Floating water chestnut

Trapa spp.
Aquatic
Alternate
Simple
White
Green

An annual, floating aquatic plant with the stems (up to 5 m long depending on water depth) rooted in the bottom mud of the water body. The leaves are produced in rosettes and are serrated, waxy and triangular. The white flowers have four petals and are produced in the centre of the floating rosette.The fruit is a hard seed pod containing a single seed; the pod has two or four extremely sharp, barbed spines or "horns".

Floating water chestnut is a prohibited invasive plant under the Biosecurity Act 2014.

Common names 
Also known as: water chestnut, water caltrop,
Family 
Trapaceae
Deciduous 
No
Flowering time 
Mainly Summer but can continue into Autumn
Native/Exotic 
Exotic
Origin 
Native to South-East Asia,
Notifiable 
Yes
State declaration 
Category 1
Council declaration 
As per State Declaration
Known distribution 

The species Trapa natans is present as an invasive species in various eastern states of the United States of America. No species of Trapa is naturalised in Australia.

Habitat 

Prefers lakes, dams, reservoirs and mudflats. Floating water chestnuts grow in stationary or slow moving bodies of fresh water up to 5 m deep.

Habit 

Floating water chestnuts (Trapa spp., water caltrop) are aquatic plants with triangular shaped, floating, waxy leaves held in a rosette; the surface leaf rosette is borne on a stem that extends to and is rooted in the mud floor of the water body; the stem may be up to 5 m in length depending upon water depth.

Floating water chestnuts (Trapa spp.) can have extremely detrimental effects on a fresh water environment. Severe infestations can produce up to three successive layers of the plants on the water surface and the subsequent reduction of light to the bottom of the water body will eliminate native plants that normally would be present. Oxygen levels in the water beneath the infestation become lower and eventually lethal to fish and other native organisms. The overall effect is the smothering and destruction of the native aquatic flora and fauna.  The plant does not provide food for bird life which consequently abandon the infestation area.

The heavy infestations of Trapa spp. can affect human activities such as fishing, boating or swimming. The plants can clog irrigation facilities. In addition, the extremely sharp seeds are capable of penetrating footwear and causing personal harm. The massed plants can also provide better habitat conditions for mosquito breeding.

Stem and leaves 

Species of Trapa produce stems up to 6 mm in diameter and are commonly about 1 m long, however the length can be up to 5 m depending on water depth. The stem is anchored to the bottom mud by very fine roots. During its growth from the bottom of the water body, the stem produces submerged leaves which are soon discarded and the stem sites from which these leaves have fallen then produce feather like roots which can be easily mistaken for leaves. Up to 10 (occasionally 15) stems can be produced by a single seed and each stem will produce a rosette of leaves on the water surface.

Floating water chestnut plants produce two types of leaves.  While the stem is growing to the surface, it produces feather-like leaves which are discarded once the plant reaches the surface.  At the surface, the plant produces a rosette of leaves which are waxy, oval/rhomboidal or triangular, glossy on the upper surfaces, coated with short, fine hairs on the lower surfaces, serrated on the margins and normally 2-3 cm long but they can reach lengths of up to 6 cm. The leaf stalks (petioles) are enlarged and spongy with air sacks which allow the rosette to float.

Flowers and fruits 

The white flowers are carried on short stalks, have four petals, are about 8 mm long, and are produced at the centre of the rosettes. The flowers are supported by bracts (sepals) and these eventually become the spines on the fruit. The flowers may self-fertilise before opening or they can be insect pollinated. Once fertilisation has occurred, the stalk droops downwards to place the flower into the water where development of the fruit then continues.

The fruit is a hard seed pod or nut (about 3 cm wide) with two or four barbed spines which are about 1 cm long. Each seed pod contains a single seed and one rosette can produce up to 20 seeds in a season.

Reproduction and dispersal 

Reproduction in floating water chestnuts (Trapa spp.) is either by seeds or vegetatively.

Seeds can be carried by water currents or animals and they can remain dormant in the mud for up to 10 years; the seeds do not survive desication. Trapa spp. also reproduce vegetatively: portions of the parent plant can break off and water currents then move them away from the original location - they then survive long enough to produce seeds. Observations indicate that as much as a 10-fold increase in the biomass of a floating water chestnut population can be achieved in a single season and each of the new plants will produce up to 20 seeds.

The heavy seeds sink to the bottom of the water body and do not travel far, however plant fragments can be carried long distances to establish new populations.

Human activity has also contributed to the wide dispersal of these plants whether through the desire for a food crop or ornamental species in dams, lagoons, ponds and aquaria.

Similar species 

Species of Trapa are not likely to be confused with other water plants.  The distinctive rosettes and the spined nuts make the plants easy to distinguish.