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• Centaurea solstitialis

Centaurea solstitialis — yellow knapweed

Facts

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

• 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 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
  • the leaves are growing only at the base of the plant (basal)

  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 (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 (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

From the dichotomous key of Flora Novae Angliae