The waterweeds (Elodea and Egeria, Hydrocharitaceae) in Canada

Abstract

A study of herbarium material of Elodea spp. and Egeria densa was undertaken to document status in Canada and to evaluate existing methods of identification. Except in Elodea longivaginata, nonflowering plants are 2 – 3 times more common than flowering plants and females are more common than males. Leaf width partially separates E. canadensis and E. nuttallii males but not females. The ratio of pistillate sepal length versus leaf width separates female E. canadensis from female E. nuttallii, but intermediates exist. Nonflowering plants with leaf widths of 1.8 – 2.2 mm are treated as unknowns. Bifid styles are characteristic of E. longivaginata. Keys, descriptions, and drawings based on Canadian material are provided. The introduced Egeria densa occurs rarely in southwestern British Columbia. Elodea longivaginata and E. nuttallii are both rare native species, the former occurring in southern Alberta and southern Saskatchewan and the latter occurring in southern Ontario and southern Québec, Nova Scotia, southern Manitoba, and southern British Columbia. Elodea canadensis is a widespread native species occurring throughout southern Canada.