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Native Plant Trust: Go Botany Discover thousands of New England plants

Centaurea diffusa — white knapweed

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Facts

White knapweed is native to southeastern Europe and is a severely invasive weed in some western states in the United states. It can form a tumbleweed if broken off after senescence. Other methods of dispersal include as a contaminant of crops, and on vehicles, clothing and animals. It is found in New England on roadsides and waste areas.

Habitat

Anthropogenic (man-made or disturbed habitats), meadows and fields

New England distribution

Adapted from BONAP data

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North America distribution

Adapted from BONAP data

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Characteristics

Habitat
terrestrial
New England state
  • Connecticut
  • Massachusetts
  • New Hampshire
  • Rhode Island
Leaf type
leaves are simple (i.e., lobed or unlobed but not separated into leaflets)
Leaf arrangement
alternate: there is one leaf per node along the stem
Leaf blade edges
  • the edge of the leaf blade has lobes, or it has both teeth and lobes
  • the edge of the leaf blade has no teeth or lobes
Flower type in flower heads
the flower head has disk flowers only, and lacks the strap-shaped flowers
Tuft or plume on fruit
NA
Spines on plant
  • the plant has no spines
  • there are spines on the plant
Leaf blade length
100–200 mm
Disk flower number
21-50
Show all characteristics

  • Flowers

    Bases of bract appendages
    bases of bract appendages are u-shaped, with a tiny projecting edge running across from one appendage to the next
    Bract color
    the bracts are not colored or tinged with pink, red or purple
    Bract cycle number
    there are three or more cycles of bracts
    Bract margins
    there are fine hairs along the bract margins
    Bract outer side hair type
    • the bracts are hairy on their outer surfaces, with curled, tangled, matted, or woolly hairs
    • the bracts are not hairy on their outer surface
    Bract outer side hairs
    • the bracts are hairy on their outer surfaces
    • the bracts are not hairy on their outer surfaces
    Bract shape
    • the main bracts are lanceolate (widest above the base, then taper narrowly towards the tip)
    • the main bracts are ovate (egg-shaped)
    Bract spines
    there is just one simple spine at the tip of the bract
    Bract tip color
    the tips are a different color from the center of the bract
    Bract tip extension appearance
    the projections from the bract tips have further projections from their edges, like a fringe
    Bract tip extension edge
    there are projections from the bract tips
    Bract tip orientation
    the bracts are pressed against the plant, or spreading out at the tips
    Bract tip shape
    the tips of the bracts acute (have a sharp point)
    Bracts
    the bracts in separate cycles are similar or gradually changing from the outer to inner cycles
    Disk flower color
    • pink to red
    • white
    • yellow
    Disk flower lobe number
    5
    Disk flower number
    21-50
    Disk flower reproductive parts
    the disk flower has both pollen- and seed-producing parts
    Disk flower shape
    the disk flower is tube-shaped (cylindrical), or gradually widening like a funnel
    Flower head number
    each flowering stem has four or more flower heads on it
    Flower head outer flowers
    at the outer edge of the flower head, the flowers have no enlarged lobe or strap, and are of similar size as those in the center of the disk
    Flower head platform
    the base has fine, short hairs on it
    Flower head position
    each of the flower heads is separate on its own peduncle (stalk), not clustered in groups
    Flower head profile
    the disk is flat or nearly flat across the top
    Flower head shape
    • NA
    • the sides of the flower head are roughly parallel, like a cylinder
    Flower type in flower heads
    the flower head has disk flowers only, and lacks the strap-shaped flowers
    Height of flower head base
    10–13 mm
    Inflorescence branching (Solidago)
    NA
    Inflorescence shape
    the inflorescence is not flat-topped but appears rounded, with some flower heads distinctly higher than others
    Inflorescence stem
    hairs are present on the stem of the inflorescence
    Ovary attachment
    the ovary is attached at the side near the base, rather than at the base
    Ovary beak
    there is no beak on the ovary
    Ovary cross-section
    • the ovary has five or more corners in cross-section
    • the ovary is compressed (flattened)
    Ovary lines or ribs
    there are no lines or ribs visible on the ovary
    Ovary profile
    • in profile, the ovary is oblong (roughly rectangular but rounded at the ends)
    • in profile, the ovary is roughly elliptical (widest in the middle, tapering to both ends)
    Ray flower reproductive parts
    NA
    Ray flowers
    0
    Reproductive system
    all the flowers on the plant contain both carpels and stamens
    Style branch number
    the style has two branches
    Width of flower head base
    4–5 mm

  • Fruits or seeds

    Number of pappus parts
    0
    Ovary length in developed fruit
    2–3 mm
    Seed hair tuft bases
    NA
    Seed hair tuft color
    NA
    Seed hair tuft details
    NA
    Seed hair tuft length
    0 mm
    Seed hair tuft tips
    NA
    Seed hairs uniform
    NA
    Seed tuft scale number
    0
    Seed tuft type
    there is no pappus on the ovary
    Top of disk flower ovary
    NA
    Tuft or plume on fruit
    NA

  • Glands or sap

    Sap
    the sap is clear and watery

  • Growth form

    Growth form
    the plant has one or more free-standing stems
    Plant lifespan
    • the plant is annual, it lacks evidence of previous years' growth
    • the plant is perennial, it shows evidence of previous year's leaves, stems or stem bases
    Spines on plant
    • the plant has no spines
    • there are spines on the plant

  • Leaves

    Hairs on underside of leaf blade
    the underside of the leaf is fuzzy or hairy
    Hairs on upper side of leaf blade
    the upper side of the leaf is fuzzy or hairy
    Leaf arrangement
    alternate: there is one leaf per node along the stem
    Leaf blade base
    • the leaf has a distinct petiole
    • the leaf has no petiole
    Leaf blade base shape
    the base of the leaf blade is cuneate (wedge-shaped, tapers to the base with relatively straight, converging edges), or narrow
    Leaf blade edges
    • the edge of the leaf blade has lobes, or it has both teeth and lobes
    • the edge of the leaf blade has no teeth or lobes
    Leaf blade hairs
    the leaf blade has tangled or woolly-looking hairs
    Leaf blade length
    100–200 mm
    Leaf blade shape
    • the leaf blade is linear (very narrow with more or less parallel sides)
    • the leaf blade is ovate (widest below the middle and broadly tapering at both ends)
    Leaf blade tip
    • the tip of the leaf blade is acute (sharply pointed)
    • the tip of the leaf blade is rounded, with no point
    Leaf disposition
    the leaves are nearly similar in size, prominence of teeth, and length of stalks throughout the stem
    Leaf spines
    there are no spines on the leaf edges
    Leaf stalk
    • the leaves have leaf stalks
    • the leaves have no leaf stalks, but attach directly to the stem
    Leaf tip extension
    NA
    Leaf type
    leaves are simple (i.e., lobed or unlobed but not separated into leaflets)
    Specific leaf type
    the leaf has a row of two or more lobes on each side of the central axis, and each lobe itself has rows of lobes on each side of the lobe's central axis

  • Place

    Habitat
    terrestrial
    New England state
    • Connecticut
    • Massachusetts
    • New Hampshire
    • Rhode Island
    Specific habitat
    • man-made or disturbed habitats
    • meadows or fields

  • Stem, shoot, branch

    Flowering stem cross-section
    the flowering stem is circular, or with lots of small angles
    Leaves on stem
    there is at least one full leaf above the base of the flowering stem
    Stem internode hair direction
    the hairs point mostly upwards to outwards
    Stem internode hair type
    at least some of the hairs on the stem are tangled, matted or woolly
    Stem internode hairs
    the stem has hairs between the nodes
    Stem wings
    the stem does not have wings on it

Wetland status

Not classified

In New England

Distribution

Connecticut
present
Maine
absent
Massachusetts
present
New Hampshire
present, invasive
Rhode Island
present
Vermont
absent

Conservation status

Exact status definitions can vary from state to state. For details, please check with your state.

Massachusetts
not applicable (S-rank: SNA)

From Flora Novae Angliae dichotomous key

4.  Centaurea diffusa Lam. E

white knapweed. Acosta diffusa (Lam.) Soják; Acrolophus diffusus (Lam.) A. & D. Löve • CT, MA, NH, RI. Roadsides, fields, waste areas, abandoned homesteads.

4×13. Centaurea diffusa × Centaurea stoebe → Centaurea ×‌psammogena Gáyer is a rare knapweed hybrid in New 
England that is frequently confused with C. diffusa. The hybrid, known from MA, 
 is morphologically variable. Involucral bract appendages are usually brown to 
black (rarely light brown), and appendage spines are absent or short (spine mostly 
1 mm long or shorter). However, an evident pappus always crowns the ovary, 
and the marginal flowers of the capitulum are falsely radiant (pappus absent and marginal flowers not enlarged in C. diffusa).

Native to North America?

No

Sometimes confused with

Centaurea calcitrapa:
apical spine of involcural bracts mostly 5-25 mm long and involucre mostly 7-12 mm wide (vs. C. diffusa, with the apical spine of involucral bracts mostly 2-3 mm long and involucre 5-5 mm wide).

Synonyms

  • Acosta diffusa (Lam.) Soják
  • Acrolophus diffusus (Lam.) A. & D. Löve

Family

Asteraceae

Genus

Centaurea