-dotted. Leaflets 20–45 mm long, 5–17 mm wide, oblong to elliptic, rounded or angled at the base and with a stalk 1–2 mm long, rounded to minutely notched at the tip but usually with a minute sharp point, the upper surface glabrous or nearly so, the undersurface sparsely to moderately short-hairy and pustular gland-dotted. Inflorescences 8–18(–25) cm long, mostly in the upper leaf axils, solitary or in clusters of 3–9, the flower stalks 1–2 mm long. Calyces with the tube 2.0–2.4 mm long, the lobes 0.2–1.0 mm long, the lowermost lobe conspicuously longer than the other 4 lobes. Corollas with the banner 4–5 mm long, 2.5–3.0 mm wide, obovate, arched, folded around the stamens and pistil, dark bluish purple to dark purplish blue. Stamens with the free portion of the filaments 3–4 mm long, the anthers yellowish orange to orange. Ovary 1–2 mm long, usually glabrous, the style 4–5 mm long, glabrous or more commonly with ascending hairs. Fruits 4–7 mm long, 1.6–2.0 mm wide, strongly exserted beyond the persistent calyx, glabrous but prominently pustular gland-dotted. Seeds 3.5–4.0 mm long, 1.4–1.6 mm wide, tan to reddish brown. 2n=40. May–June.

Scattered nearly throughout the state (nearly throughout the U.S.; Canada, Mexico). Banks of streams and rivers, margins of ponds, lakes, and sloughs, bottomland prairies, bases and ledges of bluffs, and openings of swamps and bottomland forests; also ditches, railroads, roadsides, and moist disturbed areas.

In Missouri, A. fruticosa does well in gardens and is available commercially through wildflower nurseries. However, although widespread in the United States, this species is not native to New England and the Pacific Northwest, where it is considered an invasive exotic (Glad and Halse, 1993). In some northeastern states, A. fruticosa has escaped from cultivation to become a pest in sensitive riparian areas (Lapin, 1995). The plants propagate clonally by stems and roots washed up during floods, and the seeds have delayed germination, making them difficult to control.

This wide-ranging species is quite variable in the number and size of the leaflets, shape and size of fruits, and pubescence. Numerous varieties and forms have been described (Palmer, 1931; Steyermark, 1963; Wilbur, 1975). However, patterns of variation are not correlated with ecology, geography, or other morphological characters. The infraspecific taxa intergrade freely and are often difficult to distinguish in natural populations. For these reasons none of the infraspecific taxa is recognized in the present treatment.

Several closely related species in the A. fruticosa complex are recognized in states adjacent to Missouri (Wilbur, 1975; Isely, 1998), and the question arises as to their potential presence here. The taxonomy of the group requires more intensive study, and most of the segregates may not warrant recognition at the species level, if at all. Among them, A. nitens F.E. Boynton (shining false indigo) is the most likely to be discovered in the state and should be searched for in southern Missouri. This southeastern taxon is known from adjacent counties in Illinois, Kentucky, Tennessee, and Arkansas. It is said to differ from A. fruticosa in its generally fewer (mostly 9–15), usually slightly larger leaflets that are often somewhat shiny when fresh and more rounded at the tip. It is also unusual within the genus in that pressed samples tend to blacken upon drying.

A putative hybrid between A. fruticosa and A. canescens was reported by Palmer (1953), which he named Amorpha ×notha E.J. Palmer. This report was based on a locality near Webb City (Jasper County). Both parental species occurred in the same area, and the hybrid was growing between an upland prairie and an alluvial creek valley. The hybrid resembles A. canescens in its dense whitish gray pubescence, but is taller, has shorter calyx lobes, and a strongly curved fruit. Steyermark (1963) discussed additional morphological features in support of the hybrid status of the plant. Co-occurrence of the two parents is very uncommon, thus hybridization between the two species should be extremely rare.