Quick Facts

Quick facts

• Hydrilla (Hydrilla verticillata) is a submerged aquatic herb, with branching stems up to 8 m long that are usually anchored to the soil.
• It mostly spreads vegetatively, by a variety of means.
• It is a native species that is not normally weedy, but may become problematic when nutrient levels rise and it may grow extremely rapidly and form dense tangled floating masses.
• It can restrict water flow and be a weed of irrigation and flood mitigation channels and streams, as well as reducing reservoir capacity and interfering with boating and recreational activities.
• Dense stands can have negative impacts on wetland animal and plant communities, altering habitats, displacing other aquatic plants, and thus reducing biodiversity.

What Does It Look Like?

What is it?

Hydrilla (Hydrilla verticillata) is a much-branched, wholly submerged long-lived (perennial) herb with stems growing up to 2 m or sometimes to 8 m long. It usually forms a canopy or dense mat immediately below the water surface. Stems are pale green, slender, branching above and take root freely from the lower joints (nodes). The internodes, the stem sections between the nodes, are up to 5 cm long in deeper water, but get closer together towards the surface. The internodes are usually longer than the length of the leaves except near the tip of the stem. While normally firmly rooted in the bottom mud, plants occasionally form free-floating mats. Slender, creeping lateral stems form on, or slightly below, the sediment surface. The leaves are arranged mainly in whorls (rings around the stem) of 3-8 that are mostly well-spaced along the stem. They are pale to bright green and often red-streaked on the prominent midrib and along the margins or with pink or reddish brown spots and stripes. They are thin and linear to lance-shaped, 5-40 mm long and 1-5 mm wide (the lower leaves are small, fewer and often in opposite pairs), with finely toothed or serrated edges.

Hydrilla is usually dioecious i.e. with male and female flowers on different plants. The small female flowers are about 3 mm in diameter and are borne singly on thread-like tubes originating in the forks (axils) of the leaves. They are borne just above the water surface and have six tiny 'petals' (perianth segments). Male flowers are also borne in the leaf forks (axils) but quickly detach from the plant and float freely on the water surface. The fruit is more or less cylindrical, 5-10 mm or sometimes up to 15 mm long and contains up to 6 green to dark brown, oblong to oval seeds that are 2-3 mm long (Yeo et al. 1984; Stanley & Ross 1989; Jacobs 1993; Swarbrick et al. 1995; Holm et al. 1997; Cronk & Fennessy 2001; Parsons & Cuthbertson 2001; Balciunas et al. 2002; Weber 2003; Navie 2004).

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

Flower colour

• White
• Green
• Red
• Brown

Growth form (weed type/habit)

Aquatic, Herb

Where it currently grows? Preferred habitat

Hydrilla is widespread in a variety of primarily warm freshwater habitats and is most common in tropical and temperate regions. It usually found in still or slow-moving water, such as creeks, rivers, irrigation channels, lakes, lagoons, ponds, wetlands and seasonally inundated swamps. It is often found in habitats subjected to occasional flooding and tolerates a great deal of disturbance (Sainty & Jacobs 1981, 2003; Cook & Lüönd 1982; Stanley & Ross 1989; Jacobs 1993; Holm et al. 1997; Cowie et al. 2000; Parsons & Cuthbertson 2001). Hydrilla is tolerant of cool and brackish waters and it is often found in the lower reaches and upper estuaries of coastal streams. It can grow in water varying from a few centimetres to 12 m deep. Although it tolerates a wide range of nutrient levels, it grows better on more fertile muds than on sands, and growth is also enhanced under conditions of increased nutrient levels, for example where there is runoff from agriculture. Hydrilla is also affected by pH changes, temperature and variation in day length, light intensity and light quality (Swarbrick et al. 1995; Holm et al. 1997; Parsons & Cuthbertson 2001).

Are there similar species?

There are a number of similar-looking submerged aquatic species that can grow in similar habitats. Differences between Hydrilla, which is characterised by having leaf margins that are distinctly serrated with fine translucent teeth, and the following introduced species, which are all in the family Hydrocharitaceae, include:

• Dense Waterweed (Egeria densa); has more crowded leaves with finer serrations, and also differs in its large, up to 2 cm wide, 3-petalled white flowers

• Elodea (Elodea canadensis); which has generally shorter leaves in whorls of 3 (rarely 4 or 5) with margins more finely serrated with minute dark-tipped teeth

• Lagarosiphon (Lagarosiphon) has leaves that are spirally arranged around the stem rather than in whorls as in the other species (Kleinschmidt & Johnson 1977; Jacobs 1993; Bowmer et al. 1995; Romanowski 1998; Sainty & Jacobs 1981, 2003; Ramey 2001a, b; Richardson et al. 2006).

When using vegetative parts for identification of Hydrilla it is important to be aware of the wide variation in its form and colour (Holm et al. 1997). A simple test to distinguish Hydrilla from Egeria densa and Elodea canadensis is to pull the foliage through your hand; the latter two feel smooth while Hydrilla feels harsh and scratchy (Yeo et al. 1984).

Why Is It A Weed?

What are its impacts?

Hydrilla is a native species and is rarely a problem in Australia in healthy aquatic environments (Tasmanian Department of Primary Industries and Water 2008).

It is usually ecologically beneficial to habitats e.g., a stable population will improve water quality and provides protective habitat for juvenile fish, except where environmental changes occur, such as increased nutrient status. Nutrient enrichment of waters often leads to greatly increased growth rates, rapid spread and the formation of dense masses of intertwined stems. Hydrilla can become an aggressive weed that restricts water movement and interferes with hydroelectric systems, urban water supplies, its extensive growth can clog the filters of irrigation pumps in irrigation projects, congest flood control canals and natural streams and cause flooding, block piers and tangle in boat motors and affect recreational activities, including most water sports such as swimming, fishing, sailing and motor-boating. It can clog rivers, choke channels and also reduce the holding capacity of storage ponds and dams, leading to the loss of urban and agricultural water supplies (Swarbrick et al. 1995; Holm et al. 1997; Cronk & Fennessy 2001; Parsons & Cuthbertson 2001; Balciunas et al. 2002; Stephens & Dowling 2002; Global Invasive Species Database 2006).

Dense stands can alter water quality by decreasing oxygen levels and increasing pH and water temperature (Global Invasive Species Database 2006).

Dense colonies of Hydrilla reduce sport fishing by reducing the open-water feeding and spawning areas for fish. It also affects the recreational use and tourism potential of waterways as dense masses clog waterways and impede boat passage (Cronk & Fennessy 2001).

Hydrilla can be invasive and impact on other native biodiversity. It can form a dense canopy at the water's surface that shades growth in the water column below, while large mats crowd out other aquatic plants and reduce habit for wildlife (Cronk & Fennessy 2001; Sainty & Jacobs 2003; Weber 2003). Single species stands of Hydrilla may occur due to plants releasing allelopathic substances, chemicals that inhibit the growth of other plant species (Gopal & Goel 1993; Parsons & Cuthbertson 2001).

How does it spread?

Hydrilla reproduces both via seeds and vegetatively. It usually reproduces, spreads and survives vegetatively by spreading horizontal stems (stolons and rhizomes), turions (vegetative propagules that act like dormant buds), lateral stem buds, fragmentation and short lateral branches along the upper stems. Turions are either axillary or subterranean. Axillary turions are green buds that develop in the joint between the stem and leaves, or sometimes at the ends of stems, and fall to the sediment to over-winter and sprout new growth in the spring. Subterranean turions (frequently called tubers to differentiate them from the stem turions) are buds produced on the rhizomes and develop at the terminal nodes or ends of stems that turn downward and penetrate the substrate. They are often brown but can vary from off-white to almost black. Hydrilla colonies extend locally via stolons and 'tubers', while turions and stem fragments can be transported long distances in flowing water enabling the species to spread to new areas. Small fragments of stem easily break off and readily produce roots when they come to rest in a suitable location, thus starting the formation of a new colony (Sainty & Jacobs 1981; Cook & Lüönd 1982; Yeo et al. 1984; Stanley & Ross 1989; Spencer & Ksander 1991; Rye 1992; Holm et al. 1997; Netherland 1997; Cronk & Fennessy 2001; Parsons & Cuthbertson 2001; Stephens & Dowling 2002).

Hydrilla can also be dispersed by recreational activities e.g., plant fragments carried by boats, trailers, fishing equipment, nets and possibly by waterbirds, as tubers survive digestion and regurgitation, and it has been spread as an aquarium and pond plant (Global Invasive Species Database 2006).

What is its history in Australia?

Hydrilla is now widely distributed and found on all continents except Antarctica and most of South America (Holm et al. 1997; Cronk & Fennessy 2001; PIER 2008). It was introduced to the Americas, Fiji and New Zealand (Swarbrick et al. 1995; Cowie et al. 2000; Parsons & Cuthbertson 2001) and has been listed by Soerjani (1986), as second only to Water Hyacinth (Eichhornia crassipes) in threatening world waterways and crops. Hydrilla is regarded as a serious weed in the United States of America (Jacobs 1993; Global Invasive Species Database 2006) and in the waters of south-east Asia (Soerjani 1986; Holm et al. 1997). There is considerable variation in the species throughout the world, with strains and biotypes often being recognised (Cook & Lüönd 1982; Holm et al. 1997; Parsons & Cuthbertson 2001).

Although native to Australia, there is some uncertainty as to Hydrilla's natural distribution and it is spreading in some parts of the continent (Rye 1992).

How To Manage It?

Best practice management

Since Hydrilla usually becomes weedy in response to the human-induced problem of nutrient enhancement, this factor needs to be addressed to effectively control the weed. When the cause of the environmental change has been appropriately managed, there are various controls that can be attempted if the infestation remains a problem (Parsons & Cuthbertson 2001).

Mechanical removal methods such as the use of booms, disking, dredging, hydraulic excavators, draglines, or cutting by hand or pulling can only provide temporary relief as they actually increase the spread of Hydrilla populations by fragmenting plants which gives rise to new plants. Mechanical methods are also slow and costly and can reduce fish numbers significantly (Cronk & Fennessy 2001; Parsons & Cuthbertson 2001). In some situations, a more effective control may be possible through the temporary lowering of water levels, such as draining dams, drains and channels (Swarbrick et al. 1995; Parsons & Cuthbertson 2001).

Hydrilla can be controlled effectively with several herbicides (Kleinschmidt & Johnson 1977; Gupta 1987; Swarbrick et al. 1995; Cronk & Fennessy 2001; Parsons & Cuthbertson 2001; Shearer & Nelson 2002; Weber 2003).

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)

Hydrilla is a perennial plant in which growth continues throughout the year in tropical areas, with a flush of new shoots from both vegetative organs and seeds at the end of the wet season, March to April in Australia. In cooler temperate areas, Hydrilla over-winters as dormant shoots, tubers, turions and seed. These reproductive organs commence growth in spring and grow rapidly in summer to produce a mat of interwoven stems which become dormant or die back at the end of the growing season in late autumn or early winter, depending on temperature (Swarbrick et al. 1995; Parsons & Cuthbertson 2001). Turions that developed over a few weeks in autumn break off and sink to the bottom, where they grow. Tubers are also formed on rhizomes in the autumn (Sainty & Jacobs 1981). The over-wintering propagules may also be carried long distances by flowing water and will establish new plants in the next season (Holm et al. 1997). A bank of tubers and turions in the soil may persist for several years (Weber 2003).

Flowering occurs generally during warmer months (Jacobs 1993), and continues until winter in south-east Queensland (Parsons & Cuthbertson 2001). On the tropical coastal floodplains of the Northern Territory, flowers are reported from April to October (Cowie et al. 2000). Following a bright sunny day, male flower buds detach from the plant and rise to the surface where they float for a while and then open explosively to eject pollen. Female flowers, while remaining attached to plant stems by a thread-like tube (they reach the surface by elongation of this floral tube), also float on the surface to catch the air-borne pollen (Swarbrick et al. 1995; Parsons & Cuthbertson 2001).

Where Is It Found?

Which states and territories is it found?

• NT
• NSW
• VIC
• SA
• WA
• QLD

What areas within states and territories is it found?

Hydrilla occurs in all states and territories of Australia, except the Australian Capital Territory and Tasmania, in a broad band along the northern and eastern coasts, in parts of the Murray-Darling system, and near Perth in Western Australia. It is widespread in the tropical to warm temperate regions and less common in the cooler, southern areas (Jacobs 1993; Swarbrick et al. 1995; Parsons & Cuthbertson 2001; Western Australian Herbarium undated; AVH 2008).

Where does it originate?

It is uncertain where Hydrilla is truly native (Cook & Lüönd 1982), but it is generally considered to be a cosmopolitan species with an original distribution from parts of Europe to Africa, India, south-east Asia and Australia (Swarbrick et al. 1995; Cowie et al. 2000; Parsons & Cuthbertson 2001; PIER 2008).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

WA, TAS

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

Hydrilla verticillata

Other scientific names (synonyms)?

• Elodea verticillata (L.f.) F.Muell.
• Hydrilla dentata Casp.
• Seripicula verticillata L.f
• Udora australis F.Muell.

Does it have other known common name(s)?

• Hydrilla
• Florida Elodea
• Water Thyme
• Waterthyme