THUNBERGIA alata (black-eyed susan) is the only species of this genus grown as a house plant. It is a fast-growing twining plant with attractive flowers. Although actually a perennial, it is usually treated as a temporary house plant, to be enjoyed during the flowering^ period—which normally lasts from late spring to late fall—and then discarded. Its leaves are tooth-edged, triangular to arrow-shaped, up to 3 inches long and wide, borne on slender stalks along the twining stems. Two-inch-wide flowers, which are produced on short stalks from the leaf axils, consist of an inch-long tube flaring out into five petallike lobes. Petal-lobe color varies, but in all forms there is a central deep chocolate brown "eye" (the entry point to the tube for insects); the tube itself is dark purple, and each flower is backed with a pair of |-inch-long, pale green bracts. In the most common form of T. alata, T.a. 'Aurantiaca,' the lobes are orange-yellow; in T.a. 'Alba,' white; in T.a. 'Lutea,' bright yellow.
Buy young thunbergia plants only a few weeks old in the spring. They usually twine around three or four thin sticks pushed into the edge of the pot but they will also climb up a string fixed to the side of a window. In either case, they should be cut down and discarded when flowering stops.
Light Thunbergias must have bright light with two to three hours a day of direct sunlight to flower properly.
Temperature These thunbergias will do well in any normal room temperature during their stay in the home. Nevertheless, they can tolerate much cooler conditions—down to 50°F.
Watering Water young plants moderately, enough to make the potting mixture moist at each watering, but
allowing the top half-inch of the mixture to dry out before watering again. As these swift-growing plants get bigger and begin to flower, they need more water. Throughout the flowering period water them plentifully, as often as necessary to keep the mixture thoroughly moist.
Feeding Give standard liquid fertilizer to flowering plants every two weeks throughout the year.
Potting and repotting Use a soil-based potting mixture. Young thunbergias should be moved on when they grow too big for their original pots. As soon as a number of roots begin to appear through the drainage hole in the bottom of a pot, move the plant into a pot two sizes larger. Probable maximum pot size needed: 6 inches.
Propagation Seed can be grown indoors without much difficulty if sown in early spring. Three seeds should be planted in a single 3-inch pot containing a moistened soil-based potting mixture. They will usually germinate in three to five weeks if kept in a warm room in a position where they get bright filtered light, and are watered enough to make the mixture moist, but with the top half-inch of the mixture allowed to dry out between waterings. The new plants will grow quickly and may be transferred to individual 3-inch pots and treated as mature thunbergias when they are 6 inches high.
Special points Be sure to remove faded flowers by nipping them out with the fingertips, if this is not done regularly, the flowering season will be needlessly brief.
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