Plants annual or perennial; cespitose, occasionally rhizomatous. Culms 14-100 cm, ascending to erect; nodes pubescent to hirsute. Sheaths open; auricles absent; ligules of hairs or membranous and ciliate; blades 1-5 mm wide, flat or involute. Primary inflorescences terminal, open panicles, with few spikelets, exserted or partially included in the upper sheath, apices exceeding the upper leaf blades, axillary panicles sometimes also present; cleistogamous inflorescences also present in the upper sheaths. Spikelets laterally compressed, purplish, with 2-5 florets; sterile florets above the fertile florets; rachillas prolonged; disarticulation above the glumes and beneath the florets and, subsequently, at the cauline nodes. Glumes equal or unequal, shorter than the first lemma, 1-veined, keeled; calluses hairy; lemmas 3-veined, veins villous, apices bilobed to incised, midveins sometimes extending into an awn, awns to 11 mm; paleas bowed-out, keels hairy, distal hairs 0.5-2 mm, longer than those below; lodicules 2, truncate; anthers 3, yellow or reddish-purple; stigmas pink to purple. Caryopses dorsiventrally compressed. x = 10. Name from the Greek triplasios, triple, alluding to the awn and long lobes of the type species, Triplasis americana.

Triplasis is an American genus of two species that is probably related to Tridens. The disarticulating culm, which helps disperse the cleistogenes, aids in distinguishing Triplasis from other genera.

1.   Triplasis americana P. Beauv.
Perennial Sandgrass

Plants perennial; cespitose. Culms 30-80 cm, usually erect; nodes and internodes appressed pubescent. Sheaths glabrous or pilose, margins ciliate; ligules to 2 mm, membranous, ciliate; blades to 20 cm long, usually less than 2 mm wide, filiform, scabrous adaxially. Panicles 1-5 cm long, 1-3 cm wide, occasionally reduced to a raceme. Spikelets 9-12 mm, with 2-5 florets. Glumes subequal, 3.4-4.5 mm, acuminate; lemmas 4-8 mm, lobes 4.5-8 mm, tapering to the acute apices; awns 8-11 mm, divergent; paleas 2-3 mm, keels ciliate; anthers 1.5-2 mm, yellow. Caryopses 1.5-2.5 mm, ovoid, tan. 2n = unknown.

Triplasis americana is endemic to the southeastern United States. It grows on sandy soils in prairies and woods, being less common in maritime dunes than Triplasis purpurea.

2.   Triplasis purpurea (Walter) Chapm.
Purple Sandgrass

Plants annual and tufted or perennial and occasionally rhizomatous. Culms 14-100 cm, usually ascending; internodes glabrous. Ligules to 1 mm, of hairs; blades 1-5 mm wide, flat or involute, hispid or with papillose-based hairs. Panicles 3-7 cm long, 1-6 cm wide. Spikelets 6.5-9 mm, with 3-4 florets. Glumes about 2 mm, glabrous or scabrous, apices erose; lemmas 3-4 mm, lobes shorter than 1 mm, rounded; awns shorter than 2 mm, straight; paleas about 2.5 mm, keels ciliate; anthers about 2 mm, reddish-purple. Caryopses about 2 mm long, 0.6 mm wide, tapering distally, tan. 2n = 40.

Triplasis purpurea grows in sandy soils throughout the eastern and central portion of the Flora region, extending southward through Mexico to Costa Rica. It is far more common in maritime dunes than T. americana. Plants in the Flora region belong to Triplasis purpurea (Walter) Chapm. var. purpurea.