Bacopa rotundifolia (Michx.) Wettst.
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Basis for Listing
There are scattered occurrences of Bacopa rotundifolia (waterhyssop) throughout much of the country, though it is common only in parts of the Great Plains and the Mississippi River Valley. Minnesota is situated at the northern and eastern edge of its range, which is one reason it is so uncommon here. When it was listed as a species of special concern in Minnesota in 1984, only seven populations were known and all were in vulnerable habitats in the southwestern portion of the state (Prairie Parkland Province). Cattle grazing, rock quarrying, and herbicide application were identified as potential threats. Subsequently, its situation was perceived as worsening as land-use conflicts escalated, so its status was elevated to threatened in 2013. Since 2013, a small number of plants have been found on small sandy islands in the backwaters of the Mississippi River (Eastern Broadleaf Forest Province), where it had never been known to occur before. These new discoveries may warrant another review of its statewide status.
Bacopa rotundifolia is a small aquatic or amphibious plant, with roundish opposite leaves that clasp the rather thickish stem. The leaves are 1.0-3.5 cm (0.4-1.4 in.) long and 0.7-2.5 cm (0.3-1.0 in.) wide. The flowers are white or bluish-white and develop on short recurved pedicels in the axils of the leaves. The flowers are 6-10 mm (0.2-0.4 in.) long and have 4 stamens, 5 sepals, and 5 petals. The stems are 5-40 cm (2-16 in.) long and, when stranded on mud, the stem can root at the nodes.
Bacopa rotundifolia is basically an aquatic species. In western Minnesota, it is found primarily in small rainwater pools on bedrock outcrops and occasionally along the margins of shallow prairie ponds. The water is rarely more than several inches deep, and it typically evaporates by mid-June, leaving the sediments at the bottom of the pool moist or sometimes dry and cracked. Bacopa rotundifolia is generally able to persist for a couple more weeks after the pools dry up and then completely disappears from view by July. On rare occasions, pools may refill in the fall with heavy rains, and the plants will reappear for a brief time.
In eastern Minnesota, B. rotundifolia occurs along the margins of low sandy or silty islands in the Mississippi River. These islands form by natural riverine processes and are constantly shifting. Some of them are known to harbor relatively large populations of B. rotundifolia within two or three years of formation.
Biology / Life History
During the course of the growing season, the habitat of B. rotundifolia, whether rock pools or river islands, will likely change from aquatic to terrestrial and possibly back again. It is all dependent on patterns of rainfall. This seemingly capricious change would seem to be a problem for an aquatic species, yet B. rotundifolia adapts easily. In fact, this variability may be necessary to stimulate flowering, seed production, or seed germination.
The structure of the flowers indicates B. rotundifolia is pollinated by small flying insects. Seeds are small, and dispersal mechanisms are unknown. That said, dispersal in riverine populations is rather easy to surmise; water currents or the movement of aquatic animals seems probable. Dispersal of plants isolated in rock pools is more difficult to imagine.
Conservation / Management
It is possible that before settlers converted the prairies and prairie wetlands to agricultural production, B. rotundifolia was more common and widespread in western Minnesota than it is today. It is also possible that certain well-chosen examples of prairie wetlands that have been drained or badly damaged by cattle grazing could be restored to something closely approximating their original physical and hydrologic condition. If this were to happen, we might see B. rotundifolia re-colonize those habitats and perhaps even return from a long-buried seed bank. Such a restoration effort would benefit a number of prairie wetland plants and animals and should be a high priority for conservation organizations and land management agencies. Until that happens, however, protection of existing prairie habitats is crucial. This involves protecting them from intentional or unintentional contact with herbicides and other agricultural chemicals, overgrazing, and rock quarrying.
Management of riverine populations is a different matter. Island formation in the Mississippi River is a dynamic process and somewhat unpredictable. It is neither practical nor desirable to preserve such islands in a static condition. They appear at places and times where river currents deposit suspended sediments. As currents change, the islands begin to erode and could be entirely gone in only a few years. There are many such islands in the Mississippi, though B. rotundifolia has been found on only a few and only on a fairly confined stretch of the river. This sort of island dynamic is self-sustaining, as long as the river itself is not greatly altered by dams, dikes, or dredging.
Best Time to Search
Given the vagaries of its habitat, it is difficult to predict the best time to search for B. rotundifolia. The best time for prairie populations is probably from mid-May through June. The best time for riverine populations is when water levels are low, probably from early July through the middle of September.
Conservation Efforts in Minnesota
A number of B. rotundifolia sites are on publicly owned land, where conservation measures are possible. In addition, in 2007 the Renville and Redwood County Soil and Water Conservation Districts (SWCD), in association with state and federal agencies, coordinated an initiative to compensate interested private landowners for permanently protecting the most ecologically intact rock outcrops and associated wetlands in those two counties. A total of 86 ha (212 ac.) were enrolled into perpetual conservation easements as a result of this initiative, and B. rotundifolia is one of the rock outcrop species most likely to benefit from the program. In 2009, the program was expanded to protect an additional 214 ha (530 ac.) within a five county area of the Upper Minnesota River valley. Conservation plans that outline conservation practices, such as removal of invasive species and livestock exclusion, are being developed for each of the easement sites. Habitat maintenance and improvement measures, such as prescribed burning, fencing, rotational grazing, and/or seeding, are also included in the plans.
Welby Smith, MN DNR, 1988, 2008, and 2018
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