Reed sweet grass

What does it look like?

Reed sweet grass is an upright, clump forming perennial grass that can grow to almost two metres tall, and its leaves are up to two centimetres wide. It spreads through creeping underground stems (rhizomes), which can form dense mats that stay attached to the bank but float in deeper water.

Flowering happens from spring through to summer. The flower heads are branched and can grow up to 45 centimetres long. The small spikelets are yellow/green with a purple tinge. Seeds are small and dark brown, maturing from summer into autumn.

Reed sweet grass prefers still or slow, nutrient rich water bodies (in water up to about 1–1.5 metres deep) like wetlands, drains, and streams. It grows best in temperate climates and does not tolerate heavy frost or full shade, although it will grow in light shade. The species is associated with high soil nitrogen and phosphorus levels and prefers soft substrates like silt.

Why is it a problem?

This species has a history of being invasive overseas. It responds strongly to high nutrient levels, growing faster and building more biomass than native sedges, which gives it a competitive advantage in nutrient rich environments. It tolerates grazing, but its spread can increase when grazing pressure is reduced. It is less competitive where vegetation is dense or woody.

It reproduces by seeds - which it produces prolifically - and rhizomes.

Control methods

Begin all control work at the top of the catchment and minimise site disturbance and the creation of bared areas.

Physical control
Machine dig all year round. Dispose of fragments at refuse transfer station, dry out and burn, or bury. Follow up with limited spraying.

Herbicide control

• Weed wipe (spring-autumn) using glyphosate (200ml/L + penetrant).
• Spray (spring to autumn) using glyphosate (10ml/L+ penetrant). If valuable species are at risk of spray contact, use Gallant NF (5ml/L + 5ml crop oil). Resource consent may be required to spray over water.
• Weedmat: Difficult to apply, so you need to cover all of infestation to avoid rhizome survival and leave covered for 3-4 months.

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

More information

This plant is widespread in the Hawke's Bay region.

Related links

• NZPCN listing for Reed sweet grass
• NZ Flora listing for Reed sweet grass
• Weedbusters listing for Reed sweet grass