It is wildflower season in WA's South West and the Leeuwin-Naturaliste National Park is ablaze with colour. But an invader is wreaking destruction amid the natural beauty.

For several years the arum lily, also known as the "funeral flower", has been quietly spreading through natural bush and farmland around Margaret River and the South West Capes.

This season, walkers on the popular "Access for More" section of the track between Sugarloaf Rock and the Leeuwin Lighthouse have been confronted by clumps of established lily plants with their distinctive white flowers nestled amongst indigenous species.

Genevieve Hanran-Smith, from the community-based conservation group Nature Conservation-Margaret River Region, said the problem was increasing with each season.

"In the area between Cape Naturaliste and Cape Leeuwin it's our most widespread and established environmental weed," she said.

Arum lilies are invading a popular walking track between Sugarloaf Rock and the Leeuwin Lighthouse. ( ABC News: Ingo Helbig )

"Arum Lilies are spread by birds eating the seeds and then spreading them into new areas, which makes them a particularly damaging environmental weed because they can be spread into areas of pristine native vegetation."

Originally imported from South Africa as a garden plant, the sale of the Arum Lily has been banned in Western Australia since 2006.

But this has done little to slow its advance into wetlands and coastal beauty spots along the popular tourist strip.

If left uncontrolled, the lily tubers often expand into large masses, or mono-cultures, which smother all surrounding native vegetation.

The arum lily threatens the natural species that attract walkers to the South West. ( ABC News: Ingo Helbig )

Along with the environmental impact, Ms Hanran-Smith said there was concern the changing environment would affect local tourism, wiping out the wildflowers that attract walkers to the area each year.

The Department of Biodiversity, Conservation and Attractions, the government body responsible for controlling invasive species in WA's national parks, said its management of the weed was largely focused on using chemical spraying for asset protection.

But it conceded the battle to rid WA of the invasive species had been lost.

"Given arum lily is already widespread on public and private land across the South West, eradication of the species is not feasible," it said in a statement.

Private landholders offered help

In the meantime, Ms Hanran-Smith says her organisation is working hard to control the spread of the plants on private land.

"We've probably worked with about 300 private landholders across 3,500 hectares over the last five years to get arum lilies under control.

"In those areas we make contact with every single landholder and offer to share the cost of a contractor to come and spray the arum lilies.

"It's such a widespread problem that people think why would I bother trying to do anything about it, so we're trying to demonstrate to people that if everyone works together you can successfully control it."