Tufted or rarely shortly stoloniferous annual or short-lived perennial, to 90(–120) cm tall; culms erect to geniculate, 1–4 mm wide near base. Leaf blades dull green to bluish-green when fresh, shallowly grooved, involute (although basal leaves becoming flattened with senescence), smooth, sometimes scabridulous on margins, up to 50 cm long and 2 mm wide (unflattened); ligules membranous, acute, becoming laciniate, (2–)4–6(–7) mm long. Inflorescence an open panicle, 15–25(–55) cm long and 15–25(–30) cm wide, becoming shortly exserted with maturity; primary branches clustered (commonly 4–7 at lowest whorls), spreading; secondary branching not occurring within at least the basal third of the length of lower branches. Spikelets slightly overlapping towards tips of ultimate branches, 2.5–4 mm long, green to purplish-green; glumes not widely diverging, subequal or commonly the upper longer by up to 0.5 mm, scabrous on the keels, smooth on the lateral faces, margins entire to distantly-ciliolate; lemma 2–3.3 mm long, 5-nerved, entire or 2–4-toothed, callus with a dense tuft of hairs 0.5–0.7 mm long, back and/or laterals glabrous or with sparsely scattered hairs confined to the mid-third region, awned; awn arising from upper half of lemma length, 1.2–6 mm long, straight, slightly curved or gently sinuate but never strongly geniculate; palea subequal to the lemma or shorter by up to 0.6 mm, shortly bifid; stamens 3, dehisced anthers (0.4–)0.5–1 mm long. Flowers Nov.–Feb.
Wim, VVP, GipP, OtP, Gold, CVU, GGr, DunT. Occurs in and around saline depressions on the Volcanic Plain where recorded from Portalington west almost to the South Australian border.
Three subspecies are recognised, and while ranges overlap to some degree, there appears to be ecological segregation between the subspecies. Previous records of L. adamsonii east of its currently understood limit near Portalington are more likely referable to forms of the very widespread and variable L. filiformis with reduced indumentum on the lemmas. Some of these forms may prove to be distinct taxa.
Sterile Lachnagrostis ×contracta is a presumed hybrid between L. adamsonii and L. deflexa.
