Hybanthus concolor (T.F. Forst.) Spreng.
Basis for Listing
Despite botanical surveys that targeted rare species throughout Minnesota, only two populations of Hybanthus concolor (eastern green-violet) have been found: one in Houston County and one in Fillmore County (both in the Eastern Broadleaf Forest Province). These two populations mark the extreme northwestern edge of the species’ range. Hybanthus concolor is also reported to be rare or threatened in adjacent Wisconsin and Iowa. Its rarity is likely due, in part, to being at the edge of its range. It is also likely that loss of habitat due to the clearing of bottomland forests for cropland, pasture, and road building have resulted in the loss of populations.
The two known Minnesota populations total fewer than 100 individuals and are considered vulnerable to a number of potential threats, including increasingly frequent flood events likely associated with climate change. For these reasons, H. concolor was listed as endangered in Minnesota in 2013.
The stems of Hybanthus concolor can reach 1 m (3 ft.) in height. They can occur singly or in clumps of 12 or more that arise from a fibrous root system. The leaves are alternate on the stem, broadly elliptic to ovate-oblong in outline, and have an abruptly pointed tip and a tapering base; the leaf surfaces are ± hairy. The flowers are greenish white, 4-5 mm (0.16-0.20 in.) in length, 1-3 occur in each leaf axil. The peduncles are recurved and jointed beyond the middle. The fruit is oblong-ellipsoid in shape and 1.5-2.0 cm (0.6-0.8 in.) long. In spite of the common name, this is a relatively large and leafy plant and does not closely resemble any of the true violets found in Minnesota.
The known habitats of H. concolor in Minnesota are mature mesic forests in deep stream valleys. The forests have a closed canopy consisting mostly of large Acer saccharum (sugar maple) and Tilia americana (basswood). Soils are silty loams.
Biology / Life History
Although H. concolor is related to true violets, it resembles them only in floral structure, particularly the stamens, pistils, and seedpods. The lowest petal has at least a suggestion of a spur at its base, though it is less well developed than that of the true violets. Nectar, which attracts pollinating insects, accumulates in the spur. Later in the season, cleistogamous flowers are produced. These minute flowers do not open, nor do they produce nectar. The pollen is deposited directly upon the stigma as the anthers open. Seed production is thereby assured, even if weather conditions prevent insects from visiting the spring flowers.
Conservation / Management
The forest habitat where H. concolor occurs is very sensitive to disturbance, especially any activity that would reduce the tree canopy and allow additional sunlight to reach the forest floor. Additional light causes the soil to become drier and warmer, which shifts the competitive balance towards sun loving plant species (H. concolor is a shade-loving species). The soil in which H. concolor grows (and the plant itself) is also vulnerable to direct disturbance, such as the damage caused by heavy machinery and motorized vehicles. The soil is too soft and the plants are too shallowly rooted to withstand such an event.
The position of the populations on the valley floor could make them vulnerable to flooding. This necessitates the need to protect the vegetation in the immediate watershed, including the level uplands adjacent to the steep walls of the valley.
Garlic mustard (Alliaria petiolata) has recently been found at one of the sites. This is a non-native invasive species that could, in a short time, take over a dominant position in the plant community and threaten the survival of H. concolor.
Best Time to Search
Hybanthus concolor is easily visible throughout the growing season. Flowers are normally present from early May through early June. Fruits should be present from early June through early October.
Conservation Efforts in Minnesota
Both known Minnesota populations of H. concolor occur on state forest land. The Winona County site is designated an old-growth forest, which should afford some protection from logging and road building. The Fillmore County site is in a high conservation value forest, which has a management plan that does address rare plant species.
Welby Smith (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
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.
Minnesota Department of Natural Resources. 2006. Tomorrow's habitat for the wild and rare: An action plan for Minnesota wildlife, comprehensive wildlife conservation strategy. Division of Ecological Services, Minnesota Department of Natural Resources. 297 pp. + appendices.
NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.
NatureServe. 2012. NatureServe Explorer: an online encyclopedia of life [web application]. Version 7.1. <http://www.natureserve.org/explorer>. Accessed 13 March 2012.
Reznicek, A. A., E. G. Voss, and B. S. Walters. 2011. Michigan Flora Online. University of Michigan. <http://www.michiganflora.net/species.aspx?id=2775>. Accessed 13 March 2013.