Rare Species Guide

  Synonyms

Pontederia limosa

  Basis for Listing

This small aquatic plant is rare and local throughout much of its natural North American range, which is centered in the southern Great Plains. It also occurs in Mexico and South America, but its status in those countries in not known. Within Minnesota, Heteranthera limosa (mud plantain) is very rare and apparently limited to an unusual microhabitat in a few counties in the southwestern part of the state. These habitats are small ephemeral rainwater pools that develop in depressions in quartzite and granite outcrops. It was first discovered in Minnesota at Blue Mounds State Park in Rock County in 1945. It still survives there in viable numbers, at least as recently as 2018. The exact population size is unknown because its numbers seem to fluctuate with rainfall patterns. The few other populations known to occur in Minnesota have been found in similar habitats. However, most habitats that appear suitable do not harbor this species. Heteranthera limosa was listed as a threatened species in Minnesota in 1996.

  Description

Heteranthera limosa is most easily identified when in flower. The flowers are ephemeral, however, and may appear for only a brief time when conditions are just right. The flowers have 6 blue petals and are subtended by bladeless sheaths. 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. Leaves are consistently oblong with obtuse apices and 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.

  Habitat

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 (Inner Coteau Subsection), the remaining habitat is on granite (Minnesota River Prairie Subsection). 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 insect-pollinated annual that typically flowers in July or early August 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 pattern is supported by a study suggesting the seeds of H. limosa can remain dormant for a number of years when buried in a non-flooded substrate but germinate in large numbers when the substrate is flooded (Baskin et al. 2003).

  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 quarrying.

  Best Time to Search

In a typical year, the best time to search for Heteranthera limosa is late June and early July. This species is visible throughout the growing season in years when habitat moisture is sustained; otherwise, the plants disappear when the habitat is desiccated. The exact timing of the flowering period depends on available moisture in the pools.

  Conservation Efforts in Minnesota

Known populations of H. limosa in Minnesota occur in two National Wildlife Refuges as well as a State Park. This provides some level of protection for H. limosa. However, livestock grazing is allowed at each of the sites, and the impact of grazing on H. limosa is not being monitored.   

In 2007, the Renville and Redwood County Soil and Water Conservation Districts, in association with state and federal agencies, coordinated an initiative to compensate interested landowners for permanently protecting the most ecologically intact rock outcrops and associated wetlands in those two counties. In 2009, the initiative was expanded to encompass five counties in the Upper Minnesota River Valley (Protection of Granite Rock Outcrop Ecosystem). By 2012, approximately 227 ha (560.4 acres) were enrolled into perpetual conservation easements as a result of this initiative, and H. limosa is one of the rock outcrop species most likely to benefit from the program. Conservation plans, which outline conservation practices, such as removal of invasive species and livestock exclusion, were developed for each of the easement sites. Habitat maintenance and improvement measures, such as prescribed burning, fencing, rotational grazing, and/or seeding were also included in the plans.

  Authors/Revisions

Welby R. Smith (MNDNR), 2021

(Note: all content ©MNDNR)

  References and Additional Information

Baskin, C. C., J. M. Baskin, and E. W. Chester. 2003. Ecological aspects of seed dormancy-break and germination in Heteranthera limosa (Pontederiaceae), a summer annual weed of rice fields. Weed Research 43(2):103-107.

Harris, F. 2009. Rock pools on the prairie: ephemeral pools on the prairie form amazing mini-ecosystems. Minnesota Conservation Volunteer 72(423):30-39.