pronounced: krip-toh-STEE-jee-uh gran-dih-FLOR-uh
Cryptostegia comes from the Greek κρυπτω (krypto), to hide, and στεγη (stegé), a roof, covering – hidden covering; grandiflora is Latin: grandis, large, and flos, a flower.
This is a many-stemmed shrub that can climb 30 m into tree canopies, or grow 1–3 m high when unsupported in open areas. The stems are greyish brown with a smooth bark and have two forms: a leaf-bearing branched stem and a longer unbranched ‘whip’ with fewer leaves, that extends on to adjacent vegetation. The plant exudes a milky sap if scratched or broken.
The leaves occur in pairs and are a glossy dark green. They are elliptic, 6 – 10 cm long and 3 – 5 cm wide. The trumpet-shaped flowers are quite large, up to 5 cm long and wide, with 5 light purple to white petals. The seed pods are rigid, and usually occur in opposing pairs at the end of a short stalk, but are quite common as single pods and occasionally as triple pods. The pods are up to 12 cm long and 4 cm wide. The brown seeds are flat with a tuft of long white silky hairs at one end. The roots can grow down to a depth of about 12 metres.
This native of south-west Madagascar is probably the worst weed in Australia, because of its invasiveness, potential for spread, and its economic and environmental impacts. It was introduced here into the mining towns of Queensland, and to many other tropical areas of the world, by man, because of its attractive flowers and the fact that its latex contains commercial quality rubber – hence the common name. It has impacted severely on pastoral and conservation areas of north-eastern Australia. Its main impact on the pastoral industry is the loss of grazing country, which in 1995 was estimated to cost the Queensland beef industry $18 million a year. It also increases the costs of mustering and fencing. It threatens waterways, woodlands and rainforest, including significant conservation areas such as the Wet Tropics World Heritage Area and Cape York. It also severely threatens riverine vegetation, and can replace the plants and animals that inhabit river banks, thereby affecting the water quality of streams. The whole ecological integrity of native vine thickets and riverine systems of northern Australia is under threat from this vine.
About 95% of the seed produced by rubber vine is viable. It is scattered short distances from the parent plant by wind that catches the tufts on the seed ends, or longer distances by floating on floodwaters. Most seed remains viable even after the pods have floated on fresh or salt water for over a month, potentially leading to spread between catchments. Seed can also be spread by birds, or in mud attached to vehicles, machinery and animals. Control by biological agents has proved ineffective. Rubber vine rust (Maravalia cryptostegiae) brought from Madagascar has infected most of the plants in some areas near Charters Towers, but the effect has not been great enough to stop the spread of the plant westwards. The rubber vine moth (Euclasta whalleyi) has also had very little effect. Present efforts aim at containing the plant within a containment line. The idea is to highlight infested areas and help plan control efforts. Strategies inside the containment line are focused on managing its impact. Any infestations outside the containment line are targeted for control to prevent its spread into new areas. There are currently two such containment lines in Queensland, shown in red on the sketch map. The plants pictured are behind the Picnic Bay swimming net, and in the vine scrub on the Nelly Bay foreshore.
As well as the rubber vine moth larvae, the caterpillars of Walker's Frother Amerila rubripes also feed on the plant.
