25.25   AXONOPUS P. Beauv.
Mary E. Barkworth

Plants perennial, rarely annual; cespitose, loosely tufted, or mat-forming, sometimes rhizomatous or stoloniferous. Culms 7-300 cm, not woody, often decumbent at the base, erect to ascending. Sheaths open; ligules membranous, truncate, ciliate; blades flat or convolute, usually obtuse. Inflorescences terminal, sometimes also axillary, panicles of 2-many, digitately, subdigitately, or racemosely arranged spikelike branches; branches triquetrous, spikelets subsessile or sessile, solitary, in 2 rows, lower lemmas appressed to the branch axes; disarticulation below the glumes. Spikelets dorsally compressed, with 2 florets; lower florets sterile or staminate; upper florets sessile, bisexual. Lower glumes absent; upper glumes and lower lemmas equal, membranous; lower paleas absent; upper lemmas indurate, usually glabrous, sometimes with an apical tuft of hairs, margins slightly involute, clasping the palea, apices acute to obtuse; upper paleas similar to the upper lemmas in texture. Caryopses ellipsoid. x = 10. Name from the Greek axon, axis, and pes, foot.

Axonopus is a genus of approximately 100 tropical and subtropical species, most of which are native to the Western Hemisphere. Three species are native to the Flora region; one additional species has been grown experimentally in Florida.

All the species tend to grow in open habitats, often where the soil is somewhat impermeable and slightly flooded in the rainy season. Axonopus fissifolius and A. compressus are cultivated for forage in many countries; A. compressus is also used as a lawn grass. Both species are inclined to be weedy. The presence of rhizomes or stolons is affected by environmental conditions, with plants growing in crowded conditions, e.g., lawns, rarely producing them.


SELECTED REFERENCE Black, G.A. 1963. Grasses of the genus Axonopus (a taxonomic treatment) (ed. L.B. Smith). Advancing Frontiers Pl. Sci. 5:1-186.

1
Panicles with 30-100+ branches; lower branches 10-24 cm long; culms (50)100-300 cm tall ..... 4. A. scoparius
Panicles with 2-7 branches; lower branches 1-15 cm long; culms 7-100 cm tall (2)
2
Spikelets 3.5-5.5 mm long; upper glumes glabrous; lower lemmas glabrous or sparsely pilose over the veins ..... 3. A. furcatus
Spikelets 1.6-3.5 mm long; upper glumes and lower lemmas sparsely pilose on the margins or marginal veins (3)
3
Upper glumes and lower lemmas extending beyond the upper florets, forming acute to acuminate apices; blades 3-20 mm wide ..... 2. A. compressus
Upper glumes and lower lemmas not or scarcely extending beyond the upper florets, forming obtuse to subacute apices; blades 1.5-6 mm wide ..... 1. A. fissifolius


1.   Axonopus fissifolius (Raddi) Kuhlm.
Common Carpetgrass

Plants usually cespitose, sometimes stoloniferous, nodes of the stolons often pilose. Culms 10-75 cm, erect or depressed-decumbent; cauline nodes glabrous or slightly pubescent. Sheaths compressed, mostly glabrous, margins ciliate; ligules 0.2-0.4 mm; blades 4-15 cm long, 1.5-6 mm wide, flat, mostly glabrous, margins with papillose-based cilia. Panicles terminal and axillary, 5-11 cm overall, rachises to 3 cm, with 2-7 branches; branches 2-9(12) cm, spreading or ascending. Spikelets 1.6-2.2(2.8) mm, ovoid or ellipsoid, obtuse to acute. Upper glumes and lower lemmas scarcely extending beyond the upper florets, 2-veined, margins sparsely pilose, apices obtuse to subacute; upper lemmas and paleas 1.6-2.1 mm long, 0.5-0.7 mm wide. Caryopses 1.5-1.8 mm, gray. 2n = 20, 40, 80, 100.

Axonopus fissifolius is sometimes used as a lawn or pasture grass, but it is also an invasive weedy species, often growing in moist, disturbed sites. It is native in the southeastern United States and from central Mexico south to Bolivia and Argentina. It has also been introduced into tropical and subtropical regions of the Eastern Hemisphere.


2.   Axonopus compressus (Sw.) P. Beauv.
Broadleaf Carpetgrass

Plants stoloniferous, rarely rhizomatous, rhizomes, when present, 3-5 cm. Culms 7-80 cm; nodes glabrous or pubescent. Sheaths keeled, strongly compressed, pubescent; ligules 0.3-0.5 mm; blades 3-20 mm wide, glabrous or sparsely pilose, midveins often white and prominent, apices frequently ciliate or pubescent. Panicles terminal and axillary, 4-10 cm overall, rachises to 3.5 cm, with 2-5 branches; branches 1-13 cm. Spikelets 2-3.5 mm, ovoid, ellipsoid, or lanceoloid, acuminate. Upper glumes and lower lemmas extending beyond the upper florets, 2-5-veined, marginal veins pilose, apices acute to acuminate; upper lemmas and paleas 1.5-1.8 mm, broadly ellipsoid. Caryopses 1.2-1.5 mm, gray. 2n = 40, 60, 80.

Axonopus compressus is native from the southeastern United States to Bolivia, Brazil, and Uruguay, and has become established in the Eastern Hemisphere. It is used as a lawn and forage grass but is also weedy, readily growing in moist, disturbed habitats.


3.   Axonopus furcatus (Flüggé) Hitchc.
Big Carpetgrass

Plants stoloniferous. Culms 30-100 cm; nodes glabrous or pubescent. Sheaths compressed, glabrous or sparsely to densely pilose, hairs appressed; ligules 0.3-1 mm; blades 3-25 cm long, 2-15 mm wide, margins often with papillose-based hairs near the base, scabrous distally. Panicles terminal and axillary, with 2(-4) divergent branches; branches 4-15 cm. Spikelets 3.5-5.5 mm long, about 1.5 mm wide, sessile or subsessile, ovoid-ellipsoid, acuminate. Upper glumes glabrous, 5-7-veined; lower lemmas 5-7-veined, glabrous or sparsely pilose over the veins; upper lemmas and paleas 2.5-3.2 mm, light yellow, obtuse. Caryopses 1.8-2.2 mm, obovate, yellow. 2n = unknown.

Axonopus furcatus is endemic to the southeastern United States. It grows in moist pine barrens, marshes, river banks, wet ditches, pond margins, and other such damp areas.


4.   Axonopus scoparius (Humb. & Bonpl. ex Flüggé) Kuhlm.
Carpetgrass

Plants rhizomatous, frequently also stoloniferous. Culms (50)100-300 cm, to about 1 cm thick, sometimes branching above the base; nodes glabrous. Leaves primarily cauline; sheaths often much wider than the internodes, mostly glabrous but the collars pubescent, lower sheaths compressed; ligules 0.5-2.9 mm, ciliolate; blades 15-50 cm long, (5)20-35 mm wide, bases usually wider than the sheaths. Panicles terminal and axillary, 10-50 cm overall, rachises 2-3.5 cm, with 30-100+ branches; lower branches 10-24 cm, frequently fascicled. Spikelets 2.1-2.7 mm, ovoid to oblong-ellipsoid, acute or apiculate; upper glumes and lower lemmas usually 5-veined, sparsely pilose; upper florets 0-0.4 mm shorter than the upper glumes and lower lemmas, obtuse to subacute. Caryopses usually absent. 2n = 20.

Axonopus scoparius is native from southern Mexico to Peru, Bolivia, and Brazil. In Mesoamerica, it rarely sets seed but is grown for forage and often persists after cultivation has ceased. It has been grown experimentally in Florida, but it is not winter hardy even there. Not surprisingly, A. scoparius is not established in the Flora region.