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Blue trumpet vine (Thunbergia grandiflora)

Blue Trumpet Vine is a vigorous introduced climber and a major threat to remnant vegetation in the Wet Tropics. It can smother native vegetation and remains a weed of potential national significance, although it is not a Weed of National Significance.

Key facts

Type: Weed, vine and creeper.

Native to: India.

Established in: QLD, NSW, WA, NT.

Other names: Blue Thunbergia, Bengal Clock Vine, Sky Flower, Blue Sky Flower.

Best time to act: Peak flowering is from September to December, although flowers may be seen outside these months.

Identification

  • It is a vigorous, scrambling vine with tuberous roots.
  • Its opposite leaves are usually 4–5 lobed, with leaf blades 3–12 cm long and 3.5–14 cm wide.
  • Near branch tips, 2–4 flowers grow at each node, usually pale to mid-blue with a paler throat and darker lines on the lower lip.
  • Flowers can be up to 8 cm across, and are enclosed at the base by a pair of leafy bracts covered in velvety hairs.
  • The fruit is a capsule with a rounded base about 1.5 cm wide and a beak up to 2 cm long.
  • Look-alike: Laurel Clock Vine (Thunbergia laurifolia) is very similar; its leaves are unlobed, while Blue Trumpet Vine leaves are lobed.
  • Look-alike: Thunbergia arnhemica and T. fragrans have white flowers, while T. alata has orange flowers with a black throat.

Where it grows

In Australia it occurs sporadically along Queensland's eastern coastline, particularly densely around Cairns, in places including watercourses, forest margins, open woodlands, roadsides, fence-lines, gardens and plantation crops.

Why it matters

  • It is a major threat to monsoon vine thickets and remnant tropical rainforests across northern Australia.
  • It climbs tall trees and spreads across lower vegetation, smothering the plants that support it, often native species.
  • The weight of the vine may cause mature trees to fall.

How it spreads

  • It spreads by seeds, cuttings, and fragments of stems and roots; dumping garden cuttings is considered the most likely means of spread.
  • New plants can grow from stem nodes and tuberous roots, which can become as large as a small car.
  • Seed production is described as the least effective means of dispersal.

Control options

  • Act early, because established plants develop extensive underground tubers that are difficult to remove.
  • Small plants may be dug out, but cutting vines at ground level is usually ineffective because plants regenerate from their tubers.
  • Herbicide spraying or injection has provided effective control, but one application is unlikely to achieve complete eradication; use registered herbicides and seek advice from your state or territory weed management agency.
  • Monitor treated areas and follow up as needed.

Hygiene and washdown

  • Do not dump garden waste or soil containing any part of the plant, as cuttings and stem or root fragments can establish new plants.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Restricted matter, category 3. Do not give it away, sell or release it into the environment. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources