Prickly Waternymph

Najas marina L.

Synonyms - Naiad marina, Najas latifolia, Najas gracilis, Najas major.

Family: - Najadaceae


Other Names:

Hollyleaf Naiad (US

Prickly Naiad

Spiny Naiad


An underwater, perennial plant forming a mat on the lake floor about 20 cm deep. It has green to light brown, many branched stems up to 3 m long and with occasional short teeth. The leaves partially clasp the stem and are slender, stiff and crisp with coarse spiny teeth on the edges, upper surface. It forms large submerged masses that appear black from the surface.




First leaves:


Opposite in usually appearing to be in rings around the stem (whorled)

Stipules - None.

Petiole - None.

Base - Sheathing or partially clasps the stem with no lobes (auricles) where it meets the stem.

Blade - Up to 20-80 mm long and 3-5 mm wide, spine tipped teeth on the edges, lower midrib and often 1 to several spines on the top surface. Parallel sided. 1 veined.

Stem leaves -


Often about 1800 mm long and up to 3000 mm long.

Many branches.

May have spine tipped teeth or internodes may be covered with minute prickles

Flower stem -

Flower head:


Small, single and in leaf axils and remain submerged in water. Unisexual. Male flowers flask shaped, 3-4 mm long with a large bract at the base (spatheate). Female flowers flask shaped, 3-4 mm long without a spathe.

Separate male and female plants (dioecious)

Ovary - Superior. Single (1 carpel, 1 celled). 1 basal erect ovule. Usually has 3 stigmas but varies from 2 to 4.

Calyx -

Perianth -

Sepals -

Petals -

Stamens - One, 1-3 mm long enclosed in a thin 2 lobed envelope which ruptures as peduncle lengthens at anthesis. 4 sporangiate.

Anthers - Stalkless, 2 celled. 3.5 mm long. Water borne pollen.


Capsule or nut with a thin skin (pericarp). 3-4 mm long, single seeded.


Usually 3-6 mm long by 1.2-2.5 mm wide and covered with irregularly arranged pits. Large quantities of long lived seed produced. American material appears to have smaller seeds than Australian specimens.


Attached to the lake bottom until mature.

Key Characters:

Leaves about 5 mm wide with non auriculate sheathing bases and numerous spine tipped teeth along the margins.

Female flowers mostly with 3 stigmas.

From 625.


Life cycle:

Annual (though some list it as perennial also).

The mature plant detaches when mature and floats around dropping seeds.


Optimal growth at salt concentrations of 35mM but often grows in fresh water 626.

Rarely found more than 1.5m deep in water 626.

Light compensation point for net photosynthesis at about 5 mu E m-2 s-1 626.


Pollinates underwater. Large quantities of hard seed produced which will remain in the seed bank for many years.

Some infestations have no male plants but still produce seed indicating that it may produce seed without pollination (apomictic) 627.

Flowering times:

Summer in NSW.

February to March in Perth.

February to August/November in WA.

Summer and early autumn in the SW of WA.

Seed Biology and Germination:

Seed remains viable for more than 4 years 628.

Optimum germination at 20-25 degrees C 628.

Germination generally around 10% and increases to 60% is seed coat is scarified 628.

Better germination in dark conditions 628.

Vegetative Propagules:


2 sub species.

Najas marina ssp. latior has less than 1 spine per 10 mm along the stems and lower midrib of the leaves.

Najas marina ssp. armata has more than 1 spine per 10 mm along the stems and lower midrib of the leaves.


Population Dynamics and Dispersal:

Origin and History:



NSW, NT, QLD, Vic, WA.

Lake Wyangan near Griffith. Broke inlet in WA.

Almost worldwide.


Aquatic in fresh or brackish water in coastal and inland areas. Slow moving waterways, ponds, lakes and marshes. Up to 1000 m altitude in the US.


Temperate to hot.


Plant Associations:

Water weeds.




Large, dense submerged masses interfere with boating and fishing.






Management and Control:

Apply 1 ppm diquat (5 ppm Reglone®) as necessary. The quick kill of heavy infestations may lead to deoxygenation of water causing death of fish and other aquatic organisms. In areas with aquatic life it is better to treat about a third of the area each month.

Mix 5 litres of Reglone® in a 1000 litres of water in a fire fighting unit or similar and squirt into the water under high pressure to help mix the herbicide into each 1000 cubic metre section of water. For large areas apply the herbicide to the surface and mix the water with a boat and outboard motor.

5 ppm is equivalent to 5 litres of product per 1000 cubic metres of water.

Treated water can usually be used for irrigation or stock water about a week after application and the herbicide is usually undetectable 14 days after treatment.

Clipping doesn't affect growth rates 629 or provide control.


Eradication strategies:

Eradication will require many years of control to exhaust the hard seed bank.

Herbicide resistance:

Biological Control:

Related plants:

Waternymph (Najas browniana) has narrower leaves 1-3 mm wide with inconspicuous teeth and sheathing bases with obtuse auricles < 0.5 mm long. The seed is <1.7 mm long. The stigmas are 2-3 lobed.

Thin-leaved Naiad (Najas tenuifolia) has narrower leaves 1-3 mm wide with inconspicuous teeth and sheathing bases with acute auricles > 0.5 mm long. The seed is >1.7 mm long. The stigmas are 2-3 lobed.

Plants of similar appearance:

Chara (Chara species) has no flowers

Nitella (Nitella species) has no flowers

Parrots Feather

Pondweeds (Potamogeton spp.) has flowers above the water surface.

Water Milfoil

Widgeongrass (Ruppia spp.) has bisexual flowers and usually has alternate leaves.


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Everist, S.L. (1974). Poisonous Plants of Australia. (Angus and Robertson, Sydney).

Harden, Gwen J. (1991). Flora of NSW. (Royal Botanic Gardens, Sydney). Volume . P22. Diagram.

Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P

Lazarides, M. and Cowley, K. and Hohnen, P. (1997). CSIRO handbook of Australian Weeds. (CSIRO, Melbourne). #684.1.

Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). P723.

Paczkowska, G. and Chapman, A. (2000). The Western Australia flora: a descriptive catalogue. (Wildflower Society of Western Australia (Inc), the Western Australian Herbarium, CALM and the Botanic Gardens & Parks Authority). P75.

Randall, J.M. and Marinelli, J. (1996) Invasive Plants. (Brooklyn Botanic Gardens Inc. Brooklyn). P. Photo.

Wheeler, Judy, Marchant, Neville and Lewington, Margaret. (2002). Flora of the South West: Bunbury - Augusta - Denmark. (Western Australian Herbarium, Bentley, Western Australia). P341.


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