Glade mallow (Napaea dioica)

Right now is a favorable time for assessing populations of glade mallow (Napaea dioica), our only midwestern endemic. I had a post a few years ago about the tragedy that happened with the large roadside population along U.S. 14. Although about 70% of this population was lost, the remaining stand that Kathie was able to save (mostly along the railroad near Kahl Road) is doing quite well this year.

In addition to Napaea's interest as an endemic, it is monotypic (the only species in its genus), dioecious (the only species in its family with separate male and female plants), and lacking any close relatives. Although uncommon, it is a robust plant, quite easy to cultivate in a garden. (We have a couple of volunteers that are doing quite well in the small garden in front of our house.) The confusion about its taxonomy that had existed since Linnaeus' time was straightened out by UW-Madison botanist Hugh Iltis in the 1960s (American Midland Naturalist, Vol. 70, pp. 90-109).

The population along U.S. 14 was first described by Norman Fassett in 1942 (Torreya Vol. 42, pp. 179-181). The photo below shows a typical stand. Although not seen in the photo, Black Earth Creek is on the other side of the shrub layer.

Robust population of Napaea dioica growing between U.S. 14 and the railroad.

 In addition to the large Kahl Road population, there are several large populations growing along the Creek west of the Village of Black Earth. I have also seen small populations along East Blue Mounds Creek, a tributary of Black Earth Creek. The population near Mount Horeb that Iltis studied in 1963 has probably been wiped out by new road construction.

Most sites where Napaea has been found are along stream banks and floodplains of small to medium-sized streams. Iltis considered that its sun-loving nature indicated that it was a wet prairie species.

When we first started restoration at Pleasant Valley Conservancy we had a small population growing along our marsh edge, from which we collected seeds. When we planted the Crane Prairie in 2005 we made a special effort to include Napaea in our seed mix. It has done well there, being found mostly in the wet-mesic part of the Prairie. It has also spread on its own to other areas along the marsh edge. The photo below shows a typical stand.

Napaea dioica in the Crane Prairie

It is not clear why this species is endemic. Although it is not quite as restricted as Iltis believed, according to the USDA database it is found only in DC, IA, IL, IN. MN, OH, PA, VA, VT, and WI. In most of these states there are only a few collections. The largest collections have been from Illinois and Wisconsin. Although it is not on the state list in Wisconsin, it is listed as Threatened in Minnesota.

Seeds are available from several suppliers for those who would like to try it in a garden or wet prairie planting.