Weed Information: Centaurea diffusa (knapweed, diffuse)
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Centaurea diffusa (knapweed, diffuse)

Family: Asteraceae

Description

Quite possibly the fastest spreading weed in the western U.S. at the end of the 1900`s. This species can act as an annual, biennial, or perennial. The plants are adapted to spread as a tumbleweed but are more rapidly spread by vehicles. In general this species prefers a drier site than spotted knapweed, but the two are often found together. Diffuse knapweed can have dark tipped bracts like spotted knapweed (attributed by some to hybridization between the two species) but can always be distinguished by the fact that the terminal spine of the bract is always longer than the marginal spines.

Characteristics

Flower Color
  • Purple
  • Red-purple
  • White
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
  • Pointed
Flower Symmetry
  • Flowers regular
Flower Length (Head length in the Asteraceae)
  • 7 mm (.28 inch)
  • 8 mm (.32 inch)
  • 9 mm (.36 inch)
  • 1.0 cm (.40 inch)
  • 1.5 cm (.6 inch)
Flower Width (Head width in the Asteraceae)
  • 7 mm (.28 inch)
  • 8 mm (.32 inch)
  • 9 mm (.36 inch)
  • 1.0 cm (.40 inch)
Number of Sepals
  • sepals lacking
  • more than 10 sepals
Sepal Separation
  • Completely free to base
Sepal Tip
  • Spine-like
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
  • 2 mm (.08 inch)
  • 3 mm (.12 inch)
  • 8 mm (.32 inch)
  • 9 mm (.36 inch)
  • 1.0 cm (.40 inch)
  • 1.5 cm (.60 inch)
  • 2.0 cm (.80 inch)
Fruit Width at Maturity
  • 1 mm (.04 inch) or less
  • 2 mm (.08 inch)
  • 6 mm (.24 inch)
  • 7 mm (.28 inch)
  • 8 mm (.32 inch)
  • 9 mm (.36 inch)
  • 1.0 cm (.40 inch)
Burs
  • Fruit or seed NOT bur-like
  • Fruit or seed bur-like

Leaf Arrangement
  • Alternate
Leaf Type
  • Pinnately dissected
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)
  • 4 - 10 cm (1.6 - 4.0 inches)
Leaf or Leaflet Margin
  • Lobed
  • Parted
Leaf or Leaflet Blade Base
  • Acute (cuneate)
  • Clasping
Leaf or Leaflet Tip
  • Acute
Leaf or Leaflet Blade Surface
  • Puberulent
  • Scabrous (like sand paper)
  • Tomentose
Succulence
  • Not succulent

Milky Juice
  • Juice NOT milky (watery)
Woodiness
  • Herbaceous
Spines or Thorns
  • Spines or Thorns ABSENT
  • Spines on flowers or fruits only
Aromatic (vegetative structures)
  • Not aromatic
Life Cycle
  • Annual
  • Biennial
  • Perennial
Growth Habit (while flowering)
  • Erect, spherical (bushy)
Plant Height at Maturity
  • 5 - 10 cm (2.0 - 4.0 inches)
  • 1 - 2 dm (4.0 - 8.0 inches)
  • 2 - 4 dm (8.0 - 16.0 inches)
  • 4 - 6 dm (1.3 - 2.0 feet)
Wind Dissemination
  • Entire plant mobilized (tumbleweeds)
Moisture Regime
  • Mesic
  • Arid
Chlorophyll
  • Present

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

Root or Vegetative Propagule
  • Tap Root

US State
  • Connecticut
  • Illinois
  • Kentucky
  • Massachusetts
  • Michigan
  • Missouri
  • New Jersey
  • Ohio
  • Tennessee
  • Wisconsin
  • Indiana
  • Arizona
  • California
  • Colorado
  • Iowa
  • Idaho
  • Montana
  • New Mexico
  • Nevada
  • Oregon
  • Utah
  • Washington
  • Wyoming
  • Nebraska
Canadian Province or Territory
  • British Columbia
  • Ontario
  • Alberta
  • Saskatchewan
  • Yukon

  • The Biology of Canadian Weeds: Canadian Journal of Plant Science Vol54 Pg 687
  • Common Weeds of Canada, Mulligan 1987 (ISBN 0-910053-59-9) See page: 104
  • Common Weeds of the Canadian Prairies, 1963 (Cat# A53-1136-1) See page: 29
  • Common Weeds of the United States, 1971 (ISBN 0-486-20504-5) See page: 380
  • Gilkey`s Weeds of the Pacific Northwest, 1980 (ISBN 0-88246-039-0) See page: 333
  • Northwest Weeds, 1990 (ISBN 0-87842-249-8) See page: 145
  • Selected Weeds of Oregon, 1985 and 1989 Supplement (s) Oregon State Dept. of Agriculture, See page: 74
  • South Dakota Weeds 2002 South Dakota Dept. of Ag. 523 E. Capitol, Foss Bldg. Pierre, South Dakota 57501 See page: 218
  • Weeds, 1955 (1980) Walter Conrad Muenscher, ISBN# 0-8014-1266-8 See page: 441
  • Weeds and Poisonous Plants of Wyoming and Utah 1987 (ISBN 0-941570-06-1) See page: 162
  • Weeds of Alberta, 1983 Alberta Environmental Centre Agdex 640-4, See page: 183
  • Weeds of California and other Western States, 2007 (ISBN 13: 978-1-879906-69-3) See Page: 235
  • Weeds of Canada, 1970 Canada Dept. of Ag., Pub. # 948, See page: 193
  • Weeds of Colorado, 1990 Cooperative Extension, Colorado State Univ. Bulletin 521A, See page: 169
  • Weeds of Eastern Washington and Adjacent Areas, 1972 (LOC 72-83635) See page: 280
  • Weeds of Nebraska and the Great Plains, 1994 (ISBN 0939870-00-4) See page: 103
  • Weeds of the Midwestern United States & Central Canada, University of Georgia 2010, ISBN#0-8203-3506-1 See page: 75
  • Weeds of the Northern U.S, and Canada, 1999 Royer and Dickinson, (ISBN# 1-55105-221-0) See page: 29
  • Weeds of the West, 1992 (ISBN 0941570-13-4) See page: 84
  • Weeds of the West, 2000 (ISBN 0941570-13-4) See page: 88
  • 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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