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Key to Centaurea

Asteraceae

Centaurea

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1 Phyllaries evidently spine-tipped.
  2 Leaf base not decurrent on the stem, the stem merely angled; pappus absent; corolla pink-purple.
    3 Central spine of the principal phyllaries 10-25 mm long
    3 Central spine of the principal phyllaries 1-3 mm long
  2 Leaf base decurrent on the stem as wings (only shortly so in C. benedicta); pappus present in at least the central flowers in the head; corolla yellow.
      4 Heads sessile, closely subtended and partially concealed by large foliar bract
      4 Heads obviously pedunculate, lacking large foliar bract subtending the head.
        5 Larger spine of the middle and outer phyllaries 5-9 mm long; marginal and central flowers of the head with pappus
        5 Larger spine of the middle and outer phyllaries 11-22 mm long; marginal flowers of the head lacking pappus
1 Phyllaries not spine-tipped.
          6 Plant an annual; flowers pale to medium blue, flowering Apr-Jun
          6 Plant a perennial; flowers pink to purple, flowering Jun-Oct.
             7 Phyllary appendage tapering to long, often recurved, pectinately dissected, filiform tips
             7 Phyllary appendage obtuse to acute, erect or ascending.
               8 Involucre 10-13 mm high
               8 Involucre 15-25 mm high.
                 9 Phyllary appendage evidently decurrent along phyllary margin
                 9 Phyllary appendage not or only slightly decurrent along phyllary margin.
                   10 Phyllary appendage roundish (seldom triangular), scarious, light to dark brown, undivided to irregularly lacerate
                   10 Phyllary appendage more-or-less triangular, brown to black, more-or-less wholly pectinate-margined.
                     11 Heads discoid (the peripheral floret not expanded and showy); pappus blackish, < 1 mm long; green parts of phyllaries nearly or completely covered by black appendage, the involucre thus appearing totally black
                     11 Heads radiate (the peripheral floret expanded and showy); pappus absent or rudimentary (when present usually not black); green part of phyllaries sometimes evident, or the appendage light to dark brown.
                       12 Heads relatively broad, the pressed involucre usually as wide as or wider than long; green parts of phyllaries usually covered by brown, variously pectinate fimbriate appendage, the involucre thus light to dark brown
                       12 Heads relatively narrow, the pressed involucre usually longer then wide; green parts of phyllaries not fully covered by black appendage, the involucre black and green