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• Centaurea calcitrapa

Centaurea calcitrapa — purple knapweed

Facts

Purple knapweed is native to southern Europe and northern Africa. It is an important weed in western North America, but rarely encountered in New England. It can form dense stands and is defended by the vicious spines on the mature involucres.

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 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 shape

  the main bracts are ovate (egg-shaped)

  Bract spines

  • there is just one simple spine at the tip of the bract
  • there is one spine with branches, at the tip of the bract

  Bract texture

  the bracts appear leathery or hardened

  Bract tip color

  the tips are the same color as 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
  • the tips of the bracts curve outwards and downwards from the plant

  Bract tip shape

  • NA
  • the tips of the bracts are truncate (end abruptly in a more or less straight line as though cut off)

  Bracts

  there are at least two distinct forms of bracts in different cycles

  Disk flower color

  • blue to purple
  • pink to red

  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 only one to three flower heads on it

  Flower head outer flowers

  at the outer edge of the flower head, each flower has a single enlarged lobe or strap

  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

  Flower type in flower heads

  the flower head has disk flowers only, and lacks the strap-shaped flowers

  Height of flower head base

  15–20 mm

  Inflorescence branching (Solidago)

  NA

  Inflorescence shape

  • NA
  • 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 beak

  there is no beak on the ovary

  Ovary 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 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)

  Peduncle length

  At least 0 mm

  Ray flower color

  NA

  Ray flower reproductive parts

  NA

  Ray flowers

  0

  Reproductive system

  all the flowers on the plant contain both carpels and stamens

  Scale tip

  NA

  Smaller bracts at base of bracts

  there is no smaller, outer cycle of bracts

  Style branch number

  the style has two branches

  Width of flower head base

  7–15 mm

• Fruits or seeds

  Number of pappus parts

  0

  Ovary length in developed fruit

  2.5–3.4 mm

  Seed hair tuft details

  • NA
  • the pappus hairs are hooked or barbed

  Seed hair tuft length

  At least 0 mm

  Seed tuft type

  • the pappus is made of very fine hairs or bristles
  • there is no pappus on the ovary

  Top of disk flower ovary

  NA

  Tuft or plume on fruit

  • NA
  • at least a part of the plume is made up of fine bristles

• Glands or sap

  Leaf blade glands

  the leaf blades have 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 annual, it lacks evidence of previous years' growth
  • the plant is biennial, it appears as either first year (non-reproductive) plants or second year plants with flowers or fruit
  • 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
  • the underside of the leaf is not hairy, or has very few hairs

  Hairs on upper side of leaf blade

  • the upper side of the leaf is fuzzy or hairy
  • the upper side of the leaf is not hairy, or has very few hairs

  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

  • NA
  • the leaf blade has tangled or woolly-looking hairs

  Leaf blade length

  Up to 200 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)
  • 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 acuminate (tapers to a long, thin point)
  • the tip of the leaf blade is acute (sharply pointed)

  Leaf disposition

  • the leaves are nearly similar in size, prominence of teeth, and length of stalks throughout the stem
  • the lower leaves are larger, toothier, and/or on longer stalks than the upper leaves

  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 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
  • the leaf has lobes that themselves have lobes, and these secondary lobes also have lobes; there may be more than three levels of lobes

• Place

  Habitat

  terrestrial

  New England state

  Massachusetts

  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

New England distribution and conservation status

From the dichotomous key of Flora Novae Angliae