Weed Alert: Eurasian water milfoil

Eurasian water milfoil (Myriophyllum spicatum)

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

Eurasian water milfoil is a highly aggressive and invasive submerged aquatic weed that can spread rapidly. It forms a dense cover that shades out and replaces all other aquatic plants, seriously affecting native plant and animal life. Dense mats also interfere with water sports and irrigation.

It has the potential to become a major weed of dams, lakes and reservoirs in Australia. Eurasian water milfoil prefers lakes, ponds, shallow reservoirs and slow moving water, but will grow in fast moving water. It can tolerate and thrive over a range of temperatures and water conditions, including low levels of salinity.

World Status

This water milfoil is native to Europe, Asia and northern Africa. It is now naturalised and a major weed of lakes and reservoirs in Canada and the USA. It is also considered to be a weed in its native range.

This species has not been recorded as present in Australia.

Identification

Eurasian water milfoil is a submerged perennial herb. Stems are rooted at the base and grow towards the surface. It can grow in water from 0.5 to 10 m deep, but most commonly at depths up to 3 m deep.

Stems

Hairless slender stems grow to 7 m long and 5 mm thick. They are reddish-brown to whitish-pink in colour and branch profusely near the surface to form a dense canopy.

Leaves

Leaves are usually submerged, olive-green in colour, less than 4 cm long and feather-like.  They are arranged around the stem in whorls of four and have 5-24 pairs of divisions (usually more than 12).

Flowers

Small, pinkish, four-petalled flowers occur in whorls of four around the stem. They are held above the water in an erect spike up to 8 cm tall but then lie parallel to the water surface once fruit sets.

Growth and  Spread

Plants mostly spread as vegetative fragments. During the growing season plants automatically fragment, often developing roots before they separate from the parent plant. Water movement and human activities may also cause fragmentation.

Fragments are spread over long distances by water currents and are mainly dispersed between water bodies by boating and fishing activities.

Eurasian water milfoil may die back to the base during winter and reshoots in spring.

Control

Control of Eurasian water milfoil is difficult. Mechanical harvesting can lead to rapid reinfestation due to the plant being fragmented artificially.

In dams and reservoirs where the water levels are under manual control, water level manipulation can be successful. Raising water levels ‘drowns’ the plant and lowering levels ‘dehydrates’ it.

Legislation

Myriophyllum spicatum is a Class 1 noxious weed throughout NSW under the NSW Noxious Weeds Act 1993. As such, the weed must be eradicated from the land and the land kept free of the plant. As a notifiable weed, all outbreaks must be reported to the local council within three days.

Acknowledgments

Rachele Osmond

References

• Aiken, S.G., Newroth, P.R. and Wile, I. (1979) The biology of Canadian weeds. 34.
• Myriophyllum spicatum L. Canadian Journal of Plant Science 59: 201-215.
• Hosking, J. R., Sainty, G., Jacobs, S. and Dellow, J. (in prep) The Australian WEEDbook.