Rare Species Guide

  Basis for Listing

Rotala ramosior (toothcup) has a wide distribution in North America but it appears to be common only in the southcentral states. Minnesota sits along the northern periphery of its range, and there are very few occurrences in the state. It has been known only from shallow wetlands in five east-central counties, in what is now the Twin Cities metropolitan area (Anoka Sand Plain, St. Paul-Baldwin Plains and Moraines, Big Woods, and Mille Lacs Uplands subsections). Many of the known sites have not been seen for many decades and have presumably been destroyed by subsequent urban and agricultural expansion. Many of the remaining habitats are in areas projected to experience similar development in the near future. The comparatively recent invasion of aggressive non-native plant species is also taking a heavy toll on wetland habitats. It seems plausible that undiscovered populations may exist elsewhere in the region, especially considering that this species is small and easily overlooked, even by experienced botanists. But all evidence supports the conclusion that this is a very rare species requiring special conservation action if it is to survive in Minnesota. Rotala ramosior was listed as a threatened species in Minnesota in 1996.

  Description

Rotala ramosior is the only member of its genus in Minnesota and can usually be identified easily when in flower or fruit. However, the species is quite inconspicuous and may be difficult to detect in its chosen habitat. It is a low-growing plant with a simple or diffusely branched stem. Leaves are small, opposite, entire, and without stipules. Flowers occur singly in leaf axils. The calyx has 4 short lobes with appendages in each sinus and encloses a 4-locular, many-seeded capsule. There are four small pink petals which are promptly deciduous. There are 4 stamens and 1 style, which has a capitate stigma. The entire plant may take on a red coloration late in the season.

Rotala ramosior looks very similar to Ammannia coccinea (ammannia), which is another summer annual that might occur in the same habitat. One important difference is that Rotala will have just 1 or 2 flowers in the axils of the leaves; Ammannia will have up to 5 flowers in the axils of the leaves, and the flowers will be larger.

  Habitat

The populations of R. ramosior in Minnesota typically occur on the sandy shores of small shallow lakes set in a savanna landscape. More specifically, it can be expected to occur between the normal high-water line and low-water line. Such shoreline habitats undergo seasonal fluctuations that expose broad beaches in late summer, to the apparent benefit of this species. Habitats of this type were once common on the flat sandy outwash plain of the Twin Cities metropolitan area, but they rarely developed elsewhere in the state. Because these lakes were small and shallow, they were easy to fill, especially in an era when protecting wetlands was not a priority. Now, such lakes with undisturbed shorelines and native vegetation are a rarity.

  Biology / Life History

Rotala ramosior is a small inconspicuous summer annual. It is insect-pollinated, though it is self-compatible and often self-pollinated. No published studies have identified specific pollinators, though it is likely pollinated by small bees. A large number of small seeds are produced in a good year. Seeds are dispersed short distances by passive means, though water currents may move seeds to various locations within a wetland basin. The frequency and method of active, long-distance dispersal is unknown.  It appears to experience population booms and busts that may be related to water level fluctuations on its shoreline habitat. It has been shown that the seeds are dormant when dispersed in the fall but break dormancy after a period of flooding (Baskin et al. 2002). It has also been shown that seeds of R. ramosior germinate in response to light and can remain dormant in a persistent seed bank in the soil for at least fifteen years (Baskin et al. 2019).

  Conservation / Management

Habitat destruction or degradation seems to be the biggest threat to R. ramosior. The lakeshore and pond margin habitat required by this species is in high demand for a variety of uses. It is especially vulnerable to the draining and filling activities that typically precede industrial development. Residential, commercial, and recreational developments are also claiming potential habitat and contributing to the general decrease in populations of this species. Also, these ponds are sometimes used for stormwater retention or may be dredged to provide habitat for waterfowl. This decline is severe in the face of the rapid urban and suburban growth of the greater Twin Cities area. It is critically important to identify high quality examples of this habitat type and take measures to ensure their protection.

  Best Time to Search

The best time to search for Rotala ramosior is when it has reached full growth and produced reproductive structures, from August through September.

  Authors/Revisions

Welby R. Smith (MNDNR), 2023

(Note: all content ©MNDNR)

  References and Additional Information

Baskin, C. C., J. M. Baskin, and E. W. Chester. 2002. Effects of flooding and temperature on dormancy break in seeds of the summer annual mudflat species Ammannia coccinea and Rotala ramosior (Lythraceae). Wetlands 22:661-668.

Baskin, C. C., J. M. Baskin, and E. W. Chester. 2019. Long-term persistence of summer annuals in soil seed banks of seasonally dewatered mudflats. Plant Ecology 220:731-740.

Mattrick, C. 2001. Rotala ramosior (L.) Koehne (Toothcup): conservation and research plan. New England Wild Flower Society, Framingham, Massachusetts. 30pp.