Blue trumpet vine. It is also known as blue thunbergia, or Bengal trumpet.

Thunbergia grandiflora. It was previously known as Thunbergia laurifolia. It is a member of the Acanthaceae family.

Asia, Africa, North, South and Central America, the Caribbean, Europe (Portugal), Oceania. It is recorded from Australia, Federated States of Micronesia, Fiji, Guam, Nauru, New Caledonia, Palau, Samoa, Solomon Islands, and Vanuatu. It is native to India and tropical Africa.

A climbing vine introduced and cultivated in many countries as an ornamental, but which often becomes an aggressive invasive weed, blanketing native vegetation. It invades pastures, roadsides, banks of creeks and rivers, and its impenetrable stands make it a major threat to remnant areas of tropical forests.

Vines up to 15 m, producing large tubers (up to 70 kg) (Photo 1). Stems are square when young. Leaves, variable in shape and size (but often arrow-shaped), up to 15 cm long and 10 cm wide, opposite along stems, on stalks 4-12 cm long, with several, small, pointed outgrowth from the margins, especially towards the base (Photos 2-4). Flowers are on stalks, 4-5 cm long, pale blue, violet or mauve, trumpet-shaped, 3-8 cm long and 6-8 cm across, formed from five rounded petals, one larger than the others, each with a yellow or whitish throat (Photos 2-4). There are two leaf-like bracts at the base of each flower. Fruits are cone shaped, 3-5 cm long. Seeds are flat, up to 1 cm long, and are propelled from the fruit for several metres when it splits.

Spread over short distances is by seed. Over longer distances tubers, stems and roots are moved by humans, livestock, vehicles, and water. Blue trumpet vine has been introduced throughout the tropics and subtropics as an ornamental, and then it has escaped from cultivation, accounting for its worldwide distribution.

Thunbergia grandiflora is one of the most problematic invasive plants. It can cover 100% of the ground over several hectares, excluding all native plants. In Australia, for instance, it not only blankets native vegetation (often pulling down mature trees), but its large tubers are responsible for damaging riverbanks, paths, fences, and foundations. 

Used extensively as an ornamental for gardens, often to cover fences. There is evidence of its use as a folk medicine in Asia, and as a green manure in Africa.

BIOSECURITY
The chances of introduction of blue trumpet vine are high as it is commonly used as an ornamental plant in the subtropics and tropics. Countries not yet infested should consider all likely pathways for entry and apply quarantine measures accordingly. Particular attention should be given to the fact that introductions as an ornamental may escape cultivation with ease. It is readily available on the Internet.

It is listed as an invasive species in Central and North America (Hawaii), the Caribbean, parts of Africa, and Pacific islands, including Fiji, French Polynesia, and Samoa. In Australia, it is a category 3 restricted invasive plant under the Biosecurity Act 2014. It must not be given away, sold, or released into the environment.

Thunbergia grandiflora is on the Global Invasive Species Database (2023) of information on alien and invasive species that negatively impact biodiversity, managed by the Invasive Species Specialist Group of the IUCN Species Survival Commission: (http://www.iucngisd.org/gisd/speciesname/Thunbergia+grandiflora).

BIOLOGICAL CONTROL
Not a method recommended for Thunbergia grandiflora.

CULTURAL CONTROL

Physical & Mechanical:

• Hand-pulling or digging out is possible only for isolated plants or small plantings. Established plants have extensive underground roots and tubers.
  • ensure all stem fragments, roots and tubers are removed; collect and burn them, otherwise they may resprout if left on or in the soil.
• Do not use topsoil from places where blue trumpet vine has been growing. Movement of soil from such places is a major method of spread of this weed.