1 Woody vine, climbing by adventitious roots; petals 7-10; [section Decumaria of clade Hydrangea I; or genus Decumaria]

1 Shrub; petals 4-5.

  2 Leaves pinnately lobed, the lobes toothed; inflorescence a panicle; large sterile flowers many (> 20 per inflorescence), borne throughout the inflorescence; [section undetermined (“incertae sedis”) of clade Hydrangea I; or genus Hydrangea s.s.]

  2 Leaves unlobed, merely toothed; inflorescence a corymb (except H. paniculata); large sterile flowers absent to relatively few (0-15 per inflorescence), borne around the periphery of the corymb (except H. paniculata).

    3 Inflorescence a panicle; large sterile flowers many (> 20 per inflorescence), borne throughout the inflorescence; [large shrub to small tree, to 8 m tall and 10 cm trunk diameter]; [exotic, cultivated and sometimes persistent]; [section Heteromallae of clade Hydrangea I; or genus Heteromalla]

    3 Inflorescence a corymb; large sterile flowers absent to relatively few (0-15 per inflorescence), borne around the periphery of the corymb; [small to medium shrub, to 3 m tall and 2 cm trunk diameter]; [native]; [section Hydrangea of clade Hydrangea I; or genus Hydrangea s.s.].

      4 Lower leaf surface variously pubescent, appearing white or gray; trichomes of the lower leaf surface on veins and interveinal areas; sterile flowers usually present, large and showy, usually greater than 1 cm in diameter.

        5 Lower leaf surface velutinous, pilose, or tomentose, appearing gray; trichomes usually not dense enough to entirely mask the green leaf surface; trichomes with prominent tubercles (as seen at 40× magnification); sterile flowers generally very few (0-3 per inflorescence)

        5 Lower leaf surface densely floccose-velutinous, felt-like, appearing bright white or silver; trichomes dense enough to entirely mask the green leaf surface; trichomes without tubercles, or with small and inconspicuous tubercles (as seen at 40× magnification); sterile flowers generally fairly many (2-15 per inflorescence)