Hornwort

Ceratophyllum demersum L.

Family: Ceratophyllaceae.

Names:

Ceratophyllum is a combination of the Greek word keras for horn and phyllon for leaf and refers to the horn like leaves.
Demersum is from the Latin demergo meaning to sink or plunge and refers to the submerged nature of the weed
Hornwort refers to the horn shaped leaves and wort means weed.

Other Names:

Coontail.

Summary:

Hornwort is a free-floating, rootless perennial. The whole plant forms a tangled mass that may float just below the surface or under 1 to 2 metres of water.

Description:

Cotyledons:

Two.

Leaves:

Arise in crowded whorls from each node, the number of leaves varying between 5 and 12. They tend to be more crowded towards the ends of the stems.
Stipules - None.
Petiole - None.
Blade - Dark green. 10-40 mm long. Often rough to the touch, stiff and brittle. Thickened base. Usually the leaves are twice forked but may be once or three times forked, the segments slender, 0.5 mm wide, tapered, cylindrical with two rows of minute teeth and two spines at the tips of the teeth. Teeth are obvious on one side.

Stems:

Cylindrical, slender. Flexible and cord like or stiff, branched and brittle. Up to 2000 mm long, wavy and often rough to the touch. Nodes are very crowded at the tips of the stems and are 10-30 mm apart lower down.

Flower head:

Single flowers in the leaf axils.

Flowers:

Greenish, stalkless, inconspicuous, underwater. Small, 1 mm long, unisexual (monoecious). Male and female flowers on different nodes.
Ovary - Small. 1 pendulous ovule. Single, persistent, awl shaped style.
Perianth - None apart from 9-12 tiny bracts.
Stamens - No filament.
Anthers - 12-20, oblong with 2 sharp points, stalkless. Break off and float to surface.

Fruit:

Turns black when ripe. 4-5 mm long. 1 celled, single seeded, egg shaped to elliptical nut with the spine like persistent style on top. Warty, with 2-4, soft, 9-12 mm long, bent back spines near the base that are longer than the nut.

Seeds:

Fruit.

Roots:

None. Finely divided basal stems (rhizoid shoots) occasionally anchor the plant to the soil.

Key Characters:

Dichotomous segments on leaves.

Biology:

Life cycle:

Annual or perennial. Reproduction is mainly vegetative, branches breaking away from the main plant. Although the plants commonly remain sterile, male and female flowers may be produced in the leaf axils of different nodes on the upper branches. When mature, stamens break loose and float to the surface, releasing pollen that then sinks to the female flowers. Hard skinned fruits are produced which sink to the bottom when ripe. The seed germinates in the mud in spring but does not produce any roots. It produces leaves and forms a shoot about 80 mm long. This breaks off and floats to the surface where it grows floating freely in the water. As the plant ages the lower parts of the stems may decay but growth continues from the upper branches. Flowering usually occurs in summer.
In cooler areas it grows mainly in the summer and over-winters as thickened shoots (turions) that may stay attached to the mother plant or sink to the mud bed. Axillary buds grow in the following spring when day length and temperatures increase. Those growing in the bed break free and rise to the surface to continue their growth.

Physiology:

Killed by salty water and ice.
Prefers shaded, warm, clear, gently flowing water (10 mm/sec), with a pH of 7.6-8.8.
Maximum growth rates at 300C, 2.5-5 m water depth, 10% sunlight, high nitrogen, phosphate and potash levels.

Reproduction:

Mainly vegetative but also by seed.

Flowering times:

Winter and spring in SA.
Summer in NSW.

Seed Biology and Germination:

Vegetative Propagules:

Turions, stem fragments.

Hybrids:

Allelopathy:

Population Dynamics and Dispersal:

Hooked spines on the seed catch on wading birds aiding dispersal.
Diving birds may carry small stem fragments for some distance.
Most dispersal is by natural water flows carrying turions or stem fragments.

Origin and History:

Australia. Europe. Northern Hemisphere. Cosmopolitan.

Distribution:

ACT, NSW, QLD, SA, VIC, WA.

Courtesy Australia's Virtual Herbarium.

Habitats:

In fresh or occasionally brackish, still or slow flowing waters, up to 10 m depth.

Climate:

Cosmopolitan.

Soil:

Aquatic. Prefers water bodies with a muddy bottom.

Plant Associations:

Significance:

Beneficial:

Oxygenates the water.
Fodder for aquatic birds and fish.
Removes sodium, iron, arsenic and strontium from the water.
Good fodder with 23% protein and 2742 cal/g but has a high cost for drying.

Detrimental:

The plants can form large populations that may impede stream flow, interfere with amenity use of the water or reduce the efficiency of hydro electric installations. This is made worse if it occurs in combination with Potamogeton spp.
Blocks pumps.
Taints water.
Weed of stream, reservoirs, ponds and dams.
In Ghana it has hosted a snail that carries a disease causing parasite.

Toxicity:

Not recorded as toxic, but it can accumulate toxic compounds such as arsenic.

Legislation:

Secondary and prohibited weed in TAS.

Management and Control:

Rarely requires control in natural situations.
Mechanical harvesting provides temporary control.
Covering with black plastic for 18-28 days is effective for areas of less than 300 m2 like ponds or dams.
Lowering the water level may expose the plant to desiccation.
Acrolein, paraquat and diquat usually provide the most cost effective means of control.

Thresholds:

Eradication strategies:

It won't tolerate turbidity or salinity.

Herbicide resistance:

Biological Control:

Grass carp have eliminated it from some streams in Australia.

Related plants:

None.

Plants of similar appearance:

Water Milfoils (Myriophyllum spp.) have leaf segments arising in pairs on either side of the mid rib whereas Hornwort has the segments dividing into two like the fork in a road.

References:

Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P147. Photo.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P361.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P308. Photo.

Hyde-Wyatt, B.H. and Morris, D.I. (1980) The Noxious and Secondary Weeds of Tasmania. (Tasmanian Department of Agriculture, Hobart, Tasmania). P118-119. Diagram.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #288.1.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P376-378. Photos.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.