The department’s South West Invasive Species Project Manager Dennis Rafferty said people should know what they were buying and ensure the plants were not invasive weeds.

“We have just found water hyacinth in a water feature at a Mount Hawthorn business,” he said.

“The plant had been bought at a car boot sale in the Perth northern suburbs, and the buyer did not realise that it was an invasive plant that poses a major threat to our waterways in Western Australia.

“Water hyacinth is one of the world’s worst weeds, choking up rivers, lakes and other waterways. It is a declared plant in Western Australia, prohibited from sale, and must be destroyed when found.

“If anyone thinks they have this plant or any other suspect plants, they should not dump them in creeks or drains, or in bushland where they could proliferate and spread. They should be put in a plastic bag in the sun, and the Department of Agriculture and Food should be called for advice.”

Mr Rafferty said the discovery of the plant emphasised the need for caution when buying plants at car boot sales, fetes and markets.

“They may seem a cheap option, but could prove a major cost to the State’s environment and economy if they spread to our natural habitats, farmlands and waterways,” he said.

“Department biosecurity officers do random spot checks of markets, and respond to reports of sales of banned plants. However, the public has a major role to play in exercising caution when buying or trading plants, and should report any suspect plants.”

Other serious weeds on the Declared List that might be offered at these outlets include arum lily, salvinia and Lantana camara.

Reports can be made to the department’s Pest and Disease Information Service (PaDIS) on Freecall 1800 084 881.