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Large water-starwort, Callitriche heterophylla, is a small annual aquatic herb (stems up to 20 cm long). This herb grows usually submerged in shallow slow waters such as ponds, ditches, rain water pools and slow-moving streams. It is noted for its dimorphic leaf structure; where leaves grow from an underwater apex they are long (up to 2 cm), thin, pinnate with a branched tip, whereas leaves growing above water level are smaller, oval and cup-shaped. Research indicates that cell expansion and turgor pressure dependent on environmental conditions dictates leaf shape (Deschamp and Cooke 1984).
Callitriche heterophylla flowers in spring and produces fruit that break into four parts, each yielding a seed. Although its mechanism for surviving seasonal desiccation is yet unknown, it presumably overwinters as rhizomes or dormant seeds on the bottoms of pools, or in sediments left behind when water supply dries up.
Callitriche heterophylla is one of about 12 species of Callitriche native to North America. There are two subspecies, C. h. bolanderi (Bolander’s water-starwort, limited to western North America from Alaska to California) and C. h. heterophylla (twoheaded water star-wort, found across most of Canada and the United States, and also native to Greenland). Since 1996 this species is recognized as “of special concern” in Minnesota, where it is found in only eight locations in two disjunct populations. It is sensitive to water quality degradation and herbicides, habitat loss to farmed animal grazing, invasion by non-native aquatic species, and mining of rocky outcrops which harbor pools where this herb is frequently found (Archive 2003-2013; Minnesota DNR 2014; USDA plants database 2011).