Basis for Listing

Caltha natans (floating marsh marigold) is a circumboreal aquatic species that is generally rare or local throughout its North American range. This is especially true south of the Canadian border, where it has been found at only a few sites in St. Louis County (Northern Superior Uplands Section) and Koochiching County (Minnesota and Ontario Peatlands and Northern Minnesota Drift and Lake Plains sections), Minnesota and in one county each in Wisconsin and Michigan. An additional cause for concern in Minnesota is the local extirpations recently suffered by this species. The reason for the extirpations is largely habitat loss and degradation, which is a well-documented problem for aquatic species statewide. For these reasons, Caltha natans was listed as an endangered species in Minnesota in 1996.

  Description

Caltha natans is a distinctive plant that bears only superficial resemblance to the more familiar C. palustris (common marsh marigold), which is larger and has yellow flowers. Caltha natans has 2-6 small white flowers about 1 cm (0.4 in.) across, each with 5 tepals. The leaves are 2-5 cm (1-2 in.) long and about as wide. The tip is rounded, and the base is deeply notched where the petiole is attached. The margins of the leaves are entire or with shallow and irregular indentations (crenations). The leaves and petioles commonly float in shallow water, but when water recedes, the entire plant may be stranded on mud. The stems can be as long as 50 cm (20 in.) and form roots at the nodes when they contact a soft, moist substrate. The fruit is a follicle forming a dense globose head. The seeds are very small, less than 1 mm (0.04 in.) long (Smit 1973).                                                     

  Habitat

Caltha natans occurs in shallow, slow-moving water in streams, creeks, pools, ditches, sheltered lake margins, swamps, and beaver ponds. It typically roots in mud, silt, or clay, and it spreads when stems root at the nodes. Some sites may support only a few scattered individuals, but other sites may have a dense mat consisting of many plants. In Minnesota, C. natans is associated with Glyceria spp. (manna grass), Carex spp. (sedges), Potamogeton spp. (pondweed), and Utricularia spp. (bladderwort).

  Biology / Life History

Caltha natans is a perennial herb and is insect-pollinated. The fruits are unspecialized follicles, making long-distance wind or animal dispersal unlikely (Schuettpelz et al. 2004). It is likely that short-distance dispersal is accomplished by water currents. Population maintenance is achieved by a perennial root system that is initiated by seed germination and by the stems forming adventitious roots where their nodes contact a suitable substrate.                                                                                                                                                                                                                                                                                                         

  Conservation / Management

The natural habitats of C. natans in Minnesota are typically associated with low-gradient riverine systems. Seasonal flooding accompanied by certain amounts of sedimentation and scouring are normal events in these habitats. Beaver activity can sometimes have an even greater influence on habitats than extreme weather events. Caltha natans seems to have many adaptations to cope with such a dynamic system, and yet the species is extremely rare in Minnesota even when habitat conditions seem ideal. There are obviously unknown factors that influence the establishment and survival of populations in Minnesota. This prevents specific management recommendations from being proposed. However, there are some general concerns that can be identified. Non-native invasive plant species such as Lythrum salicaria (purple loosestrife), Phalaris arundinacea (reed canary grass), and Typha angustifolia (narrow-leaved cattail) could be severe competitors in C. natans habitat. They should be controlled, though application of herbicides should be avoided if possible. Buffers of native vegetation should always be maintained between C. natans habitat and any activity that would expose bare soil. In some cases, the aquatic habitats where C. natans is found are open to use by motorized water craft and wake control may be needed. Public water access and private ownership of lakeshore property combine to make conservation management more difficult (United States Forest Service 2000). Without collective stewardship by various dispersed interests, the cumulative impacts of land use and lake level changes could negatively impact C. natans.

  Best Time to Search

The best time to search for Caltha natans is from May into September, when flowers or leaves are present.

  Conservation Efforts in Minnesota

The Minnesota Biological Survey is in progress in northern Minnesota, and C. natans is a targeted species.

  Authors/Revisions

Welby R. Smith (MNDNR), 2020

(Note: all content ©MNDNR)

  References and Additional Information

Ford, B. A. 1997. Caltha. Pages 187-189 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 3. Oxford University Press, New York, New York.

Lakela, O. 1943. Rediscovery of Caltha natans in Minnesota. Rhodora 45:53-55.

Schuettpelz, E., and S. B. Hoot. 2004. Phylogeny and biogeography of Caltha (Ranunculaceae) based on chloroplast and nuclear DNA sequences. American Journal of Botany 91(2):247-253.

Smit, P. G. 1973. A revision of Caltha (Ranunculaceae). Blumea 21(1):121-150.

United States Forest Service. 2000. Population viability assessment in forest plan revision. Questions for plant population viability assessment panel: Caltha natans. United States Forest Service, Region 9, Duluth, Minnesota.

Walton, G. B. 1994. Report for field season 1994 status survey for Caltha natans and Sparganium glomeratum in Minnesota. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 9 pp.