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

Centaurea nigra — black knapweed

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Facts

Black knapweed can be an aggressive weed. It was introduced from Europe as an ornamental plant as well as in ships' ballast.

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
  • Maine
  • Massachusetts
  • New Hampshire
  • Rhode Island
  • Vermont
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
  • the edge of the leaf blade has teeth
Flower type in flower heads
the flower head has disk flowers only, and lacks the strap-shaped flowers
Ray flower color
NA
Tuft or plume on fruit
  • at least a part of the plume is made up of fine bristles
  • there is no plume, or the plume is made up of scales, awns, a crown, or a rim
Spines on plant
the plant has no spines
Leaf blade length
50–250 mm
Disk flower number
  • 21-50
  • more than 50
Show all characteristics

  • Flowers

    Bases of bract appendages
    the bases of two bract appendages are connected by a straight line; there is no projecting edge running between appendages
    Bract cycle number
    there are three or more cycles of bracts
    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
    the bracts have no spines
    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 shape
    the tips of the bracts are truncate (end abruptly in a more or less straight line as though cut off)
    Disk flower color
    blue to purple
    Disk flower lobe number
    5
    Disk flower number
    • 21-50
    • more than 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 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 platform surface
    NA
    Flower head profile
    the disk is flat or nearly flat across the top
    Flower head shape
    the flower head is hemispherical (like the bottom half of a sphere)
    Flower type in flower heads
    the flower head has disk flowers only, and lacks the strap-shaped flowers
    Height of flower head base
    15–18 mm
    Inflorescence branching (Solidago)
    NA
    Inflorescence shape
    the inflorescence is flat-topped in profile
    Ovary attachment
    the ovary is attached at the side near the base, rather than at the base
    Ovary cross-section
    the ovary is compressed (flattened)
    Ovary hair type
    the ovary has hairs on it, but the hairs have no glands
    Ovary hairs
    the ovary has hairs on it
    Ray flower color
    NA
    Ray flower reproductive parts
    NA
    Ray flowers
    0
    Ray length
    0 mm
    Reproductive system
    all the flowers on the plant contain both carpels and stamens
    Scale tip
    NA
    Width of flower head base
    15–18 mm

  • Fruits or seeds

    Ovary length in developed fruit
    2.5–3 mm
    Seed hair tuft color
    the pappus hairs are another color
    Seed hair tuft length
    0.5–1 mm
    Seed tuft scale number
    0
    Seed tuft type
    the pappus is made of very fine hairs or bristles
    Top of disk flower ovary
    NA
    Tuft or plume on fruit
    • at least a part of the plume is made up of fine bristles
    • there is no plume, or the plume is made up of scales, awns, a crown, or a rim

  • Glands or sap

    Leaf blade glands
    the leaf blades have no glandular (translucent) dots or scales
    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 perennial, it shows evidence of previous year's leaves, stems or stem bases
    Spines on plant
    the plant has no spines

  • Leaves

    Hairs on underside of leaf blade
    the underside 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 bloom
    the underside of the leaf has no noticeable bloom
    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
    • the edge of the leaf blade has teeth
    Leaf blade hairs
    the leaf blade has simple hairs with no glands, and not tangled or wooly
    Leaf blade length
    50–250 mm
    Leaf blade shape
    • the leaf blade is elliptic (widest near the middle and tapering at both ends)
    • the leaf blade is lanceolate (lance-shaped; widest below the middle and tapering at both ends)
    • the leaf blade is linear (very narrow with more or less parallel sides)
    • the leaf blade is oblanceolate (lance-shaped, but with the widest point above the middle of the leaf blade)
    Leaf blade surface colors
    there is no noticeable color variation on the upper surface of the leaf
    Leaf blade veins
    the leaf blade has one main vein running from the base towards the tip
    Leaf blade width
    10–60 mm
    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 stalk length
    At least 0 mm
    Leaf tip extension
    NA
    Leaf type
    leaves are simple (i.e., lobed or unlobed but not separated into leaflets)
    Leaflet number
    0
    Specific leaf type
    • the leaf has a row of two or more lobes on each side of the central axis
    • the leaves are simple (i.e., lobed or unlobed but not separated into leaflets
    Teeth per side of leaf blade
    At least 0

  • Place

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

  • Scent

    Plant odor
    the plant does not have much of an odor

  • 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 bloom
    there is no powdery or waxy film on the 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
    • the stem has no hairs between the nodes
    Stem wings
    • the stem does not have wings on it
    • the stem has wings on it that run down the stem from the leaf nodes

Wetland status

Not classified

In New England

Distribution

Connecticut
present
Maine
present
Massachusetts
present
New Hampshire
present
Rhode Island
present
Vermont
present

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

8.  Centaurea nigra L. E

black knapweed. Jacea nigra (L.) Hill • CT, MA, ME, NH, RI, VT. Roadsides, fields, waste areas.

5×8. Centaurea jacea × Centaurea nigra → Centaurea ×‌moncktonii C.E. Britton [Fig. 386] has been known by the illegitimate name C. pratensis Thuill. It is known from CT, MA, ME, NH, RI, VT. It is a variable hybrid, with marginal corollas usually enlarged and falsely appearing as ray flowers (but sometimes not; usually enlarged in C. jacea, usually not enlarged in C. nigra), either lacking pappus or with vestigial bristles, and involucral bracts with brown apical appendages, those on the middle involucral bracts becoming irregularly pectinate (these irregularly lacerate in C. jacea and regularly pectinate in C. nigra). The involucral bract appendage often conceals the distal portion of adjacent involucral bracts but can be highly variable in later-generation hybrids (i.e., they can approach one or the other parent). The involucres are usually at least as wide as tall in life.

Native to North America?

No

Sometimes confused with

Centaurea nigrescens:
apical appendage of involucral bracts mostly 1–2 mm long, with 5–8 fringe segments on each margin, and outer flowers of capitulum sometimes enlarged and falsely appear as ray flowers (vs. C. nigra, with the apical appendage of involucral bracts longer than 2 mm, the larger ones mostly 4–6 mm long, with 7–15 fringe segments on each margin, and outer flowers of capitulum not enlarged).
Centaurea jacea:
involucral bracts of flower head with a terminal appendage that is light brown to brown, the appendage on the lower and middle ones irregularly lacerate, the appendage on the upper ones with a notch in the center (vs. C. nigra, with the involucral bracts of flower head with a terminal appendage that is brown to black, the appendage on the lower and middle ones regularly fringed, the appendage on the upper ones without a notch in the center).

Synonyms

  • Jacea nigra (L.) Hill

Family

Asteraceae

Genus

Centaurea