Water Thyme

Hydrilla verticillata (L. f.) Royle

Synonyms - Hydrilla angustifolia, Hydrilla dentata, Hydrilla lithuanica, Hydrilla ovalifolia, Hydrilla wightii, Hottonia serrata, Leptanthes verticillatus, Serpicula verticillata, Vallisneria verticillata.

Family: - Hydrocharitaceae.


Hydrilla is from Hydra the many headed water serpent slain by Hercules.

Verticillata Is from the Latin verticillus meaning in rings and latus meaning widely spaced and refers to the spaced whorls of leaves on the stems.

Hydrilla is the preferred common name because there is only species in this genus.

Other names:

Florida Elodea (USA)



A submerged, much branched, stoloniferous, perennial, aquatic plant with rings of 4-8, toothed leaves spaced along the stems.





Produced in whorls usually of 4-8 but occasionally 3-12, usually well spaced at 5-40 mm along the stem, submerged. Leaves at the base of the stem or on the stolon are often opposite, smaller and fewer.

Stipules - None.

Petiole - None.

Blade - Pale to bright green often with red or brown dashes, 5-40 mm long by 1-4 mm wide, 1 nerved, linear or lance shaped with forward pointing teeth on the margins. Tip with a short spine. Hairless.

Sheath - None.

Axillary scales - Usually 2, triangular, translucent, fringed on the edges.


Pale green, up to 8000 mm long shorter in shallow water, slender, many branched, submerged, erect, roots at the nodes, attached to mud by roots. Hairless. Develop over-wintering buds (ovoid hibernacula) at the stem tips and short lateral branches with fleshy storage leaves. Short stolons on or just below the mud produce tubers. Vegetative organs for reproduction turions form in the leaf axils. Occasionally the stems form a free floating mat.

Flower head:

Single flowers emerge from spathes(modified leaves) in leaf axils towards the end of branches. 2 triangular spathe bracts that split at the top as the flower emerges.

Spathes of male flowers, sub globular, with one flower, 1.5-3 mm long with 8-16 awl shaped appendages surrounding a small knob at the top.


Float on the water surface. Male and female flowers are produced on separate plants usually.

Female flowers about 6 mm in diameter, cylindrical, with a short or no stalk (peduncle), emerge from a cylindrical spathe or sheath about 5 mm long with 2 branches at the top. They rise to the surface on a slender stalk-like hypanthium, 15-100 mm long, before opening.

Male flowers on a short, 1 mm, stalk with 1 or more spathes per leaf whorl. The buds break loose from the plant and rise to the surface where the globular flowers open and the stamens release pollen explosively, scattering it within a radius of about 200 mm where it fertilises any mature female flowers which may be floating in the vicinity.

Ovary - 3 ovules, 1 styles with 3 very short stigmas.

Sepals - 3, green or partly reddish, translucent. Female, egg shaped to oblong, up to 1.2-4 mm long.

Male, Broadly egg shaped, 1.2-3 mm long.

Petals - 3, green, translucent, alternate to the sepals, narrow, parallel sided, smaller than the sepals

Stamens - 3 with short filaments, 1 mm long, opening explosively in the male.

3 staminodes or absent in the female.

Anthers -


Cylindrical, 5-15 mm long by 1.5-6 mm wide, simple or spiny with 1-6 seeds that are not released. Develops within the spathe.


Brown to green, shiny, spindle shaped tapering at each end, 2-2.5 mm long, smooth.


Fibrous, embedded in mud or free, from the lower stem nodes. Stolons.

Key Characters:

Whorled leaves. Submersed aquatic plant.


Life cycle:

Annual or perennial. Seed, turions, tubers and hibernacula germinate and form roots as water temperatures rise in spring. They form a mass of erect, intertwined, submersed stems which flower from mid summer to late autumn. Male flowers break off and float to the surface to explosively release their pollen. Female flowers form a long thin thread to take their stigmas to the surface to be fertilised. These decay and the seed is formed below the water surface. As water temperatures drop, vegetative reproduction organs such as turions and hibernacula are produced and the plant becomes semi dormant. In tropical areas Hydrilla may grow all year with a flush of growth at the end of the wet season in March to April.


Wind pollinated.

It is killed by salt concentrations exceeding 4000 mg/L.

It grows over a temperature range of 10-350C and will survive freezing over for 10 days.

Prefers areas where there are high levels of P in the bottom mud.

Grows better in acidic water.

Bicarbonate form the water is its major source of C for growth.

Optimum growth rates occur in water 1000-3000 mm deep.

Green light (which penetrates deeper into the water) promotes stem elongation and red light promotes branching.

Tubers and turions form during periods of stress due to short day lengths, reduced P and N. Tuber formation is increased as temperatures drop.

Turion germination is inhibited by green light that is present in deep water. Turion dormancy is broken by exposure to cold, long day lengths, and red light.


By seed, stolons and turions, bulbils and stem fragments.

Flowering times:

April and May in SA.

Autumn in western NSW.

Seed Biology and Germination:

Vegetative Propagules:

Turions, stolons, tubers and stem fragments with hibernacula and bulbil like buds.

Up to 1000 turions/m2 have been recorded in the USA.



It produces substances that reduce the growth of companion plants.

Population Dynamics and Dispersal:

Seed production is poor and most spread is by vegetative reproduction, with fragments breaking off the main plant and establishing elsewhere. Survival is assisted by the production of three forms of resting body, one a short lateral branch with short internodes and fleshy storage leaves, tubers formed under the mud at the end of a stolon and turions. The first two are produced in large numbers and are capable of withstanding adverse conditions in a dormant state. Stolons creep along in the mud and stems root from the nodes to form the major methods of local spread. Stem fragments and turions break off when water flows exceed 300 mm per second or there is a mechanical disturbance and move with the water flow to infest new areas. Turions and axillary buds are protected from desiccation by tough scale leaves.

In areas where it is established in Australia it rarely reaches pest proportions unless there is a change in environmental conditions brought about by man, such as the discharge of sewage effluent or the impounding of catchments.

Under good conditions it may spread very rapidly. In the USA it covered 5 ha and moved 45 Km downstream in a single year.

Shading from its dense canopy and allelopathic make it competitive enough to exclude most other plants.

Origin and History:

Australia, Mediterranean Region, Africa, Asia, South-East Asia.




In still or slow flowing, fresh to brackish water 20-12000 mm deep. Prefers open shallow water bodies.


Tropical or warm temperate areas.


Prefers P rich mud.

Plant Associations:

Potamogeton spp., Myriophyllum spp., Sagittaria spp. and Vallisneria spp.



May be used for hay, fodder or compost producing up to 42 kg/ha/day with a digestibility of 57% and crude protein of 13%.


Weed of irrigation channels, streams, lakes, rice and drainage channels.

It restricts water flow.

Interferes with hydro electric operations, urban water supplies, irrigation projects, fishing and recreational uses.


Not recorded as toxic.


Noxious weed of TAS.

Management and Control:

It is resistant to many aquatic herbicides and the resting bodies (turions) very difficult to kill and remain to re-infest the area after the chemical or mechanical removal of the original infestation.

Temporary lowering of the water levels is usually the best method of control.

Acrolein and fluridone are used for chemical control.

In ponds, bipyridyls, diuron, hormone herbicides, endothal and chelated copper have given good control.


Eradication strategies:

Often eradication is not desirable as it forms part of the native ecosystem and attention should be focussed on the change in conditions that has led to a major expansion in its growth or dominance.

Herbicide resistance:

Biological Control:

Several fish may be useful for biocontrol.

Several insects and fungi are under investigation.

Related plants:


Plants of similar appearance:

Hydrilla is superficially similar to Elodea canadensis and Lagarosiphon, which is a member of the same family.


Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P26. Photo.

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