Stems 1–3 m long, erect to more commonly arched-spreading, the branches often stiff, the bark gray to dark gray or reddish brown, often flaking or peeling in small plates or thin strips, leaving a tan to gray surface exposed. Twigs gray to reddish brown, glabrous or sparsely hairy, the axillary buds enclosed in the hollow petiole bases. Leaves short-petiolate. Leaf blades 1–9 cm long, 0.5–5.0 cm broad, lanceolate to elliptic or broadly ovate, narrowed or tapered to a usually sharply pointed tip, rounded or narrowed at the base, the margins with few (5–11 per side), widely spaced, fine or less commonly coarse, blunt or sharp teeth, sometimes entire or nearly so, the upper surface dark green, glabrous or sparsely hairy, the undersurface pale green, variously pubescent to nearly glabrous. Inflorescences small clusters or short racemes, each branch with (3–)5–9(–11) total flowers. Flowers showy, all similar and fertile. Sepals 4, ovate-triangular, tapered to a sharply pointed tip, the inner surface usually minutely hairy toward the tip, the outer surface variously glabrous or with longer incurved hairs, often densely and minutely hairy along the margins. Petals 4, 12–15(–20) mm long, elliptic-obovate, white. Stamens 20–45. Pistils usually of 4 carpels, these united nearly to the tip. Ovary inferior or nearly so, the hypanthium extending nearly to the broad tip of the ovary, 4-locular. Style 4-lobed to about the middle or only at the tip, the stigmas club-shaped to nearly linear. Fruits obconic, the surface smooth, not ribbed, dehiscing longitudinally. Seeds 2–3 mm long, narrowly oblong-cylindric, the body long-tapered to a slender wing, the other end truncate and with a small irregular crown, brown to dark brown. 50–70 species, North America, Europe, Asia.

Philadelphus is a difficult genus, with considerable taxonomic problems persisting in spite of a lengthy and detailed monograph (S. Y. Hu, 1954–1956). Species are wide-ranging and exhibit considerable morphological variation, and consequently can be difficult to identify. A number of species have long been cultivated and many of these originally were described from cultivated material. Two species are purported to occur in Missouri, the native P. pubescens, and a Eurasian taxon that has escaped from cultivation, P. coronarius. Both taxa belong to the sect. Stenostigma Koehne (S. Y. Hu, 1954–1956), and they can be thought of as New World and Old World analogs of the same species complex. In their native ranges, the “wild types” of the two apparently can be distinguished consistently by differences in pubescence of leaves and flowers, as well as bark color and flakiness. However, a number of cultivars, some possibly of hybrid origin, exist for both of the species, and these cultivars vary inconsistently for all of the features said to distinguish P. coronarius from P. pubescens. As far as can be determined from specimen labels, only the single population of P. pubescens from McDonald County reported by Steyermark (1963) is a native occurrence; all other specimens attributable to either species originated from the various cultivars that persisted at old home sites and eventually became naturalized in adjacent natural habitats. Accurate determination of these escapes has been equivocal. Because the characters of bark color and flakiness are difficult to interpret on herbarium specimens and do not seem to correlate well with other characters separating the species, the present treatment has arbitrarily used the characters in the key below to assign species determinations to the specimens vouchering the county distributions. Also, because the origin of various cultivars is poorly understood, no attempt has been made to categorize potential hybrids among the nonnative populations.

A wide variety of pollinators, including bees, flies, and butterflies, is attracted to the flowers, which secrete copious nectar and usually have a fragrance suggestive of orange blossoms. Although Philadelphus is cultivated as a hardy, disease-free ornamental, some gardeners object to the “scruffy” appearance of the vegetative shrubs.