It is a vigorous, perennial climbing vine, a blue wonder, commonly seen in many a garden. Now, however, there is evidence that it is an invader and though there are several species of thunbergia, here we will deal with the most invasive and widespread types, having been promoted as ornamental for the garden -- Thunbergia grandiflora and Thunbergia laurifolia.
Known as Bengal clockvine, Bengal trumpet or blue sky vine, these troublesome species have leaves of the same size but with different shapes. T. glandiflora has leaves which are broad at the base, narrowing to a pointed tip while T. laurifolia has oval-shaped leaves with edges that are not lobed.
Thunbergia takes its name from Swedish scientist Prof. Carl Peter Thunberg who had visited Japan and Sri Lanka during the time the country was under the Dutch and collected some plants for his studies. Thunbergia is believed to have originated in North India and Malaysia.
Thunbergia leaves form on opposite sides of the stem and are up to 15 cm long and 10 cm wide with pointed tips. The main reason for its popularity is its attractive flower which takes the shape of a trumpet with a short, broad tube. It is white on the outside with a yellowish throat and opens out into five rounded pale lavender-blue petals, one larger than the others up to 8 cm long and 6-8 cm wide.
The flower produces seed pods which are cone-shaped and 3-5 cm long. The seed is flat, up to 1 cm long covered in brown scales and when the ripe pod splits, it is catapulted several metres.
The thunbergia vine blankets native vegetation, often pulling down mature trees with its weight. It also drastically reduces light to lower levels of the smothered vegetation, limiting natural growth and killing many native plants.
The vine has extensive underground tubers, with some being as large as 70 kg.
A smothered tree will get a new lease of life if the thunbergia vine is cut at ground level, but the vine will regenerate if the tubers are not destroyed.
Thumbergia mainly reproduces vegetatively, when cuttings or fragments of stems and roots take root and send out new shoots. It is often spread through the careless disposal of garden waste or when contaminated soil is removed for landfills or other soil use. Infestations along riverbanks have been caused by root pieces breaking off and being transported downstream by floodwaters.
Plant scientists in Sri Lanka should be aware of thunbergia when assessing and formulating plans against invasive plant species, taking into account its destructive behaviour not only within the country but also in others with similar climatic conditions. This vine has been declared a noxious weed in tropical Queensland, Australia.
(The writer is an Invasive Plants Specialist in Melbourne, Australia)
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