Weed Alert: Long-leaf willow primrose

Long-leaf willow primrose (Ludwigia longifolia)

World Status | Identification | Growth and Spread | | Control | Legislation

An invasive aquatic weed threatening NSW waterways and wetlands Ludwigia longifolia can form dense colonies in slow moving and static waterways. If no control action is taken it will dominate temperate and tropical wetland and riparian systems throughout Australia.

World Status

Ludwigia longifolia is a native of South America, recorded from Brazil to Argentina and is considered a major weed in its native range.

This species was introduced to Australia as an aquatic ornamental plant and was first recorded as naturalised near Sydney in 1991. It has also been recorded from the Port Stephens and Gosford areas on the Central Coast of NSW and near Brisbane in Queensland.

Identification

Ludwigia longifolia is an erect annual shrub up to 3 m high which has a shallow fibrous root systems.

Leaves and stems

Alternate linear leaves up to 35 cm long and 2.5 cm wide, reducing in size up the stem. The stems are red, narrow, winged, 4-angled and winged, usually branching  towards the apex.

Flowers

Flowers are solitary and found in the junction of upper leaves. The petals are yellow and 2–2.5 cm long. Flowering occurs from summer to winter.

Fruit

Fruit is oblong to narrow-oblong, 4-angled, 1–3.5 cm long, 0.4–0.8 cm wide with numerous minute seeds in each chamber that are brown to yellow in colour.

Growth and Spread

Spread is by either seeds or stem fragments. Seeds are extremely small, less than 1 mm long and can be dispersed by water, wind, or human activity. Vegetative propagation occurs via rooting of stem sections.

Control

Control of L. longifolia is very difficult. Mechanical/hand removal may be the most desirable method for small isolated infestations. However care must be taken to dispose of the material appropriately; care should be taken to avoid fragmenting the plant and leaving behind pieces that could start new infestations. Correct disposal is essential to prevent further spread. All plants should be removed before seed set and destroyed by burning or deep burial to stop plant regrowth from stem sections.

Herbicides give the most effective means of control. See the NSW DPI publication Noxious and Environmental Weed Control Handbook for registered herbicides.

Preventative measures such as planting riparian vegetation to out-compete L. longifolia, reducing any nutrient inflow into the system and preventing animal and machinery access to the site will prevent further spread and disturbance of the site.

For any activities concerning control of long-leaf willow primrose, it is recommended that you contact your local weeds officer to seek advice on identification, disposal and best practice control methods before you start.

Legislation

Ludwigia longifolia is declared a Class 3 or 4 noxious weed throughout some areas of NSW under the NSW Noxious Weeds Act 1993. Where it is a Class 3 noxious weed it must be fully and continuously suppressed and destroyed. Where it is a Class 4 noxious weed, its growth and spread must be controlled according to the measures specified in a management plan published by the local control authority.

Ludwigia longifolia is also listed a class 5 noxious weed throughout nsw. As a notifiable weed, all outbreaks of ludwigia longifolia must be reported to the local council within three days. The plant must not be sold, propagated or knowingly distributed.

Acknowledgements

Written by Peter Gorham.

Comments by Stephen Johnson, Melissa Freeman and Birgitte Verbeek.