1 Axillary buds exposed above the petiole base (best observed in mature long-shoot leaves, not always visible in axils of young leaves or on short-shoot leaves); twigs of the current year villous-hirsute; seeds not caudate; style 1, clavate, 4-6 mm long; [calcareous and mafic sites]; [subgenus Deutzioides]

  2 Styles 4-6 mm long; leaf blades 2-8 cm long; [AR and eastwards]

  2 Styles 1.9-3.0 (-3.5) mm long; leaf blades (1.0-) 1.9-3 (-4.1) cm long; [c. TX and southwestwards]

1 Axillary buds contained within a distinct pouch directly below the petiole (best observed in mature, long-shoot leaves); twigs of the current year glabrous; seeds with caudate tails about as long as the embryo; styles 4, 6-16 mm long, the 4 branches 1.5-8 mm long; [ecologically more generalist collectively, including calcareous and mafic sites[; [subgenus Philadelphus].

    3 Flowers 1-3 (-9) in a cymule; stamens 60-90; styles 10-16 mm long; style lobes 0.8-1.0 mm wide

    3 Flowers 5-9 in a determinate raceme; stamens 20-50; styles 6-10 mm long; style lobes 0.3-0.9 mm wide.

      4 Bark of the current year brown, exfoliating in its second year; flowers fragrant

      4 Bark of the current year gray, not exfoliating later; flowers not fragrant or only slightly so