Stems prostrate or trailing (rooting at nodes and often forming mats), simple or branching proximally, 1-5 dm, glabrous or sparsely stipitate-glandular; rhizomes slender to somewhat thickened; bulblets absent. Leaves opposite; petiole 0.1-0.5 cm, eciliate; blade orbiculate to ovate-orbiculate, 1-3.5 × 0.5-3.5 cm, base rounded, decurrent, margins entire, plane, eciliolate, apex rounded, surfaces reddish-brown punctate, glabrous; venation pinnate-arcuate. Inflorescences axillary in medial leaves, solitary flowers. Pedicels 1-8 cm, glabrous. Flowers: sepals 5, calyx streaked with dark resin canals, 5-8 mm, glabrous, lobes ovate to deltate, margins thin; petals 5, corolla yellow, finely streaked with black resin canals, campanulate to rotate, 10-15 mm, lobes with margins somewhat irregularly erose apically, apex rounded to acute, finely stipitate-glandular (sometimes sparsely so); filaments connate ca. 1 mm, shorter than corolla; staminodes absent. Capsules not seen. 2n = 30, 32, 43 (Europe). Flowering summer. Wet meadows, seeps, pond edges, stream banks, flood plains, wet roadside ditches, mesic woods; 0-1700 m; introduced; B.C., N.B., N.S., Ont., P.E.I., Que.; Ala., Ark., Calif., Colo., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., S.C., Tenn., Vt., Va., Wash., W.Va., Wis.; Eurasia. Lysimachia nummularia has been collected as an adventive in Newfoundland. Lysimachia nummularia is part of a Eurasian complex of 38 species centered on the Indian subcontinent, whose boundaries are not well understood. North American populations of this species rarely, if ever, produce capsules. Plants of eastern Asia are reported to produce fruit; seed viability is unknown. The species reproduces by vegetative means, often forming extensive mats.
From Flora of Indiana (1940) by Charles C. Deam
Frequent in low ground along streams, ditches, roadsides, and elsewhere. I have seen it form a carpet in low, open woods along streams, crowding out all other herbaceous vegetation. When it becomes established in a pasture field, it chokes out the native grass and is very difficult to exterminate. Since the plant is not palatable to stock, my advice to land owners is to exterminate it at any cost.
Indiana Coefficient of Conservatism: C = null, non-native
Creeping, often forming mats; lvs opposite, punctate, short-petioled, broadly quadrate to subrotund, 1-2.5 cm; fls solitary in the axils, the pedicels about equaling the lvs; cal-lobes foliaceous, triangular-ovate, 8 mm; cor-lobes 10-15 mm, dotted with dark red; 2n=32, 36, 43, 45. Native of Europe, escaped from cult. into moist places nearly throughout our range, often weedy. June-Aug.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.