Heteranthera limosa (Sw.) Willd.
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Basis for Listing
This small, aquatic plant is rare and local throughout much of its range, and reaches Minnesota only at the extreme northern periphery of it range. Within Minnesota, Heteranthera limosa is very rare and apparently limited to an unusual microhabitat in several counties in the southwestern part of the state. It was first discovered at Blue Mounds State Park in Rock County in 1945 and still survives there in viable numbers, at least as recently as 2008. The exact population size is unknown because its vitality seems to alternate with moisture conditions. Heteranthera limosa was also collected at Pipestone National Monument in 1956, but it has not been relocated there since. Both of those populations occur or occurred in ephemeral pools that develop in association with outcrops of Sioux quartzite in the prairie region.
Based on vegetative characteristics alone, H. limosa could easily be confused with other closely related species. It can be reliably identified only when in flower. The flowers are ephemeral, however, and may appear for only a brief time when conditions are just right. The perfect 6-merous flowers are subtended by bladeless sheaths, and are usually blue, though they can range in color from purple to white (Gleason and Cronquist 1991). The 3 stamens are dimorphic. The lateral stamens are short and yellow with ovate anthers, while the central stamen is larger and purple or white, with a longer filament and an oblong anther (Gleason and Cronquist 1991). Leaves are consistently oblong and obtuse with parallel veins, but are otherwise quite variable. Specimens that develop submerged in water can appear quite different in form than those that develop stranded on mud.
All but one of the H. limosa populations in Minnesota occur in aquatic habitats that develop in association with outcrops of Sioux quartzite in the prairie region. Where these outcrops are exposed as flat terraces on gentle slopes, vernal pools may form between the outcrops or in depressions in the rock itself. These pools are sustained by seepage from the strata or directly from rain. Seepage pools generally retain water longer than rain pools and are the primary habitat of H. limosa. The pools are only 7-15 cm (3-6 in.) deep, and usually dry out by midsummer. When stranded in moist mud, the plants produce firm, shiny leaves and, if conditions stay moist, eventually form flowers. The plants disappear when the mud dries out
Biology / Life History
Heteranthera limosa is an annual that typically flowers in late June or early July in Minnesota. It is difficult to estimate population sizes for this species because it can seem quite abundant during wet years, but not appear at all during dry years. This suggests that the seeds of H. limosa can persist in the soil for at least a few years, although the actual length of viability of the seed bank is unknown.
Conservation / Management
Heteranthera limosa and its habitat have likely always been rare in Minnesota, but both may have suffered a decline resulting from years of overgrazing by cattle. Threats to this species include overgrazing, herbicide application for weed control, and rock mining.
Conservation Efforts in Minnesota
Known populations of H. limosa in Minnesota occur in two National Wildlife Refuges as well as a State Park, where they are protected from most threats. One population occurs on private land. No specific actions to enhance or preserve any of the populations have been undertaken at this time.
Gleason, H. A., and A. Cronquist. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. Second Edition. New York Botanical Garden, Bronx, New York. 910 pp.
Thorne, R. F. 1956. Notes on rare Iowa plants II. Proceedings of the Iowa Academy of Science 63:214-227.