Ruellia humilis Nutt.
Basis for Listing
Ruellia humilis (wild petunia) was thought to be extirpated from Minnesota until 2005, when it was rediscovered at Afton State Park in Washington County (St. Paul-Baldwin Plains and Moraines Subsection). Even after extensive field surveys, no other site has been found. The population is small and occurs in a remnant oak savanna. The habitat is being invaded by non-native plant species and needs active management if the population of R. humilis is to remain viable.
Because the only known population of R. humilis is located in an area that was formerly a pasture, there is some question as to how it became established. Due to this uncertainty, R. humilis was listed as a species of special concern in 2013, instead of the endangered or threatened listing its rarity would warrant. A more protected status may be designated in the future, if thorough searches within Afton State Park result in the discovery of additional populations.
The stem of R. humilis is multi-branched, and while it may reach a height of 60 cm (2 ft.), it is usually less than 30 cm (1 ft.). The short internodes give the plant a compact, leafy and bushy appearance. The leaves are sessile or subsessile, 3-8 cm (1-2 in.) long, with an acute or obtuse tip. The flowers are crowded in the axils of the upper leaves and are subtended by narrow bracts. Each flower is 3-7 cm (1-3 in.) long, tubular, 5-lobed, and lavender to purple in color.
Although the overall appearance of R. humilis resembles cultivated petunias, they are members of different plant families. There is no wild species in Minnesota that closely resembles R. humilis
Biology / Life History
Ruellia humilis is an insect-pollinated perennial. It spreads only by seeds, which are apparently dispersed by animals. It is known to be very drought resistant and fire-adapted. It produces several flowers, which open in succession, each lasting only one day.
Long-tongued bees (including bumblebees) are the most important pollinators of the flowers. Short-tongued bees and Syrphid flies also visit the flowers, but they are not thought to be effective pollinators
Intact and mature seeds of R. humilis require a cool overwintering season to break dormancy (Baskin and Baskin 1970), and it is likely that not all seeds germinate the first year after they are produced. One study, in an Illinois prairie, found viable seeds of R. humilis to be present in the long-term seed bank (Johnson and Anderson 1986).
The above ground parts of R. humilis are presumably adapted to grazing by native herbivores, though grazing by domestic cattle has a deleterious effect. One grazing study in Kansas (Hickman and Hartnett 2002) reported that shoots of R. humilis were taller in the ungrazed and low grazing treatments than in the moderate and high grazing treatments. Total shoot biomass, reproductive biomass, and percentage of reproductive stems of Ruellia decreased as grazing intensity increased.
Conservation / Management
At last report, the population of R. humilis at Afton State Park was well-established and could persist if its habitat is restored or at least maintained. Unfortunately, its habitat is being invaded by aggressive non-native plant species that threaten to overwhelm the population. Intensive management is urgently needed to reverse this trend. The first step is to prepare a management plan that would establish the long-term survival of Ruellia humilis as its goal.
The management plan should investigate the practicality and efficacy of dormant season prescribed burns, which could help control the invasive species. Grazing by domestic livestock as a management prescription should be avoided, as should herbicide use.
Best Time to Search
If a searcher is prepared, R. humilis can be located and correctly identified anytime during the growing season; however, the best time to search is during the months of June and July.
Welby Smith (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
Baskin, J. M., and C. C. Baskin. 1970. Replacement of chilling requirement in seeds of Ruellia humilis by gibberellic acid. Planta (Berl.) 94:250-252.
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Fernald, M. L. 1945. Ruellia in the eastern United States. Rhodora 153, 47-63, 69-91.
Hickman, K. R., and D. C. Hartnett. 2002. Effects of grazing intensity on growth, reproduction, and abundance of three palatable forbs in Kansas tallgrass prairie. Plant Ecology 159:23-33.
Johnson, R. G., and R. C. Anderson. 1986. The seed bank of a tallgrass prairie in Illinois. The American Midland Naturalist 115:123-130.
Michigan Natural Features Inventory. 2007. Rare Species Explorer [web application]. Michigan Natural Features Inventory, Lansing, Michigan.
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.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis. 320 pp.