Hill’s thistle (Cirsium hillii) COSEWIC assessment and status report: chapter 3

Species information

Name and classification

Scientific name:

Cirsium hillii (Canby) Fernald

Synonymy (Kartesz 1994):

Cirsium pumilum spp. hillii (Canby) Moore & Frankton,
Cirsium pumilum var. hillii (Canby) Boivin

Common name:

Hill’s Thistle, Hill's Pasture Thistle, Prairie Thistle, Hollow-rooted Thistle

Family name:

Asteraceae (aster family)

Major plant group:

Dicot flowering plant

Specimens collected in 1890 along the dunes of Lake Michigan at Pine Station (now probably within the city of Gary) Lake County, Indiana, were studied extensively by Reverend Ellsworth J. Hill, who concluded that they were distinct from the eastern Cirsium pumilum s.str. (Moore & Frankton 1966). On the basis of Hill's collections and notes, the Lake Michigan plants were described in 1891 by William M. Canby as Cnicus hillii. The species was then revised to Carduus hillii in 1894 by Porter, and finally to Cirsium hillii in 1908 by Fernald. Subsequently, the status of the two entities, Cirsium pumilum of the north-eastern United States, and C. hillii of the north central US, was in dispute for some time. In 1917 Petrak refused to recognize C. hillii as distinct and treated both eastern and interior plants under C. odoratum (Barton) Petrak, and in 1930 Peattie also considered them as conspecific in his Flora of the Indiana Dunes. Several other authors expressed doubts with respect to treating the two as separate species (Moore & Frankton 1966). In 1966 Moore and Frankton concluded, after analyses of 350 herbaria specimens, that two subspecies should be recognized: C. pumilum (Nutt.) Spreng. ssp. pumilum and ssp. hillii (Canby) Moore & Frankton. This conclusion was based on the lack of sharp definition in range or characters between the populations, the absence of chromosome number difference, and the strong probability that no sterility barrier existed (Moore & Frankton 1966). In 1972 Dr. Bernard Boivin relegated the entity to varietal status, publishing it as Cirsium pumilum (Nutt.) Sprengel var. hillii (Canby) Boivin. A white-flowered form is recognized, and was named forma candidum by Boivin from a collection made in 1874, prior to the species even being described, but was subsequently referred to as f. albiflorum (Scoggan) E.G. Voss, on the basis of a 1952 collection by Voss from Michigan.

Although generally accepted as Cirsium hillii today, Penskar (2002) states that, "the taxonomic standing of Cirsium hillii remains of some question, and may require further investigation to clarify if this species is truly distinct from Cirsium pumilum. Cusick (1995) in fact believes that even according C. hillii varietal or subspecific status may be dubious, and states that, "It might be more practical to subsume C. hillii under C. pumilum as a single entity".

Cirsium hillii and Cirsium pumilum can be distinguished on the basis of several characters. C. hillii is a polycarpic perennial species (The Nature Conservancy 1990) while C. pumilum is a biennial. The leaves of C. hillii are less deeply lobed and the marginal spines finer and shorter, and the spines of the outer phyllaries are shorter and narrower than those of C. pumilum. Plants of C. hillii are shorter than those of C. pumilum, and less often branched. The achenes of C. hillii are usually larger than those of C. pumilum, although exceptions have been noted (Moore & Frankton 1966).

All Canadian collections are referrable to C. hillii, with C. pumilum having never been discovered in Canada.

Type Specimen: Collected by E.J. Hill, labelled "Sandy ground, Lake Co., Indiana, July 9, 1890" and stamped "College of Pharmacy Herbarium", now resides at The New York Botanical Garden.

Description

The description for Cirsium hillii, adapted from Higman & Penskar (1999) and Cusick (1995) is as follows: A generally short (25 to 60 cm tall) perennial thistle with a deep, hollowed, clustered root system with tuberous swellings. The leafy stems are soft, ridged, and either sparsely hairy or with woolly hairs; with 1 or 2 short branches near the top terminating in one to several large heads (3.5 to 5 cm) with deep pink-purple, occasionally white, flowers. The leaves are elliptic-oblong and form a basal rosette with only a few progressively smaller leaves on the stem. The leaf margins are typically undulating to very shallowly lobed and can sometimes exhibit slightly woolly hairy tendencies on the underside of the leaf, but is often smooth on both surfaces.

Moore and Frankton (1966 & 1974) provide detailed technical descriptions of C. hillii.

Page details

Date modified: