Philadelphus is a genus of small, deciduous shrubs found throughout North America. Generally showing a rapid growth rate and attractive flowers, the genus belongs to the family Hydrangeaceae and consequently has much in common with the popular hydrangea shrubs. Most popular among the genus is the sweet mock-orange, the scentless mock-orange, and the hoary mock-orange.
The Philadelphus coronarius, or sweet mock-orange, is a large, hardy shrub that can survive winter temperatures down to -30°F (about -31°C). Like other members of the Philadelphus genus, the sweet mock-orange sheds its leaves in the winter and regrows them in the summer without displaying the colorful fall foliage of many deciduous plants. The sweet mock-orange follows the spring leaves with fragrant white flowers that cover the entire form of the rounded shrub. Within the species, there are a number of cultivars that present gardeners with variations on the basic plant; these include the yellow-leaf aureus and the dwarf nanus.
As its name suggests, the hoary mock-orange is a spindly member of the Philadelphus genus. It has the same white flowers as its fuller cousins, but the hoary mock-orange has thinner branches and leaves and grows taller. Fruits remain on the bush throughout the summer and fall, appearing after the flowers disappear.
The scentless mock-orange also has a name suggestive of its traits. This shrub sports showy white flowers, as other members of the Philadelphus genus do, but lacks their distinctive fragrance. Leaves are lighter on the bottom than they are on top, and hide reddish-brown bark. A relatively small shrub, the scentless mock-orange generally reaches a mature height of around 10 feet (about 3 m).
Traits shared throughout the genus include oval leaves that are green in the summer and simply fall in the autumn without turning brilliant colors first. Leaves are opposite, meaning they occur in pairs down thin branches arcing out from the thicker, main branches of the shrub. In most varieties of Philadelphus, they are simple leaves with non-serrated edges, although some cultivars can sport more complex leaves. Flowering generally occurs in the late spring and early summer, with small fruit capsules sectioned into four compartments that last through the autumn.
Hybrids of the species share many of the same traits, but have generally been created for one or two differences. Some, like the Virginal, have double flowers as opposed to the normal, single row of petals. As the scent of the Philadelphus shrubs is so attractive to so many, some hybrids have been created to be even more fragrant than the standard species.