Weed Information: Centaurea nigrescens (knapweed, meadow)
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Centaurea nigrescens (knapweed, meadow)

Family: Asteraceae

Description

A rhizomatous perennial with simple leaves and brownish, fringed, membranous bracts. The specific name `pratensis` means `growing in a moist meadow` and this is, in fact, the type of site where the species is commonly found. This species is a hybrid between Centaurea jacea and Centaurea nigra and as such is sometimes found listed as Centaurea X pratensis or Centaurea pratensis.

Characteristics

Flower Color
  • Red (or pink)
  • Purple
  • Red-purple
Inflorescence Type (How the flowers are arranged on the plant)
  • Corymb
  • Cyme (simple or branched)
  • Head
  • Panicle
Number of Petals
  • 0 (no petals)
  • 5 petals
  • more than 10 petals
Petal Separation
  • Completely free to base
  • United > 75% of length
Petal Tip
  • Laciniate
  • Pointed
Flower Symmetry
  • Flowers regular
Flower Length (Head length in the Asteraceae)
  • 1.0 cm (.40 inch)
  • 1.5 cm (.6 inch)
  • 2.0 cm (.8 inch)
  • 2.5 cm (1.0 inch)
  • 3.0 cm (1.2 inches)
Flower Width (Head width in the Asteraceae)
  • 1.0 cm (.40 inch)
  • 1.5 cm (.60 inch)
  • 2.0 cm (.80 inch)
  • 2.5 cm (1.0 inch)
  • 3.0 cm (1.2 inches)
  • 3.5 cm (1.4 inches)
  • 4.0 cm (1.6 inches)
  • 4.5 cm (1.8 inches)
  • 5.0 cm (2.0 inches)
Number of Sepals
  • sepals lacking
  • more than 10 sepals
Sepal Separation
  • Completely free to base
Sepal Tip
  • Rounded
  • Square
  • Toothed
Flower Sex
  • Bisexual (perfect)
Number of Styles
  • 1 style
Number of Stamens
  • 5 stamens
Stamens Attached to the Petals
  • Stamens attached to the petals
Stamens Connate (united)
  • Anthers united, syngynoecious

Carpel Separation
  • Carpels united (or one)
Number of Carpels
  • 1 carpel
Ovary Position
  • Inferior (epigynous)
Fruit Type
  • Achene
Fruit Length at Maturity
  • 8 mm (.32 inch)
  • 9 mm (.36 inch)
  • 1.0 cm (.40 inch)
Fruit Width at Maturity
  • 1 mm (.04 inch) or less
  • 2 mm (.08 inch)
  • 3 mm (.12 inch)
Burs
  • Fruit or seed NOT bur-like

Leaf Arrangement
  • Alternate
Leaf Type
  • Simple (including lobed leaves)
Stipules
  • Lacking
Tendrils
  • Tendrils absent
Venation
  • Pinnate
Petiole (leaf stem) Blade Ratio
  • Petiole absent (leaf sessile)
  • Petiole < 25% total length
Leaf Length (blade PLUS petiole)
  • 1 - 2 cm (.40 - .80 inch)
  • 2 - 4 cm (.80 - 1.60 inches)
  • 4 - 10 cm (1.60 - 4.0 inches)
  • 1 - 2 dm (4.0 - 8.0 inches)
Leaf Width
  • 5 - 10 mm (.20 - .40 inch)
  • 1 - 2 cm (.40 - .80 inch)
  • 2 - 4 cm (.80 - 1.6 inches)
Leaf Shape (simple leaves only)
  • Elliptic
  • Linear or filiform
  • Oblanceolate
  • Pinnately lobed
Leaf or Leaflet Margin
  • Dentate
  • Entire
  • Lobed
  • Ciliate
Leaf or Leaflet Blade Base
  • Acuminate (attenuate)
  • Clasping
Leaf or Leaflet Tip
  • Acute
Leaf or Leaflet Blade Surface
  • Glabrous (NO hairs)
  • Hirsute
Succulence
  • Not succulent

Milky Juice
  • Juice NOT milky (watery)
Woodiness
  • Herbaceous
Spines or Thorns
  • Spines or Thorns ABSENT
Aromatic (vegetative structures)
  • Not aromatic
Life Cycle
  • Perennial
Growth Habit (while flowering)
  • Erect, columnar (narrow)
Plant Height at Maturity
  • 2 - 4 dm (8.0 - 16.0 inches)
  • 4 - 6 dm (1.3 - 2.0 feet)
  • 6 - 8 dm (2.0 - 2.6 feet)
  • 8 - 10 dm (2.6 - 3.3 feet)
  • 1 - 1.5 meters (3.3 - 5.0 feet)
Wind Dissemination
  • Not wind disseminated
Moisture Regime
  • Seasonally saturated
  • Mesic
Chlorophyll
  • Present

Stem Cross Section
  • Round
Flowering Stem Leaves
  • Stem uniformly leafy
Main Stem Branches
  • Main stem branched

Root or Vegetative Propagule
  • Rhizomatous

US State
  • Connecticut
  • Illinois
  • Massachusetts
  • Maine
  • Michigan
  • Missouri
  • New Hampshire
  • New Jersey
  • New York
  • Ohio
  • Pennsylvania
  • Virginia
  • Wisconsin
  • West Virginia
  • Indiana
  • Maryland
  • Rhode Island
  • Vermont
  • California
  • Idaho
  • Minnesota
  • Montana
  • Oregon
  • Washington
  • Wyoming
  • Nebraska
Canadian Province or Territory
  • British Columbia
  • Nova Scotia
  • Ontario
  • Quebec
  • Newfoundland
Other
  • District of Columbia
  • St. Pierre and Miquelon

  • Gilkey`s Weeds of the Pacific Northwest, 1980 (ISBN 0-88246-039-0) See page: 331
  • Ontario Weeds, 1992 Agdex 640, Publication #505 See page: 244
  • Selected Weeds of Oregon, 1985 and 1989 Supplement (s) Oregon State Dept. of Agriculture, See page: 76
  • Weeds of California and other Western States, 2007 (ISBN 13: 978-1-879906-69-3) See Page: 245
  • Weeds of the West, 1992 (ISBN 0941570-13-4) See page: 90
  • Weeds of the West, 2000 (ISBN 0941570-13-4) See page: 94
  • Listed as a `Weed` by the Biota of North America Program, 2008 X
  • Listed in the WSSA Composite List of Weeds X
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