Basis for Listing

Plagiobothrys (popcorn flowers) is a large genus, with nearly 75 accepted taxa in North America. Nearly all are endemic to the western states. Only one taxon is found in Minnesota, and it occurs only in the southwestern corner of the state (North Central Glaciated Plains Section). Although listed in Minnesota statute as Plagiobothrys scouleri the correct name for this taxon appears to be Plagiobothrys scouleri var. penicillatus (Scouler’s popcornflower). However, its correct name and taxonomic rank are currently unresolved, which makes it difficult to determine its status in neighboring states and provinces.

Currently, P. scouleri var. penicillatus is known from only ten locations in three counties. All three counties have been the subject of general botanical surveys, and yet no new locations have been discovered for several decades. Population trends are unknown and without highly directed surveys will likely remain so.  All of the populations are in areas where human land use practices are intensifying and competing with conservation practices. Rock quarrying, cattle grazing, and herbicide application are the most prominent threats to the species and its habitat. Mining, in particular, has been a growing issue over the past decade, fueled by federal highway construction standards that now require crushed bedrock instead of gravel.  Another recent threat, evidenced in Rock County, is the conversion of rocky pastures to cornfields by excavating the bedrock. For all of these reasons, P. scouleri var. penicillatus was designated a special concern species in 2013.

  Description

Plagiobothrys scouleri var. penicillatus is a small plant, usually about ankle high (10-15 cm [4-6 in.]), with numerous wide-spreading branches arising from near its base. The branches have stiff and sharp hairs pressed close against the branch. The leaves are linear-oblanceolate, 2-4 cm (0.8-1.6 in.) long, and 1.5-3 mm (0.06-0.12 in.) wide. They are covered with short and stiff hairs, much like those on the branches. The flowers are very small, barely visible, only 2-4 mm (0.08-0.16 in.) long and white. The flowers each have a bract at their base and are loosely arranged in simple racemes, which are coiled at the top. The 4 nutlets are 1.5-2.3 mm (0.06-0.09 in.) long, lanceolate in shape, with a wrinkled surface.

  Habitat

All the Minnesota records of P. scouleri var. penicillatus are from shallow pools that form on rock outcrops. These are rather large and nearly level outcrops of quartzite set in a prairie landscape. The pools are only a few inches deep and develop in the spring from rain water or possibly snow melt. By the middle of the summer, water in the pools has usually evaporated, leaving a thin layer of moist sediments. These sediments are the medium in which P. scouleri var. penicillatus roots.

  Biology / Life History

Because P. scouleri var. penicillatus is an annual, the only part of the plant that survives the winter is the seeds. The seeds likely pass the winter in the loose and shallow sediments that accumulate at the bottom of the vernal pools. When the pools rehydrate the following spring, the seeds apparently germinate, though the details are unknown. Rainfall patterns affect the size and duration of the pools, which likely affects the seed output of P. scouleri var. penicillatus. Empirical evidence suggests that populations survive droughts and floods, which is evidence for a seed bank that can survive successive years.

While the structure of the flowers indicates that pollination is accomplished by flying insects, the flowers are very small (only 2-4 mm (0.08-0.16 in.) across), too small for most species of bees or Lepidoptera.

  Conservation / Management

The fact that P. scouleri var. penicillatus is an annual species may limit management options. The most important aspect of the species’ habitat is likely the seasonal development of vernal pools, which is highly variable and beyond the ability to ameliorate. Overgrazing by livestock and broadleaf herbicide applications in pastures are both known to harm the native flora in the type of pools where Plagiobothrys scouleri var. penicillatus occurs. The physical disturbance and “fertilization” caused by livestock tips the competitive balance to more aggressive non-native species.

  Best Time to Search

Although the best time to search for P. scouleri var. penicillatus may be influenced by the pattern of spring and summer rainfall, in a normal year, the best time is probably from about the middle of June to the middle of July.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Harris, F. 2009. Rock pools on the prairie: ephemeral pools on the prairie form amazing mini-ecosystems. Minnesota Conservation Volunteer 72(423):30-39.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the prairie parkland and tallgrass aspen parklands provinces. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 362 pp.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.