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Thunbergia grandiflora Roxb. 1820
pronounced: thun-BER-ghee-uh gran-dih-FLOR-uh
(Acanthaceae – the black-eyed Susan family)
common names: Blue Trumpet Vine
Thunbergia was named for Carl Peter Thunberg (1743–1828), a Swedish naturalist who has been called ‘the father of South African botany’ and the ‘Japanese Linnaeus’. Grandiflora, from the Latin, means ‘big flower’.
This native of India is a garden escapee, once sold extensively, but now a serious environmental weed in many parts of Australia. Its vigorous growth makes it unsuitable for gardens, although it is common on walls and fences north from Sydney. There is a great deal of it on Magnetic Island: the spread photographed is in Magnetic Street, Picnic Bay.
This is a perennial twining vine with stems 20 m or more long. The root system is tuberous, spreading with a soft, deep, thick taproot. The branches are 4-angled, the opposite leaves variable, triangular to ovate, up to 20 cm long and 14 cm wide, broad-based narrowing to a pointed tip, usually with deeply scalloped lobes towards the base; the leaf edge may be entire, toothed or lobed, on a stalk up to 6 cm long.
The inflorescence is a drooping terminal raceme bearing trumpet-shaped flowers with a tube that is pale yellow inside and expands to five rounded pale lavender-blue petals, one larger than the others. There are four stamens, all fertile. The flowers are up to 8 cm long and 6–8 cm across. The seed pod is inconspicuous, cone-shaped, 3–5 cm long with a rounded base. The seed is flat, up to 1 cm long and covered with brown scales. The seeds are catapulted several metres when the ripe pods open. Most spread, apart from that by the seeds, is by root pieces washed down watercourses, or moved about by earth-moving equipment.
Thunbergia species are a major threat to remnant vegetation in the Wet Tropics. In the past, Thunbergia grandiflora and Thunbergia laurifolia were promoted and sold in Queensland as attractive garden plants, and both became widespread in Queensland gardens. These vigorous plants soon escaped into native bushland and began to cause considerable environmental damage. The plant climbs and blankets native vegetation, often pulling down mature trees with the sheer weight of the vine. When the vegetation is smothered, this also reduces the light to the lower layers of vegetation, which drastically limits natural growth, killing many native plants. The large tubers make eradication of the pest difficult. In garden situations it will also quickly spread, and the tubers may cause damage to paths, fences and foundations.
A pretty flower, but a nasty pest.
Photographs taken at Picnic Bay 2009, 2010
Page last updated 28th February 2017