Plants perennial. Culms 50-350 cm, decumbent, often rooting from the lower nodes; nodes glabrous; internodes glabrous, filled with spongy aerenchyma. Leaves evenly distributed; ligules membranous. Inflorescences narrow, condensed, cylindrical or spikelike panicles; branches appressed but not fused to the rachises, densely and evenly short ciliate on the ridges. Spikelets dorsally compressed, narrowly lanceolate, with 2 florets. Glumes unequal, membranous, with a definite rachilla internode between the 2 glumes; lower glumes no more than 1/2 as long as the spikelet, 1-3-veined; upper glumes subequal to or longer than the lower lemmas, membranous, not saccate, 3-7-veined; lower florets sterile; lower lemmas similar to the upper glumes; upper florets bisexual, much shorter than the upper glumes and lower lemmas; upper lemmas membranous to leathery, white at maturity, margins clasping the edges of the paleas basally but not distally, apices acute; paleas similar in texture to the lemmas; lodicules 2; anthers 3; styles 2, fused. Caryopses falling free from the lemmas and paleas. x = 10. Name from the Greek hymen, membrane, and achne, chaff, alluding to the membranous upper lemma, an unusual texture for the upper lemma in members of the Paniceae.

Hymenachne is a pantropical genus of approximately five species. It is unusual in the Paniceae in that all its members grow in aquatic or swampy habitats. It differs from Sacciolepis in its aerenchyma-filled internodes, a character that is probably an adaptation to its aquatic habitat. One species is native to the Flora region.

1.   Hymenachne amplexicaulis (Rudge) Nees
West Indian Marsh Grass

Plants perennial. Culms to 3.5 m tall, 1 cm or more thick, decumbent. Ligules 1-2.5 mm, brownish; blades 15-33 cm long, 12-28 mm wide, lax, flat, glabrous. Panicles 10-40 cm long, 0.7-1.2 cm thick, spikelike, dense, sometimes lobed near the base; basal branches 1.5-5 cm, strictly erect. Spikelets 3.5-5 mm, lanceolate, acuminate. Lower glumes 1-1.7 mm, 3-veined; upper glumes 2.8-3.9 mm; lower lemmas 3.6-4.6 mm, longer than the upper glumes, attenuate to subaristate; lower paleas absent; upper lemmas 2.5-3.5 mm; anthers 1.1-1.2 mm. 2n = 10.

In the Flora region, Hymenachne amplexicaulis is known only from low, wet pastures in southern Florida and it is rare even in that state. It is more abundant in the remainder of its range which extends through Mexico to Argentina.