Water chestnut

Water chestnut commonly refers to the aquatic plant *Trapa natans*, an invasive annual species native to Eurasia and Africa.

Water chestnut commonly refers to the aquatic plant *Trapa natans*, an invasive annual species native to Eurasia and Africa. It is also used for the edible corms of *Eleocharis dulcis* (Chinese water chestnut), but in North American plant-invasion references the name usually means *Trapa natans*. The plant forms floating rosettes in shallow freshwater and can create dense mats that crowd out native vegetation.

Description

*Trapa natans* is an annual aquatic plant that produces submerged feathery leaves and floating triangular leaves arranged in rosettes. It bears distinctive hard fruits armed with sharp spines, which can remain viable in sediment for years.

Distribution and habitat

The species is native to Eurasia and Africa and was introduced to North America in the 19th century as an ornamental. It typically colonizes shallow, slow-moving freshwater habitats such as ponds, lakes, and rivers, particularly those that are nutrient-rich and quiet.

Ecological impact

In invaded waters, water chestnut can form dense surface mats that shade out native aquatic plants, block sunlight, and reduce oxygen levels through decay. These mats also interfere with recreational activities such as boating, swimming, and fishing, and can crowd out native vegetation.

Taxonomy and common-name confusion

The name "water chestnut" is used for both *Trapa natans* and the edible corm-producing species *Eleocharis dulcis* (Chinese water chestnut). These are different plants in different families—*Trapa natans* is in the family Lythraceae, while *Eleocharis dulcis* is a sedge.

Status in Canada

Water chestnut is relevant in Canada because it has been reported in Canadian waters, including areas near Quebec and Lake Champlain, and it is a management concern for Canadian and cross-border invasive-species programs.