Parthenium integrifolium L.
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Basis for Listing
This long-lived prairie species reaches Minnesota at the extreme northwestern edge of its range. It was apparently limited to only a small portion of southeastern Minnesota before European settlement, but it was abundant where it occurred. Parthenium integrifolium suffered a dramatic decline when the prairies were converted to crop production. In fact, there are fewer than 200 ha (494 ac.) of suitable habitat remaining today, compared with the original habitat of more than 200,000 ha (494,200 ac.). Other prairie states have reported a similar decline, but the species appears to be secure in the East, where it has adapted to marginal habitats that are not heavily exploited for agricultural purposes.
Parthenium integrifolium is a tall, robust plant that is quite conspicuous when in full flower. The numerous small heads form a distinctive flat-topped floral structure, with densely crowded, whitish, wooly flowers. In a vegetative state the species can generally be identified by the basal leaves, which are thick, rough, toothed, and long-stalked. The stem leaves are also toothed, and they are alternate on the stem, becoming increasingly smaller with shorter petioles (stalks) near the summit, eventually becoming sessile (losing the petiole altogether).
In Minnesota, P. integrifolium is restricted to mesic habitats in remnant prairies and savannas of the type that developed in the southeastern portion of the state. It is often found in association with other declining prairie species such as Arnoglossum plantaginea (tuberous Indian-plantain), Asclepias sullivantii (sullivant's milkweed), and Silphium laciniatum (compass plant).
Biology / Life History
Parthenium integrifolium is a long-lived, herbaceous perennial. Flowers are insect-pollinated and seeds are dispersed short distances by the wind, and possibly in the fur of mammals. There is considerable size variation in this species, which is likely a result of genetic differences (Rollins 1950).
Conservation / Management
Parthenium integrifolium seems to be intolerant of herbicides, cattle grazing, and repeated haying. Cattle grazing must be carefully managed to prevent over-grazing, which can result in a loss of native biological diversity and the extirpation of certain sensitive species. It can also create conditions for invasion by non-native species that displace native species and negatively affect ecosystem processes. Prescribed fire can be a useful tool to control invading brush and reduce thatch, but the timing is critical. The small patches of prairie that remain can be safely burned only during the dormant season, usually in early spring before the native prairie species appear above ground. Annual hay mowing presents another potential challenge. Seeds of P. integrifolium, like those of most prairie plants, mature in late summer and autumn. If the prairie is mowed before the seeds mature, the species is prevented from producing and dispersing seeds, which can eventually lead to a population collapse.
Conservation Efforts in Minnesota
A legislatively funded survey of many railroad rights-of-way was conducted under the direction of the Minnesota Biological Survey in 1997-1998. As a result of this survey, the Minnesota DNR acquired two prairie strips on abandoned railroad rights-of-way that support populations of P. integrifolium. The prairies have been designated as Scientific and Natural Areas and are being managed for the perpetuation of rare native plants and natural features.
Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas. 1,402 pp.
Rollins, R. C. 1950. The Guayule Rubber Plant and its relatives. Contribution from the Gray Herbarium 172:3-73.
Voss, E. G. 1996. Michigan Flora. Part III: Dicots (Pyrolaceae-Compositae). Cranbrook Institute of Science Bulletin 61 and University of Michigan Herbarium, Ann Arbor, Michigan. 622 pp.