INVASIVE plants crowd out native species even 30 years after being cleared from the land, ecologists say.

A study based in the rural west coast found indigenous species have yet to return to areas cleared of the invasive rhododendron in the 1980s.

Researchers say the findings prove removing the problem plant – introduced to the UK in the late 1700s – is not enough and grounds workers must reintroduce native flowers themselves if they are to return at all.

The issue is said to be key to protecting biodiversity.

Dr Janet Maclean of Aberdeen University said: “Rhododendron has spread throughout the UK, affecting around 827,000 hectares, and is particularly widespread across western Scotland and Snowdonia.

“Eradication programmes cost around £8.6 million a year and the results of this study show that – as well as removing rhododendron – land managers should also consider clearing mats of common mosses from the ground and reseeding with typical woodland grasses and flowering plants.

“Cleared sites are far richer than invaded sites. But for them to resemble uninvaded sites, removing rhododendron is not enough.

“This is an important discovery, because it means that many sites may need further intervention to meet their conservation goals.”

Aberdeen researchers worked with teams from the James Hutton Institute and Scottish Natural Heritage in the woodlands of Argyll, Kintyre and Lochaber.

They found sites cleared of rhododendron in 1984 still lacked the native plants normally found in Atlantic oak woodlands.

Instead of primroses, violets, wild garlic, ferns and grasses, only thick mats of mosses and liverworts returned. Ecologists believe the deep shade rhododendron casts is responsible.

Maclean said: “Rhododendron dramatically reduces the amount of light reaching the woodland floor throughout the year, causing local extinction of native grasses and herbaceous plants. Mosses and liverworts cling on because they can tolerate the darkness.

“When rhododendron is removed, common mosses and liverworts quickly recolonise. By the time grass and other plant seeds arrive, the thick mossy mat prevents them from germinating, so the site never recovers its previous rich flora.”

The research is published in the Journal of Applied Ecology.