Montana Field Guide

PLANTS: Small, annual (sometimes biennial) forbs with fibrous roots and a single stem that can branch above the middle, 10 to 50 cm tall. Sources: Lesica et al. 2022; Pringle in Flora of North America [FNA] 2023.

LEAVES: Basal and cauline (stem) present. Basal leaves present during flowering and anthesis; blades elliptic to spatulate-obovate, 15-70 by 5-20 mm, apex rounded to subacute. Leaves arranged opposite on stem. Lower stem leaf blades elliptic to lanceolate and upper stem blades linear, in shape, 1-4 cm long. Sources: Lesica et al. 2022; Pringle in FNA 2023.

INFLORESCENCE: Dense, cymose (corymboid-like) with two lateral axes, producing yellowish to pink flowers – salverform in shape. Calyx: 5, linear, 4-6(7) mm long, and united at base. Corolla: 5 (sometimes 4), 10-17 mm long. Petal lobes are (3)4-6(8) Stamens: Exserted. Anthers twisted. Pistil: Single with 1 style and an ovate-elliptic stigma. Sources: Lesica et al. 2022; Pringle in FNA 2023.

TAXONOMY & NOMENCLATURE
The native North American species have been transferred from the genus Centaurium to Zeltnera - in the Flora of North America and Flora of the Pacific Northwest - 2nd Edition. The taxonomic shift is based on molecular phylogenetic studies (Mansion 2004; Mansion and Struwe 2004; Pringle in FNA 2023).
The revised treatments separate Centaurium and Zeltnera on the following morphology:

Centaurium
* Non-native species to North America
* Basal Leaves: rosette usually present during flowering
* Flowers: white to yellowish-pink
* Pistil with 2 stigmas crescent-shaped (elliptic to ovate to orbiculate) – as long as or longer than wide.
* Capsule cylindric

Zeltnera
* Native species to North America
* Basal Leaves: rosette absent by flowering
* Flowers: pink, rose-violet, or white.
* Pistil with 2 stigmas that are crescent to fan-shaped or 1 stigma with 2 fan-shaped lobes – as wide as or wider than long
* Capsule fusiform (ovoid to ellipsoid)

Centaurium comes from the Greek word kentauros meaning "centaur" which alludes to the plant’s supposed medicinal use by Chiron in Greek mythology (Pringle in FNA 2023).

Across its range, flowering is summer to early fall (Pringle in FNA 2023).

Members of the Gentian Family (Gentianaceae) have opposite leaves, glabrous foliage, and flowers with 4 to 5 tubular petals of equal size. Traditionally, the genus Centaurium is distinguished from other family members by a salverform flower – where lobes spread at right angles to the tube, a long and narrow corolla tube, and the of lack membranous tissue between the petals.

The Manual of Montana Vascular Plants (Lesica et al. 2022) retains a traditional taxonomic treatment:

Common Centaury - Centaurium erythraea, non-native
* Leaves: Small rosette of basal leaves present during flowering and anthesis
* Inflorescence: Densely branched, many flowers
* Seeds: Brown

Western Centaury - Centaurium exaltatum, native, SOC
* Leaves: Basal rosette of basal leaves that wither by flowering time; thus, appearing all cauline
* Inflorescence: Little branched, few flowered
* Seeds: Dark reddish brown to nearly black

Across its range, Common Centaury grow in disturbed meadows, fields, roadsides, and other open habitats that are disturbed in the valleys of Montana (Lesica et al. 2022; Pringle in FNA 2023).

Plants reproduce by seed.

FRUIT & GENETICS
The fruit is a capsule. Capsule is cylindric, 7-15 mm long. Seeds are. Seeds are diploid with a chromosome number of 2n = 40 (Europe) or 42 (Spain) (Pringle in FNA 2023).

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association Contacts Webpage.
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides