Montana Field Guide

PLANTS: A herbaceous perennial with erect, simple or branched stems that grow to heights of 30-150 cm (FNA 2006). Plants villous to scabrous with septate hairs and loosely tomentose or more or less glabrate. Source: FNA 2006.

LEAVES: Basal and lower stem leaves are petiolate with oblanceolate, 5–25 cm long blades. Leaf margins are entire to shallowly dentate to irregularly pinnately lobed. Upper stem leaves become gradually smaller and are linear to lanceolate with entire or dentate margins. Source: FNA 2006.

INFLORESCENCE: Corymbiform. Few, but relatively broad flowering heads are borne on leafy-bracted stems. Flowering heads are composed of ray (not disk) florets with purple (rarely white) petals. Involucres are ovoid to campanulate or hemispheric and 15–18 mm high (usually about as wide as high). Involucral bracts are light to dark brown, more-or-less triangular, and margins coarsely dentate to evenly fringed (fimbriate), but not spine-tipped. Source: FNA 2006.

Flowering May to November (FNA 2006).

The Centaurea jacea complex has been the subject of much controversy (FNA 2006). These plants are widely distributed in Europe and variable in easily noticeable characteristics of their flowering heads, florets, and fruits (cypselae). Within this complex, numerous species have been named, but all are capable of interbreeding, and natural hybridization has resulted in intermediates that variously combine the features of their parents. These intermediates have been considered as species or as infraspecific taxa within the parental species. This has resulted in tangled nomenclature, further complicated by names misapplied in their use and infraspecific names that are inadequately indexed.

For the Centaurea jacea complex, MTNHP follows the Centaurea treatment by Keil & Ochsmann in FNA, Vol. 19 (2006). 3 species (Centaurea jacea, Centaurea nigra, Centaurea nigrescens) and 1 nothospecies [formed by direct hybridization of 2 species and not by other hybrids], Centaurea xmoncktonii are recognized in MT. However, all could be treated as 1 species, Centaurea jacea with 3 subspecies (jacea, nigra, nigrescens) and 1 nothosubspecies hybrid, C. jacea ssp. xpratensis.

The complex of Centaurea jacea, Centaurea nigra, Centaurea nigrescens, Centaurea xmoncktonii share characteristics of: a) Perennial, b) Involucres 15-18 mm tall, c) Lower portion of the involucral bracts have entire margins and their upper portion have fringed appendages that are little to not decurrent along the margin and are not spine-tippped. They are distinguished by the following characteristics.

Centaurea jacea: Involucral bracts are light to dark brown, roundish (seldom triangular), scarious, and more-or-less undivided to irregularly lacerate. Florets are disc flowers that lack a pappus.

Centaurea nigra: Involucres appear totally black and are usually about as wide as high. Involucral bracts are dark brown to black; any green portion is mostly covered by the black fringe of adjacent bracts. Bracts are more-or-less triangular with margins evenly fringed into numerous wiry lobes. The body of the bract is tomentose or glabrous. Florets are disc flowers with a blackish pappus of 0.5-1.0 mm long bristles that easily detach.

Centaurea nigrescens: Involucres are relatively narrow, usually longer than wide. Involucral bracts appear black and green; green portions are mostly not covered by the black fringe of adjacent bracts. Bracts are more-or-less triangular with irregularly dentate or lobed margins. Florets have no pappus or have many bristles of unequal length that easily detach.

Centaurea xmoncktonii: Involucres are relatively broad. Involucral bracts are light to dark brown, more-or-less triangular, and have variable margins from coarsely dentate to evenly fringed. Sterile florets are somewhat expanded and exceed the length of the 15-18 mm long fertile florets. Florets are of ray flowers and have none or 0.5-1.0 mm long pappus bristles that easily detach.

Roadsides, riverbanks, pastures, meadows, forest openings, and waste areas at elevations from 0 to 1,000 meters (FNA 2006).

POLLINATORS
The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus bifarius, Bombus centralis, Bombus fervidus, Bombus rufocinctus, Bombus ternarius, Bombus terricola, Bombus occidentalis, Bombus griseocollis, Bombus impatiens, Bombus insularis, and Bombus suckleyi (Thorp et al. 1983, Johnson 1986, Colla and Dumesh 2010, Koch et al. 2012, Williams et al. 2014, Tripoldi and Szalanski 2015).

Flowering heads are composed of about 40 to 100, purple (rarely white) ray florets (FNA 2006). Sterile florets are somewhat expanded and exceed the length of the 15-18 mm long fertile florets. The fruit (cypselae) is tan, 2.5-3.0 mm long and finely hairy. The pappus is absent or there are 0.5-1.0 mm long bristles that easily detach.

Meadow knapweeds represent an array of mutually interfertile intermediates derived by hybridization and backcrossing among the various diploid and tetraploid individuals that make up the Centaurea jacea complex (FNA 2006). The plants variously combine features of Centaurea jacea and Centaurea nigra and possibly Centaurea nigrescens (FNA 2006). Meadow knapweeds are often found without either parent in the immediate vicinity (FNA 2006).

Meadow knapweeds appear to respond to herbicides differently from their other Centaurea relatives (personal communication from a weed coodinator).