The most recent systematic treatment of the genus was authored by Elizabeth McClintock and was published on November 5, 1957 in the "Proceedings of the California Academy of Sciences", Fourth Series, Vol XXXIX, No. 5, pp. 147-256, with 10 maps & 6 plates. It was published by The Academy in 1957 in San Francisco while Elizabeth McClintock was Assistant Curator of Botany at the California Academy of Sciences. The taxonomic hierarcy below is derived from Dr. McClintock's monograph.

Section Hydrangea

• Habit usually erect
• Foliage deciduous
• Leaf texture chartaceous
• Floral bracts usually lanceolate

Subsection Americanae - seeds not caudate

• Species H. arborescens-inflorescence a flat topped cluster, leaves not lobed
  • subspecies 'arborescens'
  • subspecies 'discolor'
  • subspecies 'radiata'
• SpeciesH. quercifolia-inflorescence a pyramidal cluster, leaves pinnately lobed

Subsection Asparae - seeds caudate

• Species H. sikokiana - leaves pinnately lobed
• Species H. involucrata - bracts broadly ovate, capsules pubescent
• Species H. aspera - bracts lanceolate, capsules glabrous
  • subspecies 'aspera'
  • subspecies 'robusta'
  • subspecies 'strigosa'
  • subspecies 'Sargentiana'

Subsection Calyptranthe - seeds with rounded wing, petals falling together as a cap or calyptra, plants scandent, aerial rootlets

• Species H. anomala
  • subspecies 'anomala'
  • subspecies 'petiolaris'

Subsection Petalanthe - seeds not caudate, styles in fruit longer than conical disk of capsule

• Species H. hirta
• Species H. scandens
  • subspecies 'scandens'
  • subspecies 'liukiuensis'
  • subspecies 'chinensis'
  • subspecies 'kwangtungensis'

Subsection Hetermallae - seeds caudate, styles in fruit shorter than conical disk of capsule

• Species H. paniculata
• Species H. heteromalla

Subsection Macrophyllae - ovary inferior in anthesis, becoming half superior in fruit

• Species H. macrophylla
  • subspecies 'macrophylla'
  • subspecies 'serrata'
  • subspecies 'stylosa'
  • subspecies 'Chungii'

Section Cornidia

• Habit usually scandent
• Foliage evergreen
• Leaf texture coriaceous
• Floral bracts rounded

Subsection Monosegia - inflorescence single terminal rounded cluster; sterile present or absent

• Species H. Seemannii
• Species H. integrifolia
• Species H. Steyermarkii
• Species H. asterolasia
• Species H. Preslii
• Species H. Oerstedii
• Species H. peruviana
• Species H. diplostemona

Subsection Polysegia - inflorescence series of rounded clusters one above another; sterile flowers absent

• Species H. tarapotensis
• Species H. Jelskii
• Species H. Mathensii
• Species H. serratifolia

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