1. Philadelphus coronarius L. (sweet mock orange)

Pl. 427 g, h; Map 1908

Twigs with the bark often reddish brown, commonly flaking or peeling by the second growing season. Leaf blades with the undersurface moderately to densely hairy along the main veins, glabrous or very sparsely hairy between the veins. Inflorescence mostly racemes with 5–7 flowers, sometimes appearing as clusters. Hypanthium glabrous or very sparsely hairy. Sepals 4–8 mm long, glabrous or very sparsely hairy on the outer surface, glabrous or densely and minutely hairy along the margins. Style 4–7 mm long at flowering, becoming slightly elongated by fruiting, 4-lobed only toward the tip, the stigmas 2–3 mm long. Fruits 6–8 mm long, 5–6 mm wide. 2n=26. May–June.

Introduced, uncommon and sporadic in Missouri (native of Europe, Asia; introduced in the eastern U.S. and adjacent Canada west to Minnesota and Missouri). Bottomland forests, mesic upland forests, and banks of streams and rivers; also roadsides, railroads, and alleys.

Philadelphus coronarius has been widely cultivated in Europe and the United States, for so long in fact that the exact area of wild origin for the introduced plants is not known. Many varieties and forms have been named and numerous cultivars have been developed, including several dwarf and double-flowered forms (S. Y. Hu, 1954–1956). The species was first reported for Missouri by Mühlenbach (1979) from his collections in the St. Louis railyards.

Another species of Philadelphus has been noted to persist at old homesites, but apparently has not spread from original plantings and thus is not yet recognized formally as a member of the Missouri flora. Philadelphus inodorus L. (scentless mock orange) is a southeastern species that has long been cultivated and has escaped sporadically throughout the eastern half of the United States. Hu (1954–1956) noted specimens from plants cultivated in Hannibal (Marion County) and the city of St. Louis early in the 20th century, and a recent collection from a National Guard training site in Newton County confirms the long persistence of the taxon at homesites. This species most likely will be recorded as naturalized in Missouri in the future. It differs from P. pubescens in its flowers with a glabrous to sparsely hairy calyx and hypanthium, and from P. coronarius in its clusters of 1–3 flowers (vs. 5–7) per branchlet. It also differs from both taxa in its scentless (vs. fragrant) flowers. Hu (1954–1956) treated P. inodorus as comprising several varieties, which are now considered trivial variants unworthy of formal taxonomic recognition by most botanists. The Missouri materials mostly seem to correspond to P. inodorus in the strict sense (var. inodorus of Hu).