New research has shown that one of southern Australia's worst weeds, bridal creeper, continues to damage native plants even after it has been killed.

Bridal creeper, a native of southern Africa, is an attractive plant once much loved by gardeners. Now it is one of southern Australia’s worst environmental weeds. It smothers native vegetation and its huge tuber mats prevent germination of native plants.

This invasive plant has been successfully targeted by biological control programs across Australia with control of up to 95 per cent, but until now little has been known about what happens after the weed is controlled or killed. The assumption has been that the native bush would regenerate.

New research has shown that this is not necessarily so. Instead, restoration and revegetation projects need to manage the lingering environmental changes associated with weed invasion.

Peter Turner, from the University of Western Australia and CSIRO Entomology, is studying the effect of bridal creeper in his PhD.

“Generally native plants are adapted to low nutrient soils while exotic invasive species prefer more fertile soils,” Mr Turner says. “But bridal creeper has no problem invading low nutrient soils and taking over.

“Our research has shown that once bridal creeper is removed it leaves behind more fertile soil than when it first invaded and these changes in soil nutrients could favour invasion by other weeds rather than the regeneration of the native bush.

“We have shown that soil in areas invaded by this weed in south west Australia have higher levels of available nutrients than nearby weed free areas.”

A bridal creeper invasion replaces native woody shrubs and increases the rate of nutrient cycling. It is deciduous at the end of spring and as its foliage contains higher phosphorous and nitrogen levels than natives, the decaying foliage adds these nutrients to the soil. Its tuberous root mat then traps these nutrients.

“One important finding of our research is that restoration and regeneration work at sites freed from bridal creeper need to take into account the increased soil fertility or it could just be a case of exchanging one weed for another,” Mr Turner says. “Sites with high conservation value need to be identified so that the biological control actions can be used in conjunction with other restoration techniques.”