Rottboellia cochinchinensis

Name and classification

Rottboellia cochinchinensis (Lour.) W. Clayton
(=R. exaltata L. f.)
Family Poaceae, Tribe Andropogoneae

Common names

itchgrass, raoulgrass

Disseminule

segment of rachis internode with embedded sessile spikelet and pedicel, with or without pedicellate spikelet

Description

Spikelets heteromorphic, awnless, sunken into inflated internode. Internode base truncate, with central peg. Sessile spikelets dorsally compressed, 3.5–7 mm long, 2–3 mm wide, of 1 fertile floret and 1 basal sterile floret. Lower glume indurate, as long as spikelet, 9–11 nerved; upper glume keeled above, boat-shaped, following concavity of internode. Sterile lemma and palea chartaceous, as long as upper glume. Fertile lemma hyaline, following concavity of internode. Pedicel fused to internode. Pedicellate spikelets sterile, ovate, 3–8 mm long, often separately deciduous. Caryopsis oblong-ovate, gibbous, 3–4 mm long, 2–2.2 mm wide.

Identification considerations

Characters that may help to distinguish the genus Rottboellia from the similar genera Manisuris, Coelorachis, and Hemarthia are pedicel fused to the swollen internode, and heteromorphic and awnless spikelets. Spikelet length is a character that may help distinguish R. cochinchinensis from among the three other species in the genus.

Similar species

Rottboellia selloana Hack. (non-FNW)

Distribution

South Africa and much of tropical Africa, the Arabian peninsula, tropical and subtropical Asia, Australia, and the Pacific Islands; tropical South America, Central America, and throughout the Caribbean, United States

native to southern Asia

Habitat

varied because of its wide geographic distribution, ranging from wet places to open, well-drained soils, and from low altitudes to higher altitudes; a weed of ditch banks, cultivated fields, thickets, teak forests

General information

Rottboellia cochinchinensis is a rapidly spreading, tufted annual grass, to 3.5 m tall. It is a weed of rotation crops and perennial crops, most seriously in the Philippines, East Africa, the shores and islands of the Gulf of Mexico, and the Caribbean. It is often the first colonizer of disturbed land. Sharp hairs on the leaf sheaths penetrate the skin of laborers, causing infections. The grain is about the size of a rice grain, making it difficult to remove from rice seed. Propagation is solely by grain.

inflorescence segments

inflorescence segments

disseminules comprising internode with embedded spikelet and pedicel; far right: disseminule with pedicellate spikelet

disseminules comprising internode with embedded spikelet and pedicel; far right: disseminule with pedicellate spikelet

views of disseminule showing pedicel and lower glume of embedded spikelet (left) and rachis internode (right)

views of disseminule showing pedicel and lower glume of embedded spikelet (left) and rachis internode (right)

caryopses in ventral view (left) and dorsal view (right)

caryopses in ventral view (left) and dorsal view (right)

A, portion of inflorescence; B, disseminule showing lower glume; C, disseminule showing internode; D, caryopsis in ventral view; E, caryopsis in dorsal view; drawing by Lynda E. Chandler

A, portion of inflorescence; B, disseminule showing lower glume; C, disseminule showing internode; D, caryopsis in ventral view; E, caryopsis in dorsal view; drawing by Lynda E. Chandler