By Emma Brennand
Earth News reporter
The New Zealand Pirri pirri bur is a prickly customer for UK habitats
Non-native garden plants pose a threat to the countryside including National Parks, a report reveals.
The study into invasive plants published by charity Plantlife highlights 92 species which are on the brink of becoming invasive in the UK.
These foreign plants are spreading through natural seed dispersal and fly-tipping of garden waste.
Many gardeners are unaware of the potential threat their decorative plants may pose, says the charity.
Invasive alien species costs the economy £2 billion each year, and are thought to place the greatest pressure on native British wildlife.
The study published by Plantlife was commissioned by Natural England, as part of a project to search for alien species that might one day threaten native ones.
The report lists 599 non-native freshwater and terrestrial plants that pose a potential threat to British wildlife.
Of these, 92 species were considered to pose a 'critical risk' to invasion, including the New Zealand Pirri-pirri-bur and the North American False-acacia.
Banning the sale of such 'at risk' plants is the best strategy for preventing them taking hold, the report suggests.
As well as being cost effective, the approach might best limit their escape to the wild, and any damage to the environment.
Invasive plants compete with native plants for light, space and nutrients.
Ponds and aquatic areas are particularly susceptible as alien plants can become a drain on the light and oxygen levels in the water effecting both plants and animals.
In addition, non-native plants do not tend to produce food appealing to native animals, reducing the space for local wildlife.
Six other species, including the Holm oak, Himalayan knotweed, Large-flowered waterweed, Pickerelweed, Tree of heaven and Turkey oak, were listed as 'ones to watch'.
The holm oak is well established in woodlands across the UK
The report also highlights 20 key sites it judges are 'in peril' from plant invaders.
Many of these sites are considered national treasures, including the Somerset Levels, the Lake District and Pembrokeshire National Parks.
Several non-native plants are already affecting the Lake District National Park.
The New Zealand pigmyweed Crassula helmsii is found water bodies there including Coniston, Grasmere and Windermere.
Out of Control
Labour intense control measures include removing plants, applying herbicides as well as burning vegetation, and in the case of aquatic habitats dredging and chemically spraying.
However, these measures are not always successful.
The False-acacia can regrow when cut down
For example, the non-native rhododendron Rhododendron ponticum is now considered too widespread to fully eliminate.
The leaves and stems of this dense shrub contain toxic chemicals including 'free' phenols and diterpenes that are unpalatable to native animals.
In addition, the chemicals also infiltrate the soil, poisoning it and making it inhabitable for native plants.
As a result, rhododendron threatens the survival of rare native plants including the endemic Vulnerable Lundy cabbage (Coincya wrightil).