Global description
Eragrostis japonica is an annual herb in loose clumps, very variable in size, from 10 cm to 1.5 m high, with a humidity requirement for development. The thatch are glabrous, erect, simple or rarely branched at the nodes. The leaves have glabrous sheath, flat blade or with a slightly wound margin, linear, of variable length. The inflorescences are loose panicles, oblong in shape, often reddish, 5 to 50 cm long and 1.5 to 10 cm wide. The small spikelets with short stalks and quite densely grouped are easily disarticulated.
General habit
Loosely tufted annual grass of very variable size from 10 cm to 1.5 m high.
Underground system
Dense fibrous roots.
Culm
The culm is glabrous, erect, hollow and soft, simple or rarely branched at the nodes.
Leaf
The leaves are simple, alternate. They have a glabrous sheath, a flat lamina or with slightly rolled, scabrous, linear margin of variable length, can reach up to 30 cm long and 3 to 6 mm wide; ligule is represented by a short truncated membrane, about 0.5 mm high, ciliated at its top.
Inflorescence
The inflorescences are loose panicles, oblong in shape, or more or less contracted, often reddish, 5 to 50 cm long and 1.5 to 10 cm wide; the ramifications are solitary, or more or less close to sub-verticilliate briefly stalked, erect and applied against the axis or more or less obliquely spreading.
Spikelet
The spikelets are small, quite densely grouped, with short pedicel, usually shorter than the spikelets themselves. They are elliptic to oblong, 1.5 to 3 mm long, comprising of 3 to 8 flowers (often 4 or 5 flowers) with very fragile rachis that disarticulate between flowers from the top, with persisting glume, unequal to subequal, oblong, uninervate, keeled, obtuse or notched at the top: lower glumes,0.5 to 0.6 mm long, upper glumes 0.7 to 0.8 mm, lemma of the same shape as the glumes, 0.8 to 1 mm long, with 3 protruding ribs, smooth, bicarinated palea, slightly shorter than the lemma; 2 stamens.
Fruit
The grain is very small, oblong, about 0.4 mm long, smooth, brown in colour.