Trapa natans
(waterchestnut)

T. natans is a productive, annual, floating-leaved plant which has been cultivated globally for the nutritious nut it produces (Hummel and Kiviat, 2004). It is an extremely important food crop in China and India and is protected in Europe (Hummel and Kiviat, 2004), but in its introduced range, it grows in thick stands that displace native vegetation and affect water quality. Thick beds of water chestnut can cause significant declines in dissolved oxygen that negatively affect sensitive fauna (Hummel and Findlay, 2006). The nearly impenetrable mats are of virtually no use to wildlife and interfere with boating, fishing and swimming, while the large, spiny nuts can cause injuries to swimmers (ISSG, 2005). T. natans sets abundant seed, making it difficult to eradicate once it is introduced (Les and Mehrhoff, 1999).

The genus Trapa is cultivated worldwide for the harvest of its large, nutritious nut. It currently occupies a wide yet discontinuous native range across Europe, Asia, and Africa, and has been introduced to North America and Australia. It was more widespread in Tertiary times than it is currently (Ithaka Harbors Inc, 2008). The variety T. natans var. natans, with its four-spined nutis widely distributed in Eurasia, Africa and the northeastern United States, whereas T. natans var. bispinosa (also known as T. bicornis, T. bicornuta, or T. japonica) a two-spined variety, grows in China, Japan, India and Southeast Asia (Hummel and Kiviat, 2004). It is preferentially associated with low-energy, high-nutrient systems (USDA-NRCS, 2008).

T. natans has largely been spread as a result of intentional plantings. There have been many reports of escape from cultivation, and the species was originally introduced as an ornamental (Les and Mehrhoff, 1999). T. natans remains well-established in the North Eastern United States to this day (Hummel and Kiviat, 2004). The plant annually produces nuts that sink to the sediment and germinate. The rough spines of the fruit make it generally unpalatable to wildlife, reducing the likelihood of the species being spread this way. Instead, seeds disperse passively, being carried by water currents as they drop to the sediment surface (Boylen et al., 2006). The spines of the fruit also allow it to spread over longer distances as a hitchhiker, when it clings to boats and gear (Hummel and Kiviat, 2004).

T. natans is found world-wide in full sun and low-energy, nutrient-rich fresh waters (Hummel and Kiviat, 2004). It is commonly found in waters with alkalinity ranging from 12 to 128 mg/L of calcium carbonate (O’Neill, 2006), and dislikes calcium-rich waters (PFAF, 2000). Mixed reports exist on the depths of water typically inhabited by T. natans. Some sources report the plant can grow in water up to 5 m deep (Pemberton, 2002), others report that T. natans can be found in depths ranging from 0.3 to 3.6 m (Hummel and Kiviat, 2004), whereas others report a maximum depth of 0.6 m (PFAF, 2000). Hummel and Kiviat (2004) report that the species is found most abundantly in water around 2 m deep and in soft substrate. It also prefers slightly acidic water (PFAF, 2000), although germination can occur in water with pH ranging from 4.2 to 8.3 (Hummel and Kiviat, 2004). The species is disturbance-tolerant; it has been shown that sewage inputs create favourable conditions of increased alkalinity for the plant, and that increased nitrogen is correlated with increased petiole and fruit biomass. T. natans does not tolerate salinity; its seeds will not germinate when NaCl concentrations exceed 0.1% (Hummel and Kiviat, 2004).

Where conditions are favourable, T. natans can cover almost 100% of the water surface and shade up to 95% of sunlight (Hummel and Kiviat, 2004). Thus, the effects of the species on native vegetation in its adventive range are significant. Water chestnut is considered an invasive, destructive species, and has been implicated in the loss of many other plant and animal species. In the Hudson River, for instance, the plant has replaced water celery (Vallisneria americana ), clasping pondweed (Potamogeton perfoliatus .) nonindigenous Eurasian watermilfoil (Myriophyllum spicatum .). However, the shelter created by the rosettes is beneficial for duckweeds (Lemna minor , Spirodela polyrhiza. and Wolffia spp.) and filamentous algae. Other emergent species that grow above the waterline, including cattail (Typha angustifolia ), pickerelweed (Pontederia cordata), and spatterdock (Nuphar advena ) are unaffected by the presence of T. natans (Hummel and Kiviat, 2004).

Many natural enemies in the native range of T. natans have been documented by Pemberton (1999). The plant is native to the Old World, and many enemies (insects, fungi, viruses) are found throughout its native range. Of the currently explored enemies, he reports that the most common and damaging species in Asia is the weevil Galerucella birmanica which causes complete defoliation of entire populations and is also somewhat host-specific (oligophagous). Hummel and Kiviat (2004) report observations on natural enemies in the plant’s alien range. T. natans is productive and is occasionally a nuisance in its native range, therefore, natural enemies are extremely important to keep populations in check. A major reason behind why the plant is so problematic in its introduced range is precisely because of release from predation (O’Neill, 2006).

Animal feed, fodder, forage

• Fishmeal

Environmental

• Landscape improvement
• Revegetation
• Wildlife habitat

Fuels

• Biofuels

General

• Botanical garden/zoo
• Ritual uses
• Sociocultural value

Human food and beverage

• Flour/starch
• Nuts
• Sugar

Materials

• Chemicals
• Essential oils
• Fertilizer
• Fibre

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

The distinct floating rosette makes this aquatic species easier than most to detect soon after invasion. 

T. natans is unlikely to be readily confused with native plants in its adventive range. The distinctive floating rosette of leathery green leaves and the production of large horned nuts means it is highly morphologically distinct from other floating-leaf species. The two most commonly recognized varieties are distinguished based on the number of spines. The Eurasian/European varieties always have four spines, whereas the Asian varieties (Trapa natans var. bispinosa) have two spines.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Since T. natans remains valued for its nutritional and cultural uses, and since it is still a plant of botanical interest, educational programs must be directed to educate the public about the dangers this plant poses outside of its native range. Teaching users how to clean equipment in a way that decreases the chance of transmission is one way to lessen the impact of human-mediated transport. Several of the United States have legislated the regulation of the purchase, transportation, and introduction of this species.

It is much easier and more effective to attempt to control this plant early in its introduction timeline. Small populations are effectively controlled by hand pulling, preferably prior to the production of the propagating nuts. If the infestation is allowed to persist, it will probably grow quickly. It has been reported that this species is capable of increasing its biomass by ten times in a single year (Groth et al., 1996). Large infestations must be controlled by mechanical harvesters or herbicides and can be quite costly (O’Neill, 2006). 

Numerous educational campaigns have been directed at informing the public about the danger of aquatic invasive species in states of the USA in which T. natans is particularly problematic commonly distribute informational materials about its identity as well as instructions on how to report new invasions. Other educational campaigns have been directed toward informing the public about how to clean equipment in order to prevent the movement of invasive species.

It has been reported that this species was eradicated from the state of Virginia, USA (ISSG, 2007).

Nuts, though large and not as portable as propagules of other aquatic invasive species can remain dormant for up to 10 years, so it is extremely important to decrease the instances of accidental introduction by addressing humans as vectors. Additionally, since the plant is capable of producing ramets and engaging in vegetative clonal expansion via plant fragments, establishing guidelines on how to properly clean equipment, dispose of water, and identify target plants will probably decrease instances of accidental transportation and release.

Since the seeds of T. natans can remain dormant for up to 10 years, annual control efforts for at least that long must be undertaken in order for there to be a chance of eradication (O’Neill, 2006). Large beds must be mechanically harvested, but this will provide relief for only one growing season (O’Neill, 2006). Smaller areas of infestation can be addressed with hand pulling, although care must be exercised that all parts of the plant be removed, lest fragments remain to mature and produce fruits (Hummel and Kiviat, 2004). Ultrasound has also been proposed as a possible method of control. After treatment of the stem with ultrasound for 10 seconds, a mortality rate of 97.6% was reported (Wu, 2007).

Plants can spread locally as nuts and fragments drift in water currents, but most attention should be given to addressing forms of human-mediated transport. A number of the United States have enacted legislation limiting the introduction, sale, transportation and trafficking of the species in an attempt to limit the rate of accidental or intentional introduction (USDA-NRCS, 2008).

Much attention has been given to discovering methods of biological control. Grass carp Ctenopharyngodon idella has been used to control water chestnut (Hummel and Kiviat, 2004). However, grass carp are non-selective herbivores that will almost certainly harm native species. Much research has been forwarded on the use of herbivorous insects from the plant’s native range (Pemberton, 1999). Of the explored species, the leaf beetle Galerucella birmanica has shown the most promise. Although concerns regarding its specificity were forwarded early on in the research process, it has since been shown that although capable of completing its life cycle using native Brasenia schreberi, G. birmanica exhibits a strong preference in the laboratory and in the field for T. natans, with only occasional “spill-over” of beetles onto B. schreberi (Ding et al., 2006). 

Some control of water chestnut has been documented with subsurface applications of triclopyr and 2,4-D amine. However, the maximum control achieved was only 66% (Poovey and Getsinger, 2007). Due to its limited efficacy, if chemical control is used, it should be accompanied by other forms of physical control and removal.

08/04/08 Original text by:

Alison Mikulyuk, Wisconsin Dept of Natural Resources, Science Operations Center, 2801 Progress Rd, Madison, WI 53716, USA

Michelle Nault, Wisconsin Department of Natural Resources, 2801 Progress Rd, Madison, WI 53716-3339, USA