Stems stout, horizontal and long-creeping, branched, partly erect at tip, rarely climbing tree trunks. Leaves ± monomorphic, widely spaced, erect to arching. Petiole dull yellow or grayish brown or light brown, 10-55 cm, finely scaly proximally. Blade broadly linear to elliptic-lanceolate, 1-pinnate throughout, with conform terminal pinnae, 25-70 × 5-28 cm, base truncate, glabrous. Rachis lacking indument abaxially. Pinnae articulate to rachis except for terminal pinna, subsessile to short-stalked; larger pinnae ± straight, linear to linear-elliptic or linear-lanceolate, 3-15 × 0.5-1.8 cm, fertile pinnae often slightly smaller and contracted; margins serrulate; costae with indument of scales abaxially. 2 n = 72.

Swamps, marshes, wet prairies, and adjacent moist pine woods or hammocks; 0 m; Fla.; Central America; South America.

Plants of Blechnum serrulatum occurring in open sun are often dwarfed and stiffly erect. Those occurring in brackish conditions or perennially flooded areas may become hemiepiphytes.