Basis for Listing

There are relatively few confirmed herbarium records of Bidens discoidea (discoid beggarticks) from Minnesota. Most are from lakes and marshes in the northeastern part of the state (Laurentian Mixed Forest Province). There are also two records from the Mississippi River in the southeastern corner of the state (Eastern Broadleaf Forest Province), but they date back to 1926 and have never been relocated. Although a literal interpretation of this herbarium record indicates that B. discoidea is very rare and declining in numbers, it is also possible that the species has been overlooked due to its superficial resemblance to two common species of Bidens (beggarticks). This is most likely true in Mississippi River habitats south of the Twin Cities, where river species have been poorly surveyed and documented. It is also notoriously difficult to assess trends in plant populations over time, especially when the species occurs in poorly defined and widespread habitats as is the case here.  Given the small number of occurrences, and until a targeted botanical survey has been completed, it was deemed necessary to list Bidens discoidea as a species of special concern in 2013.

  Description

The stems of B. discoidea arise singly to a height of about 1.5 m (60 in.). They may have multiple branches and may root at the lower nodes. The leaves are opposite on the stem, have a distinct petiole, and 3 leaflets. Each leaflet is lanceolate in shape, has serrated margins, and an acuminate tip. The leaf surface is hairless or has a few antrorse hairs on the veins below; the leaves are up to 10 cm (4 in.) long and 3 cm (1 in.) broad. The inflorescence is a loose cyme, with several flower heads; the heads lack ray flowers, so they appear to be without petals. The disk flowers are relatively few, producing a disk only 5-6 mm (0.20-0.24 in.) across. In Minnesota, B. discoidea is most likely to be confused with B. frondosa (leafy beggarticks) and B. vulgata (common beggarticks), except the flower heads of B. discoidea have fewer bracts (3-5), and the bracts lack cilia on the margins.

  Habitat

In Minnesota, B. discoidea seems to occur in a wide range of wetlands, such as marshes, wet meadow/carr, pond margins, and riverine sloughs. However, due to its scarcity, there are thought to be specific habitat requirements that are yet unknown. Seasonal fluctuations in water levels may be an important characteristic of suitable habitat, while strong currents or wave action could be counter indicative. Microsites where this species has been seen (growing) include on mossy and partly submerged logs or stumps. Beaver-created habitats may be important to this species, though there are currently too few records to know for sure.

  Biology / Life History

The flowers of B. discoidea attract flying insects (nectaries are clearly visible in the flowers). And yet, flowers in the lateral heads are typically cleistogamous, meaning that they are always self-pollinated. The central heads do have chasmogamous flowers, which are available to pollinating insects. However, even the chasmogamous flowers are often self-pollinated, which may be the cause of reportedly low levels of genetic variability within populations (Roberts 1983).

The fruits (achenes) of B. discoidea have barbed bristles, which are thought to catch in the fur or feathers of animals, thereby effecting dispersion. However, because of structural characteristics, the fruit of B. discoidea may depend more on water for dispersal. In fact, the seeds of B. discoidea can float on the surface of water for more than 6 months and still remain viable (Roberts 1983). The seeds typically germinate on woody debris and sediments left at the edge of lakes, streams, and ponds when water recedes. This may be more than coincidence. It is possible, or even likely, that B. discoidea depends on the annual cycle of rising and receding water to maintain populations.

  Conservation / Management

This species bears more than a superficial resemblance to two common species of Bidens, so it is possible that non-discriminating floristic surveys have overlooked it in the past. This is most likely true in Mississippi River habitats, south of the Twin Cities. There are recent records of B. discoidea from the Mississippi River in Wisconsin, Iowa, and Illinois, however, none from Minnesota since 1926. There is no obvious reason that a thorough plant survey of the river would not turn up new records of B. discoidea for Minnesota. This is a pattern seen with other river species and should provide an impetus to conduct a serious and highly directed plant survey of the Minnesota portion of the Mississippi River. Until such a survey has been completed, it will be difficult to make meaningful management or conservation recommendations.

  Best Time to Search

When planning searches, keep in mind that B. discoidea is a late season plant, which may not even appear before August. It may also be useful to know that plants in fruit will look about the same as plants in flower, so searches can be conducted any time from about August 15 until September 20.

  Authors/Revisions

Welby R. Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Crow, G. E., and C. B. Hellquist. 2000. Aquatic and wetland plants of northeastern North America. Volume 1. Pteridophytes, Gymnosperms, and Angiosperms: Dicotyledons. University of Wisconsin Press, Madison. 448 pp.

Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 352 pp.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.

Roberts, M. L. 1983. Allozyme variation in Bidens discoidea (Compositae). Brittonia 35(3):239-247.

Strother, J. L., and R. R. Weedon. 2006. Bidens. Pages 205-218 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 21. Oxford Univeristy Press, New York, New York.

United States Forest Service. 2008. Superior National Forest Rare Plant Guide [web application]. United States Forest Service, Duluth, Minnesota. <http://www.fs.fed.us/r9/forests/superior/documents/SwampBeggar.pdf>. Accessed 10 July 2009.