Orobanche uniflora L.
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Basis for Listing
Although Orobanche uniflora (one-flowered broomrape) has been found in every continental state in the United States and in many Canadian provinces, it is considered rare or vulnerable in 17 states and five provinces (NatureServe 2015). Its U.S. distribution is interrupted in much of the central Great Plains grasslands, probably because it is more often found in wooded areas across its range.
Orobanche uniflora is very rare in Minnesota. Little more than that was known when it was listed as a species of special concern in 1984. Only eight populations were known at that time and only half of those were expected to be extant. Furthermore, all of the populations were widely separated and quite small. At issue were the woodland and bluff prairie habitats that were becoming exceedingly rare in southeastern Minnesota (Eastern Broadleaf Forest Province), so habitat availability is likely a limiting factor for the species. In addition, the parasitic nature of the plant likely restricts it to habitats where host plants have large and healthy populations. In a fragmented landscape, it may be very difficult for O. uniflora to find such habitats to colonize. These factors, as well as the outcome of subsequent field searches conducted by the Minnesota Biological Survey, which found only eleven additional populations, resulted in the status of O. uniflora being elevated to threatened in 2013.
There are three species of Orobanche in Minnesota. All are herbaceous annuals that lack leaves and green pigmentation (chlorophyll), and the stems of the plants are primarily subterranean. In two of the species, O. fasciculata (clustered broomrape) and O. uniflora, all that is seen aboveground are the flower stalks (pedicels), with a single flower at the end of each. It looks as if each flower is coming out of the ground individually.
In the case of O. ludoviciana (Louisiana broomrape), all the flowers appear to be attached to an upright above-ground stem, and they are packed tightly together to form a dense spike. This species looks quite different from the others and should be easy to identify.
In order to distinguish O. fasciculata from O. uniflora it may be necessary to gently remove some soil from the base of the plant. In both cases you will see the top of the underground stem, or at least the top of one branch of the stem. It will be vertical, and all the flowers will appear to come from there. At the point where each flower stalk is attached to the stem is a firm bract, which will look like a small stiff leaf.
In the case of O. uniflora, the bracts are hairless, there will be only 1-3 flowers from each branch of the stem, and the flowers will be violet-tinged. In the case of O. fasciculata, the bracts are hairy, there will likely be 3-10 flowers from each branch, and the flowers will be pinkish to creamy-white.
Orobanche uniflora has been found in a variety of habitats, soils, and light conditions but, unlike other members of this genus, it appears to be primarily a forest species. Range wide the species is found in woods, thickets, bluffs, and streambanks. In Minnesota, it is associated with woodland and bluff prairie habitats. Slope aspect may be north- or south-facing. Soils range from coarse and sandy gravel to silt-loam. The plant has been found in full sun and in deep shade. In woodland settings, canopy trees and shrubs may be Acer saccharum (sugar maple), Tilia americana (basswood), Quercus spp. (oaks), or Populus spp. (poplar aspen), with Salix spp. (willows) or other shrubs and conditions may be moist to dry-mesic (mesic hardwood forest). In prairie settings, associates are typical prairie grasses and forbs. Since O. uniflora is a non-photosynthetic parasitic species, the plant community that occupies its habitat may not matter. The presence or absence of its preferred host species may be more important.
Biology / Life History
Orobanche uniflora is a vascular plant that lacks true leaves and green pigmentation (chlorophyll) and is incapable of photosynthesis. Instead, it is an obligate parasite on the roots of other plants in order to obtain the carbohydrates needed to sustain it. While some Orobanche species have a specific host plant species or genus, O. uniflora is non-specific and seems to use a variety of host plants including, but not limited to, Saxifraga spp. (saxifrages), Helianthus spp. (sunflowers), and Solidago spp. (goldenrods).
Conservation / Management
Since its habitats are variable, and it seems to use a variety of host plant species, it is somewhat difficult to know how to manage for O. uniflora. We can, however, make some assumptions regarding habitat loss, degradation, and fragmentation. Given the extremely small number of populations known to occur in Minnesota, it will be imperative to protect all remaining populations and habitat from development and disturbance if this species is to survive in the state. Management of habitat may include prescribed burning where appropriate to habitat type, such as in bluff prairies and dry woodlands. Periodic dormant-season burns help to control woody encroachment. It may also be necessary to control ecologically invasive species, particularly such aggressive invaders as Rhamnus cathartica (common buckthorn) and Alliaria petiolata (garlic mustard). Also, motorized vehicles and domestic livestock should be excluded from these habitats.
Where O. uniflora occurs in prairie habitats, there are several programs and resources available to land managers and landowners to help protect and manage remaining prairie parcels, including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.
Best Time to Search
The best time to search for O. uniflora is from the end of May to the end of June. All recorded observations of the species in Minnesota have been found within this time frame.
Conservation Efforts in Minnesota
Several of the habitats where O. uniflora occurs are on public lands that are being managed for conservation purposes.
Welby Smith, MN DNR, 1988, 2010, and 2018
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