Previously included in Cassia and still often sold as such, these species have been reclassified as Senna. Grown for their lavish show of yellow, five-petaled flowers that look something like those of potentilla. Blossoms are followed by seedpods that may create litter; to reduce pod production, prune lightly after bloom. Rangy, rank growers should also be cut back periodically to encourage more compact growth. Good for screens, massing, background plantings. Prefer well-drained soil.
S. alata. CANDLESTICK SENNA. Evergreen shrub. Zones CS, TS; USDA 9-11. Native to many tropical regions of the world. May reach 30 ft. tall and half as wide, but more likely to be 610 ft. tall and 34 ft. wide in gardens. Bright green leaves divided into 14 to 28 oblong leaflets, each 212 in. long. Golden yellow, inch-wide flowers in big, spikelike clusters appear from fall into winter. Prune hard after bloom.
S. artemisioides. FEATHERY CASSIA. Evergreen shrub. Zones LS, CS, TS; USDA 8-11. Native to Australia. To 35 ft. tall and wide, with attractive light, airy structure. Gray leaves divided into six to eight needlelike, 1-in.-long leaflets. Bears bright yellow, 34-in. flowers in clusters of five to eight in winter and spring, with bloom often continuing into summer. Heavy seed production. Very drought tolerant but looks better with moderate to regular water.
S. bicapsularis. CHRISTMAS SENNA. Evergreen shrub. Zones LS, CS, TS; USDA 8-11. Native to tropical Central and South America. Recovers after being killed to ground by frost. To 10 ft. tall and wide. Bright green leaves with six to ten roundish, rather thick leaflets, each to 34 in. long. Bright yellow, 12-in.-wide flowers in spikelike clusters from midautumn to midwinter or until cut down by frost. Prune severely after flowering. Resembles S. pendula glabrata, which can be quite invasive; buy from a trusted source to be sure you are getting the true S. bicapsularis.
S. corymbosa. FLOWERY SENNA. Evergreen shrub. Zones LS, CS, TS; USDA 8-11. Native to South America. Rangy growth to 10 ft. tall, 1012 ft. wide. Dark green leaves with six narrow, oblong, 1- to 2-in. leaflets. Rounded clusters of 112-in. bright yellow flowers, spring to fall. Self-sowing can be a problem.
S. lindheimeriana. VELVET-LEAF SENNA. Perennial. Zones LS, CS, TS; USDA 8-11. Native to Texas, Arizona, Mexico. Usually slender and upright to 23 ft. tall, but may reach 6 ft. tall, 2 ft. wide. Velvety gray leaves are divided into 8 to 16 narrowly oval, pointed leaflets, each 12 in. long. Rich yellow, 112-in.-wide flowers in late summer and fall. Grows well in caliche soils. Not a good choice for high-rainfall areas or heavily watered gardens; it will rot if soil is too wet.
S. marilandica. WILD SENNA, MARYLAND SENNA. Perennial. Zones US, MS, LS, CS; USDA 6-9. Native from Pennsylvania to Florida, west to Iowa, Kansas, and Texas. Slender and usually unbranched; grows 46 ft. tall, 23 ft. wide. Feathery bright green leaves are divided into 8 to 16 oval, inch-long leaflets. Tall clusters of brownish yellow flowers top the stems in summer. Dies to ground in winter.
S. splendida. GOLDEN WONDER SENNA. Evergreen shrub. Zones CS, TS; USDA 9-11. Native to Brazil. To 912 ft. high and 610 ft. wide, with bright green leaves divided into four oval, pointed leaflets to 3 in. long. Deep golden yellow, 112-in.-wide flowers come in loose clusters at branch ends from autumn into winter. This common name has been applied to a number of sennas of varying growth habits; other golden wonder sennas have bright yellow flowers and are strongly horizontal in branch pattern, growing 58 ft. high and spreading to 12 ft. wide. All sennas known by this name produce many seeds and should be severely pruned after flowering.
S. wislizeni. CANYON SENNA. Deciduous shrub. Zones LS, CS, TS; USDA 8-11. Native to West Texas, New Mexico, Arizona, and Mexico. To 58 ft. tall and 510 ft. wide. Attractive upright branches hold small green leaves divided into four to six leaflets, each 1 14 in. long. Clusters of bright yellow, inch-wide flowers appear all summer. Little water.