Stems erect, simple or branched, 2.5-10 dm, glabrous; rhizomes thickened; elongate bulblets developing in distal axils in late season. Leaves usually opposite or subopposite (rarely alternate); petiole absent or 0.1-1 cm, eciliolate; blade elliptic-lanceolate to lanceolate, 3-10 × 0.5-2 cm, base cuneate, decurrent, margins entire, slightly revolute, eciliolate, apex acute to acuminate, surfaces punctate, glabrous; venation single-veined to obscurely pinnate. Inflorescences terminal, racemes, 10-30 cm. Pedicels 0.5-2 cm, glabrous. Flowers: sepals 5, calyx streaked with black resin canals, 2-6 mm, minutely stipitate-glandular on margins (or glabrous), lobes ovate to lanceolate, margins thin; petals 5, corolla yellow with reddish base, streaked with black or maroon resin canals, rotate, 5-7.8 mm, lobes with margins entire, apex acute to rounded, stipitate-glandular adaxially; filaments connate 0.4-1 mm, shorter than corolla; staminodes absent. Capsules 3-3.5 mm, dark-punctate, glabrous. 2n = 84. Flowering summer. Swamps, flood plains, fens, bogs, stream banks, pond and lake margins, wet ditches, other similar habitats; 0-1000 m; St. Pierre and Miquelon; B.C., Man., N.B., Nfld. and Labr., N.S., Ont., P.E.I., Que.; Conn., Del., D.C., Ga., Idaho, Ill., Ind., Iowa, Ky., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Okla., Oreg., Pa., R.I., S.C., Tenn., Vt., Va., Wash., W.Va., Wis. Lysimachia terrestris has been introduced in cranberry bogs and is occasionally found on muddy lake shores of the Pacific Northwest (British Columbia, Oregon, Washington). A fairly widespread hybrid between Lysimachia terrestris and L. thyrsiflora has been widely reported and named L. ×commixta Fernald. The parents may or may not be found in the vicinity of hybrid populations, which can form extensive colonies through vegetative reproduction of rhizomes or bulblets. J. D. Ray (1956) indicated that the hybrids are 'relatively infertile,' with abnormal pollen grains.
From Flora of Indiana (1940) by Charles C. Deam
An infrequent plant on mucky borders of lakes, marshes, and sloughs, and more rarely on wet, sandy borders of lakes. We have one specimen from the very wet marly border of a lake. Instead of flowering, this species sometimes develops bulblets in the axils of the leaves; also sometimes the lower leaves are alternate when normally they would be opposite. A form of this species occurs in which the flowers are in the axils of foliaceous bracts. Our specimen from Pulaski County is of this form.
Erect from long, stoloniform rhizomes, 4-8 dm, the stem glabrous, often branched but usually with a single raceme; lvs opposite, punctate, narrowly lanceolate, the principal ones 5-10 cm; elongate bulblets develop in the axils late in the season; raceme erect, 1-3 dm, many-fld, its bracts linear or subulate, 3-8(-10) mm; pedicels 8-15 mm; cor-lobes narrowly elliptic, 5-7 mm, yellow, marked with dark lines; 2n=84. Open swamps and wet soil; Nf. to S.C., w. to Minn., Sask., Io., and Tenn. June-Aug. A vigorous sterile hybrid with no. 7 [Lysimachia thyrsiflora L.], called L. أommixta Fernald, has numerous peduncled racemes from the middle axils, as well as a terminal raceme; it propagates vegetatively, and sometimes occurs in the absence of the parents.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.