Cryptostegia grandiflora

(Rubber Vine)

Rubber Vine belongs to an interesting Family of plants called milkweeds. Milkweeds (Asclepiadacaea) are so called because of their milky sap (latex). While most are perennials, Rubber Vine (Cryptostegia grandiflora) is a vigorous climbing shrub.

Originally from Madagascar, Rubber Vine's latex is a source of rubber, but in Australia this plant is a Weed of National Significance.

In Queensland and the Northern Terrritory this plant is a serious problem. There have also been worrying outbreaks in northern Western Australia, although they were controlled.

Some members of the milkweed Family are toxic and Rubber Vine is suspected of poisoning livestock. However, it is its invasiveness which earns it the reputation as one of Australia's worst weeds.

It smothers vast amounts of bushland, even climbing and smothering 30 metre tall trees. The native vegetation is eventually destroyed. But it's not just bushland. This plant has a serious impact on pastoralism too, with the loss of grazing land estimated to cost the Queensland beef industry more than $18 million.

Rubber vine is a threat particularly to waterways, woodlands and rainforests, including the significant conservation areas such as the Wet Tropics World Heritage Area and Cape York.

There is also significant potential for this plant to spread southward into northern New South Wales.

Description

Rubber vine is a climbing shrub with glossy, dark green, oval-shaped leaves that appear in pairs along the stem. From August to December, it produces large trumpet-shaped flowers up to 50 mm long and wide, with light purple and white petals.

The seed pods are up to 120 mm long and 40 mm wide, containing anywhere from 340 to 840 flat brown seeds with tufts of long, white, silky hairs. Seed drop occurs from February to April. Fortunately the seed is not long-lived, and if sufficient rain does not occur the seeds will die after a year. Rainfall needs to be between 400 and 1400 mm, and mainly during summer.

The plant is thought to live for up to 80 years.

Control

Broadscale strategies are put in place to control and eliminate this weed. Control often involves a combination of different methods: biological, chemical, fire and mechanical.

The rubber vine rust disease Maravalia cryptostegiae, which was introduced as a biological control agent in 1990, causes leaves to turn yellow and drop off. The disease thrives during the wet season, but is not so active in the dry.

A moth (Euclasta whalleyi) whose caterpillars feed on rubber vine leaves is another biological control agent but its success can vary with the weather conditions.

Foliar spraying with herbicide is effective but expensive, especially with enormous infestations. Cutting down and then painting the cut surface of the remaining plant with herbicide is quite effective.

Fire kills Rubber Vines easily but the fire needs to be hot and follow-up burns are required.

Mechanical techniques (ploughing) can help, but it is usually only used to get access to more of the infestation.

What we can do?

Don't grow it or plant it, and if the plant appears on your property, report it and remove it as soon as possible.

And don't plant its close relative Cryptostegia madagascariensis either! It's a garden plant that is now a naturalised weed in Western Australia, the Northern Territory and Queensland. It has slightly smaller flowers that are a deeper pink colour, but otherwise both plants look very similar.

Information sources:
Auld, B.A., & Medd, R.W., Weeds: An illustrated botanical guide to the weeds of Australia, Inkata Press
www.weeds.crc.org.au
www.deh.gov.au
Top photograph courtesy of Kate Blood, Weeds CRC

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