Mercer Grass

Also known as:
Water Couch, Knot Grass

General description

Creeping, perennial, mat-forming grass to 50 cm high. Stolons long, rooting frequently, with rhizomes. Leaves 40-150 x 3-8 mm, tip pointed, bluish-green, soft and lax, slightly hairy above and below, rolled and hairy at base. Ligule up to 4 mm, membrane-like, whitish-translucent, often torn. Auricle absent. Sheath with reddish-purple tinge. Distinctive seedhead of 2 diverging, erect racemes, 25-45 mm long; with softly hairy spikelets, Summer.

Damp margins and most freshwater waterbodies, wetlands, bogs; in frost-free areas.

Seeds, stolons and rhizome fragments, all via water and livestock pelts and hooves (possibly seeds in dung). Contaminated diggers, dumped vegetation, soil movement.

Forms dense mats, smothers most marginal species, prevents recruitment. Blocks waterways, causes flooding. Rotting vegetation affects water quality.

Site management

Recommended approaches

1. Weed mat (small spots only): leave for 2-3 months
2. Spray terrestrial sites (100ml glyphosate (non-selective) + 20ml penetrant/10L).
3. Spray aquatic sites (100ml glyphosate (non-selective)/10L, use glyphosate formulation approved for use over water).
4. Spray (60ml haloxyfop (selective) + 50ml crop oil/10L). Requires resource consent over water.

Caution: when using any herbicide or pesticide PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.

Similar species

Pennisetum clandestinum

Kikuyu is a hairier and more robust species. It has a longer leaf blade and a very hairy sheath and ligule.

RPMS status

Not a legally declared pest plant
mercer grass - Main species image