brown knapweed

Brown knapweed is an exotic, herbaceous, weedy plant. It is native to Europe, western Asia, and North Africa. The precise timing and location of its introduction to North America is uncertain. It was likely introduced as a contaminant of forage seed or intentionally as a forage plant. It now occurs in the United States in the east from Maine to Maryland, west to Minnesota and Illinois, and south along the Appalachian Mountains to North Carolina. It occurs on the West Coast from northern Washington to northern California. There are just a handful of scattered records between these two ranges. It also occurs in southern Canada from Nova Scotia to Ontario and in British Columbia.

Brown knapweed is found in grasslands, meadows, fields, pastures, woodland openings, open woodlands, forest clearcuts, roadsides, ditches, and disturbed areas. It grows under full or partial sun, in moist soil.

Brown knapweed spreads rapidly along roadsides, agricultural fields, overgrazed pastures, and other disturbed areas. It can create dense stands that crowd out native species. The roots exude toxic chemicals into the soil that inhibit the growth and survival of competing species (allelopathy).

Brown knapweed is an erect, perennial forb that rises on basal leaves and one to a few stems, often from a short rhizome. It can be 1′ to 5′ (30 to 150 cm) in height, but it is usually no more than 4′ (122 cm) tall.

In the first year, a rosette of basal leaves is produced. Basal leaves are stalked, hairy, inversely lance-shaped or elliptic in outline, and 2″ to 10″ (5 to 25 cm) long. The margins may be irregularly shallowly lobed, shallowly toothed, or unlobed and untoothed (entire). The surfaces are rough to the touch due to short, stiff, branched hairs, and they are sparsely covered with long, soft, shaggy, unmatted hairs, or short, soft, matted or tangled, woolly hairs. As the plant ages, the leaves become increasingly less hairy.

In the second and subsequent years, one to a few flowering stems are produced. The stem is erect or ascending and openly branched near the top. It is rough to the touch due to short, stiff, branched hairs. It has long, soft, shaggy, unmatted hairs, and short, soft, matted or tangled, woolly hairs. As the plant ages, the stem becomes increasingly less hairy.

Lower stem leaves are alternate and similar to basal leaves. The leaves become progressively smaller, shorter stalked, and less lobed as they ascend the stem. Middle and upper stem leaves are mostly stalkless. The leaf blade either does not or only slightly extends down the stem. Upper stem leaves are unstalked, linear to lance-shaped, and minutely toothed or entire. The leaf blade does not extend down the stem.

Flowering occurs from July through October. The inflorescence is a single flower head or a loose, branched, cyme-like cluster of a few flower heads at the end of the stem and each branch. Each flower head is 1″ to 1¼″ (25 to 32 mm) in diameter.

At the base of each flower head is an egg-shaped to bell-shaped, ⅝″ to 11⁄16″ (15 to 18 mm) in diameter whorl of bracts (involucre). Each bract of the involucre (phyllary) is closely appressed at the base, with a rounded, expanded appendage at the tip. The appendage is erect, light brown, and overlapping. The margin is pale, thin, papery, erect, and coarsely toothed. It does not have sharp spines or black curved bristles “eyelashes” at the tip. The green base is usually concealed by the expanded, light brown appendages. This is the feature that gives the species its common name.

There are 40 to 100 or more disk florets and no ray florets. The outermost disk florets are enlarged, sterile, and purple, rarely white. They may appear ray-like, but closer examination shows them to be short tubes with 5 long lobes. The inner florets are ⅝″ to 11⁄16″ (15 to 18 mm) long and fertile. They are purple, often becoming paler to white at the center. The sterile florets are longer than the fertile florets.

The fruit is a dry, tan, finely hairy, 1⁄16″ to ⅛″ (2 to 3 mm) long seed capsule (cypsela) with a single seed. There is no tuft of bristles (pappus) at the tip.

1′ to 5′ (30 to 150 cm)

Purple

Moist. Grasslands, meadows, fields, pastures, woodland openings, open woodlands, forest clearcuts, roadsides, ditches, and disturbed areas. Full to partial sun.

July through October

Sources

2, 3, 4, 5, 22, 28, 29, 30.

Native to Europe, western Asia, and North Africa

Several subspecies of Centaurea jacea are recognized in Europe—9, 11, or 12, depending on the source. North American sources do not differentiate the subspecies in North American material.

MyNatureapps; Identifying Brown Knapweed, Centaurea jacea
MyNatureApps

Brown Knapweed (Centaurea Jacea) / Brownray Knapweed - 2012-05-23
Westdelta

Chaber łąkowy - Centaurea jacea
jacoleko

Alfredo Colon
8/3/2022

Location: Albany, NY