Sky flower (Thunbergia grandiflora) packs a late summer color punch just when our gardens desperately need one. In late July or early August, just as the crapemyrtle blossoms start to fade and zinnias begin to melt away, this vine produces glorious clusters of 3-inch-wide, periwinkle-blue flowers. As if caught in a perpetual yawn, these bell-shaped blossoms show off creamy white or buttery yellow throats.
If you’re already familiar with sky flower’s little cousins, black-eyed Susan vine (T. alata) or orange clock vine (T. gregori), then you already know about this genus’s propensity to twine. Give any thunbergia some sort of support, and with a smidgen of training, it will cheerfully crawl, twist and scamper up anything in its monomaniacal quest for the sun.
What makes this group exceptional among vines is that they twine clockwise. Most “spin” in the opposite direction, and the tropical twining plant Loasa is unique because it coils both ways. Some say thunbergia’s unusual directional trait is how sky flower (also known as Bengal clock vine) and its tangerine-colored relative orange clock vine got their common names.
Sky flower is often considered an annual vine, but is some areas of the South it isn’t always. This native from India naturally gets a kick out of hot and humid weather, and doesn’t give a hoot about an occasional cold snap. It often survives in parts of Zone 7 and much of Zone 8.
However, expect it to die back after the first hard frost, and before cutting it to the ground, harvest seed pods as backup insurance for next season. Be patient the following spring. This vine is a slow starter, but just about the time you’ve either forgotten about it or have given up hope, sky flower should begin a full-throttle skyward ascent, usually sometime around late June or early July.
In addition to its acrobatic ability to twine on itself, sky flower’s 6- to 8-inch-long, coarse-textured leaves add interest to the garden while you’re waiting for the vine’s late-summer bloom. Keep in mind that because sky flower is such a robust climber, it needs strong support. Brick walls, wrought iron fences and arches fill the bill, as well as sturdy wooden fences. Also, don’t rule out well-built pergolas, especially if you want shade in the summer but sunshine during the winter.
Sky flower nonchalantly grows to 50 feet in length in its homeland. However, expect it to grow slightly less than that in most Southern gardens. Of course, anything this robust needs nourishment, and sky flower is a heavy feeder. It thrives in loamy, organic-rich soil and demands regular watering. It also requires full sun and will sulk if exposed to partial shade, especially in early spring when it’s trying to work up to its phenomenal growth speed.
If there’s a problem with sky flower, it’s our fault. Commercial growers hesitate to cultivate it because it blooms so late in the season, yet the best time to plant one is in the early spring. People often aren’t willing to purchase a few stems and leaves in March or April when garden centers are bursting with colorful, already-blooming plants for our immediate gratification. Unless it’s a showy bougainvillea that looks fabulous dangling in a pot, no one wants to buy a vine at the beginning of a growing season.
While there are plant farms that occasionally grow limited numbers of sky flower (there’s even a white version named ‘Alba’ that sometimes shows up), if you really have your heart set on trying to raise this beauty your best bet is to find someone who’s willing to share cuttings or seeds with you. Be forewarned that seeds often have slow to irregular germination times of 14–21 days. Keep the medium slightly moist, just covering the seed with potting soil, and maintain temperatures between 70 to 75 F.