Rare Species Guide

  Basis for Listing

Allium cernuum (nodding wild onion) is a small woodland herb, which appears to be rare or absent in large areas within its broad transcontinental range. This is especially true in the northern portion of its range, where it is rare in Saskatchewan, Ontario, Minnesota, Wisconsin, Michigan, Iowa, and Illinois (Nekola 1990). It is apparently absent from North Dakota, much of the Great Plains, and parts of the intermountain region. Allium cernuum was unknown in Minnesota until 1982, when it was discovered in Lake Louise State Park in Mower County. In 1984, when it was listed as threatened, it was still known from only that one location (Ownbey and Morley 1991).

Subsequent plant searches discovered more than 40 additional locations in the southeast corner of the state (Eastern Broadleaf Forest Province). This is a positive outcome of listing the species. It is now known to be more common and widely distributed in Minnesota than was formerly believed, so threatened status is no longer necessary. That said, special concern status is still needed given the species’ restricted range in the state and the threats posed by timber harvest, livestock grazing, and invasive species; hence A. cernuum’s designation was changed to special concern in 2013.

  Description

Allium cernuum is a perennial herb, with pink to deep roseate or white flowers and a strong onion odor. It is sometimes confused with the common and closely related A. stellatum (prairie wild onion), but its distinguishing feature is a flowering umbel, nodding on an arched stem. In both species the umbel is nodding in bud, but in A. stellatum the stem becomes erect in anthesis (flowering), while in A. cernuum it remains nodding. Allium  cernuum is also distinguished by leaves that are thin and flat, not thick and round; flowers that are campanulate, not stellate; and a bulb that is elongate, not ovoid (Jacobsen 1979).

  Habitat

Most populations of A. cernuum in Minnesota occur on north- or northwest-facing slopes above a creek or river, though they can also be found on west- or northeast-facing slopes and shaded cliff edges. It rarely occurs on southeast-facing slopes. The substrate is often rich and moist silty and often rocky soil, often in the vicinity of exposed limestone, dolomite, or sandstone bedrock. The dominant vegetation at these sites is most often southern maple-basswood forest stands of Quercus rubra (northern red oak), Acer saccharum (sugar maple), Tilia americana (basswood), and occasionally Pinus strobus (white pine). The ground layer is typically composed of mesic forest species, such as Asarum canadense (wild ginger), Cystopteris bulbifera (bulblet fern), and Solidago flexicaulis (zigzag goldenrod). In Minnesota, A. cernuum prefers full or partial shade, rather than open areas. 

  Biology / Life History

One of the first plants to appear in the early spring, A. cernuum emerges as a dense tuft of lime-green leaves. In June, it sends up flower stalks with single inflorescences, whose buds open in late June or July. Many, but not all, plants produce inflorescences in a given year. Although a typical clump produces from 2 to 8 flowering culms, as many as 12 or more flowers may be produced from large clumps, which can contain over 36 tightly clustered bulbs. Fruits are produced from late July through early September.

  Conservation / Management

Allium cernuum appears to be rare or uncommon throughout the region, and the factors that influence its rarity are not entirely known. At the present time, the best way to preserve this rare species is to maintain and protect the forest habitat where it occurs. Special consideration needs to be given to any activity that could result in significant canopy gaps. A gap caused by the death of a single canopy tree would not cause a problem, but larger gaps would increase light and temperature on the forest floor. This would tend to dry and warm the soil, which could disrupt the competitive relationships between plant species and shift species composition. Any land uses employing heavy machinery may potentially damage the soil, especially on slopes. Livestock grazing could also degrade A. cernuum habitats and directly injure plants.

  Best Time to Search

The best time to search for A. cernuum is when it is in flower, from late June through August, and in fruit, from late July through September. Without flowers or fruits it is very difficult to distinguish A. cernuum from its close relative A. stellatum (autumn onion).

  Conservation Efforts in Minnesota

The Department of Natural Resources’ (DNR) Minnesota Biological Survey has been completed in the range of this species in the state. Several populations occur in State Parks, where they are protected from most threats. Other populations are in State Forests, and Wildlife Management Areas, where resource management plans ought to incorporate habitat protection needs for this species.

  References and Additional Information

Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.

Jacobsen, T. D. 1979. Numerical analysis of variation between Allium cernuum and Allium stellatum (Liliaceae). Taxon 28:517-523.

McNeal, D. W., Jr., and T. D. Jacobsen. 2002. Allium. Pages 224-276 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 26. Oxford University Press, New York, New York.

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, Division of Ecological Resources. 2008. Rare species guide: an online encyclopedia of Minnesota's rare native plants and animals [Web Application]. Minnesota Department of Natural Resources, St. Paul, Minnesota. Accessed 1 July 2009.

Nekola, J. C. 1990. Rare Iowa plant notes from the R. V. Drexler Herbarium. Journal of the Iowa Academy of Science 97(2):55-73.

Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis. 320 pp.