Cultivated (and escaped) Philadelphus, also known as “syringa” (also the generic name of the lilac, in the Oleaceae), are often cultivars or hybrids and naming them may be difficult.
1. Stamens 60–90; styles (including the lobes) 9–16 mm long, the lobes 0.8–1.4 mm wide; flowers mostly 1–3 in cymes or racemes, occasionally more.
1. Stamens 25–50 (less if flowers are “double”); styles 4–10 mm long, the lobes 0.3–0.9 mm wide; flowers mostly 5–9 in racemes or panicles.
2. Leaf undersurfaces glabrous or sparsely hairy with usually appressed-ascending hairs, and sometimes villous in main vein axils; hypanthium and back of calyx lobes glabrous or sparsely villous; bark usually reddish brown, soon exfoliating in flakes or strips.
2. Leaf undersurfaces moderately to densely pubescent with usually twisted hairs; hypanthium and backs of calyx lobes moderately to densely strigose or villous (rarely glabrous or nearly so); bark usually gray, tight.
3. Flower whorls normal (petals 4, stamens 25–50).
3. Flowers “double,” many of the stamens modified into petaloid staminodia that grade from the inside to the outside of the flower from very narrow (nearly stamen-like) to broad (petal-like) staminodia (petals and petal-like staminodia usually more than 16).
All species found in Philadelphus
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. June 17, 2019. https://michiganflora.net/genus.aspx?id=Philadelphus.