The genus Thunbergia is made up of about 200 species from warm areas of central and southern Africa, Madagascar, and Asia. The genus is named after the 18th century Swedish botanist and explorer, Carl Peter Thunberg. Thunbergia is a twining perennial vine that is often cultivated for its attractive flowers.
Black-eyed Susan vine is native to tropical eastern Africa, and is widely cultivated and naturalized in other tropical regions of the world, where it can eventually climb to 20 feet. As an annual in the colder zones, it will grow from 3-8 feet tall in a single season.
The orange-yellow or white solitary flowers on long peduncles up to 2 inches long, have 5 spreading petal lobes and dark purple throats. Ovate-triangular toothed green leaves up to 3 inches long.
Blooms in summer to fall, but best bloom is often in late summer and fall after the hot summer temperatures moderate. They will bloom all year long if night temperatures are above 60° F. and they get full sun in the winter months.
The plants are not very drought tolerant and should be watered regularly for optimum growth. Fertilize every other week with a balanced fertilizer diluted to ½ the strength recommended on the label. Occasional trimming helps keep the plant growing vigorously and helps to promote more blooms. No serious insect or disease problems. Watch for scale, spider mites and whiteflies on indoor plants.
Black-eyed Susan vines are cottage garden favorites. They look good in window boxes, mixed containers or climbing on trellises, arbors, fences or other structures around the home, and as a groundcover. Also effective in hanging baskets where the vine can twist around the basket supports or in patio containers with a small trellis.
The vines can grow to five or six feet high if given good support. Clock vines don't bloom well during high summer heat and humidity, but they will make up for lost time when the weather cools off, blooming well into October.
Sow seeds indoors 6-8 weeks before the last frost date, for earlier blooms. Cover the seed with about one fourth inch of growing media. Germination takes 10 to 21 days at 70-75° F., and seedlings grow very slowly. Wait until the soil has warmed up and night temperatures remain above 50° degrees to transplant outdoors.
Place transplants six inches apart about three inches from a support. Supports will be covered quickly after the plants become established. Soil should be moist and high in organic matter. Sunny mornings and afternoon protection are appreciated.
They can be easily grown from seed sewn directly in the garden after last frost date, or may be grown in containers that can be overwintered indoors in a warm sun room. Plants grown from seed are more vigorous than those from cuttings.