Bengal Clock Vine
Thunbergia grandiflora
Bengal Clock Vine (Thunbergia grandiflora) is a spectacular tropical evergreen vine from India that produces some of the largest flowers of any Thunbergia species — stunning, trumpet-shaped blooms up to 8 cm across in shades of lavender-blue to violet-blue with a pale yellow throat. The sheer size and abundance of the flowers make this one of the most impressive tropical vines, but it has become a serious INVASIVE species in some tropical regions.
• Produces enormous, trumpet-shaped, lavender-blue flowers up to 8 cm across — the largest flowers in the genus Thunbergia
• Blooms prolifically from summer through fall in flushes
• A vigorous evergreen vine reaching 6–10+ m in tropical conditions
• The flowers are a stunning lavender-blue to violet with a pale yellow throat
• INVASIVE in some tropical regions including parts of Australia, Hawaii, and tropical Asia
• Named "Clock Vine" because the flower buds are twisted like the spiral of a clock spring
• Also called "Skyflower" for its beautiful blue blossoms
• Hardy from USDA Zone 10
Taxonomy
• Named after Carl Peter Thunberg (1743–1828), the Swedish botanist and student of Linnaeus who explored South African and Japanese flora extensively
• The species name grandiflora means "large-flowered" — an accurate description of its enormous blooms
• The common name "Clock Vine" refers to the twisted flower buds, which spiral like a clock spring before opening
• Also known as "Skyflower" and "Blue Trumpet Vine"
• Widely planted throughout the tropics as an ornamental vine for its spectacular flowers
• Has naturalized and become invasive in some tropical regions — particularly problematic in the wet tropics of northeastern Australia (Queensland), Hawaii, and parts of Southeast Asia
• In Australia, it is classified as a Class 3 declared weed under Queensland legislation
• The vine's aggressive growth allows it to smother trees and native vegetation in suitable tropical conditions
• Has been used in traditional Ayurvedic medicine in India for skin diseases and joint pain
• The genus Thunbergia contains approximately 100 species, primarily from Africa and tropical Asia
Stems: Stout, woody, green to grayish, quadrangular (square in cross-section), twining vigorously around supports. Young stems are green and slightly hairy. Mature stems become thick and woody.
Leaves: Opposite, broadly ovate to cordate, 10–20 cm long and 8–15 cm wide, dark green, slightly hairy, with coarsely toothed margins and a pointed to heart-shaped base. Leaves are large and bold, creating a tropical texture.
Flowers: The magnificent feature — large, trumpet-shaped, 6–8 cm across, with 5 broad, rounded petals in shades of lavender-blue, pale violet-blue, to deeper violet, with a conspicuous pale creamy-yellow throat. The flowers are produced in pendulous, axillary cymes of 2–6 blooms. The buds are tightly twisted before opening (like a clock spring, hence "clock vine"). Flowers are short-lived but produced in abundance from summer through fall.
Fruit: Beaked capsules containing several seeds. In tropical regions, self-seeds aggressively.
Roots: Extensive root system. Can spread by root suckers, contributing to its invasive potential.
• Thrives in warm, humid, tropical conditions — requires temperatures above 10°C
• Best in full sun to partial shade — flowers most prolifically with at least half a day of sun
• Prefers moist, fertile, well-drained soils rich in organic matter
• In tropical regions, the vine is extremely vigorous and can grow 3–5 m in a single season
• INVASIVE in northeastern Australia (particularly the Wet Tropics of Queensland), Hawaii, and parts of Southeast Asia and Central America
• The vine's rapid growth allows it to smother canopy trees, blocking sunlight and killing the host tree
• Spreads by wind-dispersed seed and vegetatively by stem fragments and root suckers
• Control in invasive situations requires cutting vines at the base, treating stumps with herbicide, and follow-up monitoring
• Should NOT be planted near native bushland in tropical regions
• In cooler or drier climates, the plant is much less aggressive and can be safely grown
Site Selection: Full sun to partial shade. Plant where the stunning blue flowers can be admired — on a large pergola, strong fence, or substantial trellis. Ensure the support is sturdy enough for a heavy, woody vine.
Soil: Fertile, moist, well-drained soil enriched with organic matter. pH 5.5–7.0.
Planting: Plant container-grown vines in spring. Space 2–3 m apart. Water well during establishment.
Container/Greenhouse: In temperate zones, grow in a large container with a trellis in a warm greenhouse or sunroom. Use a rich, well-draining potting mix.
Watering: Regular, deep watering during the growing season. Keep soil consistently moist but not waterlogged. Reduce water in winter.
Fertilization: Feed monthly during the growing season with a balanced or bloom-boosting fertilizer.
Pruning: Prune after flowering. Cut back by up to half to control size. Can be cut back hard to a main framework.
Hardiness: USDA Zones 10–12. Damaged below 5°C. Killed by frost.
Invasive Caution: Do not plant in the wet tropics of northeastern Australia, Hawaii, or other tropical regions where this species is known to be invasive. Choose alternative vines in these areas.
Propagation: By semi-hardwood cuttings in summer, layering, or seed.
Fun Fact
Bengal Clock Vine produces flowers so enormous and vibrantly blue that they look almost artificial — up to 8 cm across in shades of lavender-blue that seem to glow against the dark green foliage. The buds spiral tightly like a clock spring before unfurling, giving the plant its "clock vine" name. But in the wet tropics of northeastern Australia, this beautiful vine has become a monster — it grows up to 20 meters into the rainforest canopy, smothering native trees under a blanket of leaves and eventually killing them. The plant is now classified as a declared pest in Queensland.
Learn more
