Derivation
Brachyachne (Benth.) Stapf, in D.Prain (ed.), Fl. Trop. Africa 9: 20 (1917) in clavis; from the Greek brachys (short) and achne (scale, chaff), alluding to lemmas shorter than glumes.

ambigua- Latin for uncertain. Species that may be readily confused with others or do not necessarily belong in the genus in which they have been placed or are intermediate in characters between other genera.

Published in
Bull. Tokyo Sci. Mus. 18: 11 (1947).

Habit
Annual, tufted. Stolons absent. Basal leaf sheaths glabrous. Culms geniculately ascending or decumbent, 30–55 cm tall. Leaf-sheaths 0.5–2 cm long. Ligule an eciliate membrane, 0.3 mm long, truncate. Leaf-blades 1–10 cm long, 1 mm wide. Leaf-blade surface glabrous. Leaf-blade apex acuminate.

Inflorescence
Inflorescence digitate, with spicate branches. Spikes 1–5, unilateral, 1.5–7 cm long, bearing 10–20 fertile spikelets on each. Rhachis flattened. Spikelet packing broadside to rhachis, lax, regular, 2-rowed.

Spikelets
Spikelets appressed, solitary. Fertile spikelets 1-flowered, comprising 1 fertile floret, without rhachilla extension, lanceolate or oblong, laterally compressed, compressed strongly, 2–3.5 mm long, breaking up at maturity. Spikelets disarticulating below each fertile floret. Floret callus glabrous.

Glumes
Glumes deciduous, similar, with lower wider than upper, firmer than fertile lemma, parallel to lemmas. Lower glume lanceolate, 2–3.5 mm long, equalling upper glume, coriaceous, 1-keeled, wingless, 1-nerved, midnerve scaberulous or scabrous. Lower glume lateral nerves absent. Lower glume apex obtuse or acuminate. Upper glume lanceolate, 2–3.5 mm long, 140% of length of adjacent fertile lemma, coriaceous, 1-keeled, wingless, 1-nerved, midnerve scaberulous or scabrous. Upper glume lateral nerves absent. Upper glume apex obtuse or acuminate.

Florets
Fertile lemma ovate, laterally compressed, 1.8–2 mm long, membranous, 3-nerved. Lemma surface pubescent, hairy on nerves. Lemma hairs 0.25–0.3 mm long. Lemma apex acute, muticous. Palea 100% of length of lemma, 2-nerved. Grain with adherent pericarp, oblong, laterally compressed, 1–1.6 mm long. Embryo 50% of length of grain.

Continental Distribution:
Tropical Asia, Australasia.

Australian Distribution:
Western Australia, Northern Territory.

Western Australia: Gardner. Northern Territory: Darwin & Gulf.

Classification. (GPWG 2001):
Chloridoideae: Cynodonteae

Notes
Introduced; confined to the N of the N.T. within 350 km of Darwin, and NE W.A. near the border, also recorded from Java and Lesser Sunda Is., Papua.
A weed of rice crops and disturbed ground in northern Australia.