Echinodorus, commonly known as burhead, is a genus of aquatic and semi-aquatic flowering plants in the family Alismataceae, order Alismatales. In the most current circumscription the genus contains a single accepted species, Echinodorus berteroi, native to the Americas; a large number of taxa formerly placed here have been transferred to the closely related genera Aquarius, Helanthium, and Albidella following molecular-phylogenetic revisions.
Plants in the genus are annual or perennial herbs that grow emersed, as floating-leaved forms, or seasonally submerged. Leaves range from linear to ovate and may be glabrous or stellate-pubescent; rhizomes are present in some species. The inflorescence is a raceme or panicle of one to eighteen whorls, and the flowers are bisexual with conspicuous white petals, 9–25 stamens, and 15–250 or more pistils spirally arranged on a convex receptacle, which matures into the characteristic prickly fruit-head that gives the genus its name.
Despite the narrowed modern circumscription, the name Echinodorus remains strongly associated in horticulture with a broader group of popular aquarium plants, particularly the so-called Amazon sword plants. Echinodorus berteroi and the now-reassigned Aquarius cordifolius (formerly E. cordifolius) are widely cultivated in aquaria and garden ponds worldwide for their attractive strap-shaped or heart-shaped leaves and general hardiness.
Etymology
The name Echinodorus derives from the Ancient Greek echius ('rough husk') and doros ('leathern bottle'), alluding to the ovaries, which in some species bear persistent styles that form a spiny or prickly fruit-head.
Distribution
The genus is native to the Americas, spanning tropical to sub-tropical regions. Cultivated members of the broader Echinodorus aggregate (including species now placed in Aquarius) are grown in aquaria and ponds worldwide.
Cultivation
Echinodorus are marsh and bog plants naturally tolerant of submerged growth. They prefer strong light and a deep, nutrient-rich substrate, and thrive across variable water chemistry within tropical to sub-tropical temperature ranges. Propagation is by division or by adventitious plantlets that form on submerged flowering stems; if an inflorescence is kept emersed and humid, flowers and seeds develop readily, and seeds can be germinated in damp warm sand. Supplemental CO2 benefits vigorous growth. The Amazon sword plant group is among the most popular of all aquarium plants for its hardiness and ornamental form, and micropropagation protocols have been developed for rapid commercial production.
Taxonomy Notes
The circumscription of Echinodorus has changed substantially. Karel Rataj's last major revision recognised 62 species, 2 subspecies, and 2 varieties. Subsequent molecular phylogenetic work demonstrated that the genus as broadly defined was not monophyletic; most species were transferred to the reinstated genera Aquarius (Christenh. & Byng), Helanthium, and Albidella, leaving Echinodorus sensu stricto with a single species, E. berteroi. Species in the group are notably variable depending on whether they are grown emersed or submerged, and hybridisation — both natural and artificial — is common; many aquarium trade forms retain named cultivar or subspecific designations.
