Centaurea solstitialis L.

yellow knapweed

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New England Distribution

Adapted from BONAP data

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North America Distribution

Adapted from BONAP data

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Facts About

Widely introduced in North America, yellow knapweed is a serious weed of rangelands in the western states. It forms huge colonies and is very difficult to eradicate, replacing native plants and reducing biodiversity as well as forage quality. When horses eat it over a prolonged period it causes a neurological disorder called equine nigropallidal encephalomalacia, or "chewing disease." It is only occasionally found in New England.

Habitat

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

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
  • basal: the leaves are growing only at the base of the plant
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
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
  • there are spines on the plant
Leaf blade length
10–150 mm
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 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 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 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 tips of the bracts curve outwards and downwards from the plant
    Bract tip shape
    the tips of the bracts are rounded
    Bracts
    the bracts in separate cycles are similar or gradually changing from the outer to inner cycles
    Disk flower color
    yellow
    Disk flower lobe number
    5
    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 only one to three flower heads on it
    Flower head platform
    the base has fine, short hairs on it
    Flower head platform surface
    NA
    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
    13–17 mm
    Inflorescence branching (Solidago)
    NA
    Inflorescence shape
    NA
    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 hair type
    the ovary has no hairs on it
    Ovary hairs
    the ovary has no hairs on it
    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 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
    Smaller bracts at base of bracts
    NA
    Style branch number
    the style has two branches
    Width of flower head base
    7–15 mm
  • Fruits or seeds
    Ovary length in developed fruit
    1.5–2.5 mm
    Ovary width in developed fruit
    Up to 0.7 mm
    Seed hair tuft color
    the pappus hairs are white or off-white
    Seed hair tuft details
    the pappus hairs are hooked or barbed
    Seed hair tuft length
    2–4 mm
    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
    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
    Spines on plant
    • the plant has no spines
    • there are spines on the plant
    Underground organs
    there is a thickened taproot 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
    • basal: the leaves are growing only at the base of the plant
    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 branched hairs
    • the leaf blade has tangled or woolly-looking hairs
    Leaf blade length
    10–150 mm
    Leaf blade shape
    • the leaf blade is linear (very narrow with more or less parallel sides)
    • the leaf blade is oblong (rectangular but with rounded ends)
    Leaf blade tip
    • the tip of the leaf blade is acute (sharply pointed)
    • the tip of the leaf blade is obtuse (bluntly pointed)
    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
    • the leaves are simple (i.e., lobed or unlobed but not separated into leaflets
  • 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
    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 has wings on it that run down the stem from the leaf nodes

Wetland Status

Not classified

New England Distribution and Conservation Status

Distribution

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

Conservation Status

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

Massachusetts
unranked (S-rank: SNR)

Native to North America?

No

Sometimes Confused With

Centaurea melitensis:
apical spine of involucral bracts 5-8 mm long and disk corollas glandular (vs. C. solstitialis, with the apical spine of involucral bracts mostly 10-15 mm long and disk corollas eglandular).

Synonyms

  • Calcitrapa solstitialis (L.) Lam.
  • Cyanus solstitialis (L.) Baumg.
  • Leucantha solstitialis (L.) A. & D. Löve

Family

Asteraceae

Genus

Centaurea

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Information from Dichotomous Key of Flora Novae Angliae

12.  Centaurea solstitialis L. E

yellow knapweed. Calcitrapa solstitialis (L.) Lam.; Cyanus solstitialis (L.) Baumg.; Leucantha solstitialis (L.) A. & D. Löve • CT, MA, NH, RI. Roadsides, fields, waste areas.