Trifolium aureum Pollich (yellow clover)

T. agrarium L., an officially rejected name

Pl. 408 f, g; Map 1810

Plants annual or biennial, taprooted. Stems 20–60 cm long, erect or ascending, not rooting at the nodes, often much-branched, with short appressed hairs. Leaves petiolate toward the stem base to short-petiolate above, the longest petioles to 12 mm, shorter than the leaflets. Stipules about as long as to somewhat longer than the associated petiole, oblong-lanceolate, fused to about the midpoint, the free portions narrowly long-tapered at the tip. Leaflets 15–25 mm long, 6–8 mm wide, all sessile or nearly so, oblanceolate to obovate or elliptic, angled at the base, broadly and bluntly pointed to rounded or shallowly notched at the tip, often with a minute sharp point at the very tip, the margins finely toothed above the midpoint, the surfaces usually glabrous. Inflorescences 10–25 mm long (elongating with age), 12–14 mm wide, ovoid to cylindric dense spikelike racemes, sometimes becoming flat-topped with age, the stalk 10–50 mm long. Flowers 10–40(–80), short-stalked, the stalk spreading or becoming reflexed at fruiting. Calyces 2–3 mm long, tube 0.8–1.0 mm long, glabrous, the (longest) teeth 2–3 times as long as the tube, unequal (the lower teeth 2–3 times the length of the upper ones), slender and moderately (shorter teeth) long-tapered (longer teeth), lacking a prominent network of nerves and not becoming inflated at fruiting. Corollas 5–8 mm long, longer than the calyx lobes, bright yellow, turning brown with age, the banner somewhat incurved, broadly obovate, broadly and shallowly notched at the tip, strongly parallel-nerved, especially with age. Fruits 3.0–3.5 mm long, oblong-ovoid, stalked, the outer wall membranous to papery, 1-seeded. Seeds 1.0–1.2 mm, ovoid, pale yellowish green to yellowish brown, shiny. 2n=14, 16. June–September.

Introduced, uncommon, known thus far only from Jefferson and St. Charles Counties (native of Europe, Asia; introduced widely in the U.S. and Canada). Fallow fields and open disturbed areas.

Trifolium aureum also has been called large hop-clover, hop-clover, and palmate hop-clover. Yellow clover was an early introduction into North America. George Washington is known to have ordered seed of this species from Europe in 1786 (Pieters, 1920).

Zohary and Heller (1984) recognized two subspecies, which differ mainly in subtle details of the leaflet apices and style position. Missouri specimens are all T. aureum ssp. aureum. The ssp. barbulatum Freyn & Sint. ex Freyn is endemic to portions of southeastern Europe and adjacent Asia. Steyermark’s (1963) report of a specimen from Christian County could not be confirmed during the present research.