Draba cana Rydb.
Hoary Whitlow Grass
Basis for Listing
Draba cana (hoary whitlow grass) is an arctic disjunct not known to exist in Minnesota until it was discovered here in 2001. As of 2015, there were three records, all from crevices and shelves on cliff faces adjacent to water bodies in the boreal forest region (Border Lakes and North Shore Highlands subsections). This habitat preference essentially limits the potential range of the species in Minnesota to the northeast corner of the state. It seems unlikely that the species has been systematically overlooked or that many additional populations will be found. This leads to the conclusion that D. cana is very rare in Minnesota; hence, it was designated endangered in 2013.
The species is also listed as endangered in Wisconsin (Wisconsin Department of Natural Resources 2013) and threatened in Michigan (Reznicek et al. 2011).
Draba cana is a perennial that grows to a height of 10-30 cm (4-12 in.). The stems are covered with simple and branched hairs. The basal leaves have a short petiole and a narrow blade about 2 cm (0.8 in.) long and are covered with hairs; they form a rosette at the base of the plant. There are also 3-10 leaves scattered along the stem, which are similar but often smaller than the basal leaves. There are 15-47 flowers arranged in a compact raceme at the top of the stem, which may elongate in fruit. The flowers have four small green sepals and four slightly larger white petals. The fruits are flat, slender, hairy, and about 1 cm (0.4 in.) long. The fruits are held close to the stem and point straight upwards; they may be flat or slightly twisted (Al-Shehbaz et al. 2010). The most distinctive feature of D. cana is the dense covering of hairs on all parts of the plant, which gives it an ashy gray or blue-green color.
Based on available evidence, the habitat of D. cana in Minnesota is rock crevices and small ledges on cliffs. One cliff site is north-northwest-facing, another south-facing, and the last south-southeast-facing.
The crevices and ledges where the plants are rooted may be fairly dry, not wet or mossy. They may have small amounts of loose and gravelly “soil” or just rock chips. Some habitats, or portions of habitats, are in shade for portions of the day, others are in direct or filtered sunlight nearly all day.
Habitats in Wisconsin are reported to be exposed dolomite cliffs (Wisconsin Department of Natural Resources 2013). Habitats in Michigan are reported to be limestone cliffs (Reznicek et al. 2011). In fact, the species is often reported to occur on basic rock, rather than acidic rock. The general bedrock types in the region of Minnesota where D. cana has been found are likely to be weakly alkaline or pH neutral. Basic materials such as calcite or limestone do occur in the region but are quite localized. It is unknown whether the rarity of D. cana in Minnesota has anything to do with specificity for a certain type of bedrock substrate.
Biology / Life History
Draba cana is a perennial insect-pollinated plant species. Apparently, it reproduces only by seed; it does not grow from fragments, suckers, or off-shoots. The seeds have no specialized structures that might indicate how they are dispersed. It appears they are released from the pod while still on the plant and then simply respond to gravity. It seems likely that certain small mammals and birds gather or eat the seeds, which may result in the seeds being dispersed within the habitat or between habitats.
Conservation / Management
The expansion of mineral mining in northeastern Minnesota could impact rare plants that occur exclusively on bedrock habitat.
Recreational rock climbing could directly threaten this species. Although this particular threat seems remote at this time, it does need to be considered.
Best Time to Search
Draba cana can be easily identified when in flower or fruit, from early June through the middle of August. In an especially dry year, plants can be expected to senesce earlier than in a moist or normal year.
Welby Smith (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
Al-Shehbaz, I. A., M. D. Windham, and R. Elven. 2010. Draba. Pages 269-347 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico Volume 7. Oxford University Press, New York, New York.
Michigan Natural Features Inventory. 2007. Rare Species Explorer [web application]. Michigan Natural Features Inventory, Lansing, Michigan.
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.
Reznicek, A. A., E. G. Voss, and B. S. Walters. 2011. Michigan Flora Online. University of Michigan. <http://www.michiganflora.net/species.aspx?id=642>. Accessed March 7, 2013.
Voss, E. G. 1985. Michigan Flora. Part II: Dicots (Saururaceae-Cornaceae). Cranbrook Institute of Science Bulletin 59 and the University of Michigan Herbarium. University of Michigan, Ann Arbor, Michigan. 727 pp.
Wisconsin Department of Natural Resources. 2013. Wisconsin's rare plants. <http://dnr.wi.gov/topic/endangeredresources/plants.asp?mode=detail&speccode=pdbra112q0>. Accessed 7 March 2013.