Description from Flora of China
Herbs, subshrubs, or shrubs. Stems never twining. Leaves petiolate or sessile; leaf blade entire. Flowers 1 to few, pedicellate or sessile in leaf axils, or several in terminal spikes or capitula; bracts tiny. Sepals free, equal or subequal. Corolla rotate, funnelform, or salverform; limb ± entire to 5-lobed, midpetaline bands often pilose outside. Stamens included or exserted; pollen globose, rugate, not spiny. Disc cupular or absent. Ovary glabrous or pilose, 2-loculed; ovules 2 per locule. Styles 2, filiform, free or united basally, each 2-cleft; stigma filiform, terete or slightly clavate. Capsule globose or ovoid, usually 4-valved. Seeds 1-4, smooth or minutely tuberculate, glabrous.
About 100 species: all native to North America (S United States) and South America, with two widely naturalized in the tropical and subtropical regions of the eastern hemisphere; two species in China.
Van Ooststroom's (Meded. Bot. Mus. Herb. Rijks Univ. Utrecht 14: 1-267. 1934) monograph of Evolvulus is by and large accepted today.