Its flowers are brightening the landscape across Scotland right now, but a top scientist has said the rhododendron "swarm" poses the country's biggest single ecological threat and needs to be tackled with a mass eradication programme.

With its masses of bright purple flowers, rhododendron ponticum is for many one of the joys of early summer in Scotland. For the past six weeks the plant has put on a gaudy show, delighting visitors to beauty spots such as Loch Lomond, Cowal, Torridon and the Great Glen.

But now, ahead of a Scottish Government control strategy to be published later this year, James Fenton, the National Trust for Scotland's ecologist, has called for large-scale government action to tackle the plant, saying getting rid of it should be a top priority. He wants the plant eradicated in Scotland, apart from in private gardens.

Long known to be an invasive alien species, recent studies have found Scotland's wild rhododendron ponticum is a creation of Victorian plantsmen. It is a mix of Spanish and North American species, developed to withstand Scotland's harsh climate and planted by estates for decoration and game cover.

But the Victorians created a monster. Scientists now say it has formed a "complex variable hybrid swarm" - in effect, a new species perfectly adapted to our climate.

It spreads exponentially, Fenton said, with its flowers producing millions of seeds. It kills woodland by stopping trees setting seed. It destroys peatlands, with mosses, ferns, insects and the animals that eat them driven out. Fenton said a dedicated directorate to deal with invasive alien plants is needed.

This would direct permanent teams across the country to tackle superponticum, with the powers to order teams in to clear it against landowners wishes.

Fenton has admitted this will cost many millions of pounds over the coming decades, but said the consequence of inaction will be higher cost for removal in the future and eventually a natural disaster.

He added: "It is probably the biggest ecological issue for Scotland. If we did nothing and came back in a thousand years, the landscape would be just one dark rhododendron forest."

A recent estimate suggested that in Argyll, where Fenton lives, there are 11,000 acres of superponticum. He believes the real figure is double that. Forestry Commission Scotland is committed to its removal on its estate, but Fenton said it is also spreading on open ground, including mountainsides in Torridon and Islay's peat bogs.

The plant has been eradicated from some areas, including Knoydart in the West Highlands, where £250,000 in grants paid for a ten-year programme using local workers. Fenton said an eradication programme would boost the economy of rural areas.

A Scottish Government spokesman said: "Successful eradication of rhododendron populations requires control work to be co-ordinated across all the land holdings in an area and for planning to be long-term to address regrowth.

"Forestry Commission Scotland and Scottish National Heritage are now preparing a national rhododendron control strategy, which will be published later this year."