Plants perennial, normally gla-brous or very indistinctly papillose on veins of leaf blades abaxially, with fusiform, vertical rootstock. Stems erect, branched distal to middle, 80-150(-200) cm. Leaves: ocrea deciduous or partially persistent at maturity; blade lanceolate or oblong-lanceolate, normally 20-55(-70) × 2-7 cm, base cuneate, occasionally rounded or truncate, margins entire, flat to slightly crisped, apex acute. Inflorescences terminal, occupying distal 2 of stem, dense or interrupted in proximal 1/ 2, broadly paniculate, branches usually straight or arcuate. Pedicels articulated in proximal 3, filiform, 5-13 mm, articulation barely evident, not distinctly swollen. Flowers 15-25 in whorls; inner tepals orbiculate or orbiculate-ovate, rarely ovate-deltoid, 4-7(-7.5) × 3.5-7 mm, base truncate or slightly cordate, margins entire or weakly erose, flat, apex obtuse to subacute; tubercles 3, equal or subequal, normally less than 2 times as wide as inner tepals. Achenes usually reddish brown, 3-4.5 × 1.5-2.5 mm. 2n = 20.
The name Rumex orbiculatus commonly was applied to this North American species. After study of the Linnaean type of R. britannica, J. E. Dawson (1979) concluded that that name is the earliest valid one for this taxon.
In early North American floristic literature, Rumex britannica commonly was misidentified as R. hydrolapathum Hudson, a closely related European species also belonging to subsect. Hydrolapatha Rechinger f. (K. H. Rechinger 1937); that species differs from R. britannica in having more triangular inner tepals with an acute apex. The name R. acutus Linnaeus was misapplied to R. britannica by W. J. Hooker ([1829-] 1833-1840, vol. 2) and other botanists.
Disjunct populations have been reported from California and Louisiana. The California record (from Plumas County) was confirmed by J. E. Dawson (1979); the records from Louisiana need confirmation.
From Flora of Indiana (1940) by Charles C. Deam
Usually in boggy or marshy places but sometimes in a habitat that is rather muddy, such as about ponds and in swamps. Infrequent. No doubt all the reports of it from southern Indiana should be transferred to some other species. In 1932, E. B. Williamson found a plant along Pigeon River in Lagrange County that had a leaf with a blade 35 inches long.