Thunbergia is a genus of vines in the family Acanthaceae. Several of the species are widely grown for their flowers. They are grown as perennials in tropical regions, or as annuals in cooler climates. The most commonly grown species is Thunbergia alata, the black-eyed susan vine. These flowering plants can rapidly grow to 8 ft (2.4 m) in length, and generally have 1 in (2.5 cm) orange flowers with dark centers, along with 3 in (7.5 cm) leaves shaped like arrows or hearts.
Black-eyed susan is also the name of the plant rudbeckia. The thunbergia black-eyed susan vine is commonly known for its orange flowers, but there are other cultivars with flowers ranging in color from red to bright yellow to white. The plants are frequently grown on arbors or trellises, on which they grow by twining. Alternatively, they are also grown in hanging baskets.
Thunbergia alata prefers full sun or part shade. If the night temperature is above 60 F (15.6 C), they will bloom year round. They are more likely to produce profuse flowers if grown in soil that is well drained and moist. The plants bloom from late spring into the fall.
Plants can be started from seeds or cuttings, but are usually more vigorous when started from seeds. They will self-sow in warmer climates. The seeds can be planted indoors or outdoors, but some horticulturalists recommend starting them inside. They generally take 10 to 21 days to germinate if grown in warm, moist soil. The young black-eyed susan seedlings quickly grow to become full-sized plants.
Another species of thunbergia that is popular as a horticultural plant is Thunbergia grandiflora, also known as clockvine or sky flower. This twining vine is also of tropical origin, and is grown as a perennial in warmer regions and as a greenhouse plant in cooler areas. Clockvine has 2.5 to 3 in (6.3 to 7.5 cm) tubular, bright blue or violet flowers with 8 in (20 cm) heart-shaped leaves. This type of thunbergia prefers some shade during the summer in tropical climates.
Clockvine takes a year to get established, and then it grows rapidly if in moist soil. The plant can be aggressive and rapidly cover a trellis or fence. Thunbergia grandiflora can grow to be greater than 20 ft (6 m) long. The vine may require pruning to keep it within bounds, particularly in climates such as that of Hawaii. If frozen, it can take a year to recover.