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Key to Eupatorium

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1 Leaves generally in whorls of 3-7 (very rarely all of them opposite), most of them > 2 cm wide; involucre 6.5-9 mm high, the flowers pale pink to purple  Eutrochium

1 Leaves generally opposite, sometimes in whorls of 3-4 (if so the leaves usually < 2 cm wide), or some of them alternate; involucre mostly 2-6 mm high, the flowers mostly white, rarely blue (rarely the involucre 6-11 mm high, then the flowers white).

2 Leaves pinnate or pinnatifid, divided into linear or capillary segments, these 0-5 mm wide (or to 10 mm wide in Eupatorium ×pinnatifidum)  Key to Eupatorium, Key A: leaves pinnatifid or pinnate into linear or capillary segments (Dog-fennels)

2 Leaves simple or palmately 3 (-5)-lobed (if palmately lobed, the leaves or lobes > 5 mm wide).

3 Leaves palmately 3 (-5)-lobed  Eupatorium cannabinum

3 Leaves simple.

Show caption*© Scott Ward       4 Leaves long-petiolate, the petioles of larger leaves > 10 mm long.

Show caption*© Scott Ward         5 Leaf blades deltate or rhombic, held vertically; [of FL]  Eupatorium mikanioides

Show caption*© Aidan Campos         5 Leaf blades lanceolate, held horizontally; [widespread]  Eupatorium serotinum

4 Leaves sessile or short-petiolate, the petioles < 9 mm long.

6 Florets (3-) 5 (-7) per head  Key to Eupatorium, Key B: leaves simple, flowers usually 5 per head

6 Florets 7-14 per head.

7 Leaf bases fused  Eupatorium perfoliatum

7 Leaf bases tapering to a cuneate base

8 Leaves uniformly truncate, usually uniformly coarsely serrate, widest within the lower quarter  Eupatorium resinosum

8 Leaves truncate to cuneate (at least at top of stem), usually finely serrate on upper leaves, sometimes coarsely serrate on lower leaves, widest slightly below the middle  Eupatorium ×truncatum