Quick Facts

Quick facts

• Hydrocotyl (Hydrocotyle ranunculoides) is a stoloniferous herb that grows on damp land and out over water, forming dense mats.
• It is characterised by round, circular or broadly kidney-shaped leaves on long stalks, and clusters of inconspicuous small flowers.
• It grows vigorously and interferes with navigation, recreation and stream ecology.
• Hydrocotyl currently occurs only in the Perth region but has the potential to spread to waterways and wetlands in southern Australia.
• Weed risk is increased by Hydrocotyl being an aquarium and pond plant in Australia.

What Does It Look Like?

What is it?

Hydrocotyl (Hydrocotyle ranunculoides) is a hairless, perennial herb with long, creeping stolons that root profusely at the nodes and grow over land and out over the water. In water the leaves are emergent or sometimes floating. The leaf blades are circular or broadly kidney-shaped, 1.5–5 cm or more long and 2–12 cm wide, with small rounded teeth and shallowly to deeply 7–11-lobed (sometimes 3–6-lobed), and deeply notched with a radial slit at the base where it is joined underneath to a long leaf stalk (petiole). The stolons are trailing stems or runners that have leaves and roots at the nodes. The roots can be up to 60 cm long trailing in the water, or they anchor the mats into the riverbanks or stream bed (Wheeler 1987; Agriculture Protection Board of Western Australia 1993; Sainty & Jacobs 2003).

The inconspicuous, small, white, whitish green, or creamy yellow flowers (3–4 mm across) are arranged in umbels (umbrella-like clusters) where the more or less equal length (1–2 mm long) slender stalks of 5–10 individual flowers arise in a cluster at the top of a slender, 2–6 cm long peduncle or main inflorescence stalk that grows below the leaves.

The fruit is more or less circular in outline, 2–3 mm across, laterally compressed and breaking into ribbed segments (Wheeler 1987; Agriculture Protection Board of Western Australia 1993; Sainty & Jacobs 2003).

For further information and assistance with identification of Hydrocotyl contact the herbarium in your state or territory.

Flower colour

White, Green, Yellow

Growth form (weed type/habit)

Herb, Aquatic

Where it currently grows? Preferred habitat

Hydrocotyl grows in permanent, freshwater rivers, creeks, streams and water storages, often in wetlands, as found on the coastal plain near Perth. On the Canning River, upstream from the Kent Street Weir this introduced Hydrocotyle grows with Typha under Melaleuca woodland. It can line riverbanks with colonies forming mats on the water surface extending 3 m or more into the river. Hydrocotyl prefers still to slow moving water and is killed by salty water (Wheeler 1987; Agriculture Protection Board of Western Australia 1993; Hussey et al. 1997; Sainty & Jacobs 2003; Richardson et al. 2006).

Are there similar species?

There are many native and several introduced species of Hydrocotyle in Australia that can look similar when they are not examined closely. Care is required not to confuse Hydrocotyl with native species. It is usually an aquatic stoloniferous herb growing in water with the leaves emergent or floating whereas most native species are terrestrial, growing in damp situations. Another aquatic species, Hydrocotyle lemnoides, is endemic to the Perth region but it is distinguished from Hydrocotyle ranunculoides by having much smaller (2–4 mm wide), floating leaves. The introduced Hydrocotyle bonariensis has large leaves but they are more leathery, shiny and peltate (that is, the stalk is attached to the middle of the lower surface of the blade and not to the notched margin, as in Hydrocotyle ranunculoides) (Wheeler 1987; Hussey et al. 1997; Sainty & Jacobs 2003; Richardson et al. 2006; Kodela 2007, pers. comm.).

Why Is It A Weed?

What are its impacts?

Hydrocotyl was included in the list of 71 species that were nominated by state and territory governments for assessment as Weeds of National Significance (WONS). Following an assessment process, Hydrocotyl was not included as one of the 20 WONS. However, it remains a weed of potential national significance.

Agriculture: Dense floating mats of Hydrocotyl interfere with navigation, irrigation and river ecology. They can also constitute a safety hazard if humans or animals fall through a mat or are dragged under by the river current (Agriculture Protection Board of Western Australia 1993).

Native ecosystems: By forming extensive mats over the water, Hydrocotyl disrupts the ecology and the recreational uses of waterways. It has the potential to develop into a serious environmental, economic and recreational threat to the Canning River Regional Park and other lakes and waterways (Canning River Regional Park Management Plan 1997-2007).

How does it spread?

Hydrocotyl spreads both vegetatively and by seeds. If the mat is disturbed, sections of the runners can readily break off and float away to regenerate into new plants and mats. Seeds may be spread by waterfowl (Agriculture Protection Board of Western Australia 1993).

What is its history in Australia?

Hydrocotyl may have first arrived in Australia as a common aquarium and pond plant. It was first observed in 1983 in Bannister Creek (part of the urban drainage network) in the Canning River Regional Park near Perth, and by 1991 this weed had extended throughout the drainage network into the river and adjacent wetlands (Ruiz-Avila & Klemm 1996; Canning River Regional Park Management Plan 1997-2007). In September 1992 it was reported to cover about 30% of a 7 km section of the Canning River in water containing higher than 'normal' levels of phosphorus (Sainty & Jacobs 2003).

How To Manage It?

Best practice management

The spread of this weed should be prevented by early intervention, using manual or mechanical removal, and herbicides (Sainty & Jacobs 2003). Affected areas require ongoing surveillance and follow up weed control treatment.

Non-chemical control: Prevention: Unwanted aquarium plants of Hydrocotyl should not be dumped into rivers and lakes. Any parts removed from fish tanks or garden ponds should be dried and burnt (Agriculture Protection Board of Western Australia 1993).

A government-community program with an integrated weed management strategy (using a combination of physical, chemical and ecological controls) is discussed by Ruiz-Avila & Klemm (1996). A control strategy for the Canning River Regional Park was published by the Swan River Trust (Klemm et al. 1993).

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au .

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Hydrocotyl is perennial and grows rapidly during the warmer months, when mats can extend several hundred metres along river banks. Plants tend to stay small (about 1 square metre) when not anchored to the bank. Flowering occurs during spring to autumn (mainly November to February). The plants remain dormant but leafy over winter (Sainty & Jacobs 2003).

Where Is It Found?

Which states and territories is it found?

WA

What areas within states and territories is it found?

Hydrocotyl is only recorded from the Perth region of Western Australia, with most records from along the Canning River (Hussey et al. 1997; Sainty & Jacobs 2003; Canning River Regional Park Management Plan 1997-2007).

This vigorous weed has the potential to spread to other waterways in other temperate regions. It is a potential weed of all freshwater environments, being an aggressive invader of marshes, wetlands, waterways and the edges of still and slow-moving water (Richardson et al. 2006).

Where does it originate?

Hydrocotyl is native to the Americas and possibly also southern Europe (Cannon 1968).

It has become more widespread in Europe, probably being largely introduced as a plant for aquaria and garden ponds but now naturalised (e.g., see European and Mediterranean Plant Protection Organization 2006). Some references suggest it is native to North America but has become naturalised in Central and South America (e.g. Richardson et al. 2006).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

SA, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Hydrocotyle ranunculoides

Other scientific names (synonyms)?

Does it have other known common name(s)?

Pennywort, Hydrocotyle, Water Pennywort, Buttercup Pennywort