Invaders that threaten our native plants
Words: Steve Head, Coordinator of the UK Wildlife Gardening Forum
It is said that a weed in the garden is a just plant in the wrong place, but what about the unwelcome plants that thrive and spread outside our gardens? Many are not really weeds, but have been planted by gardeners in the past, unaware of their invasiveness.
Plants that jump the garden fence
Garden plants that 'jump the fence' and spread in the countryside are becoming an increasing problem, not only to our native plants and habitats, but to major industries like forestry, tourism, agriculture and construction.
A recent government-commissioned report came up with a staggering annual economic cost of £1.7 billion. The biggest economic impact is on food production, but only about a fifth of the total £1 billion cost of non-native species impact is due to weeds, much more being due to imported pathogens and animals.
The major troublemakers
The same report estimated average annual losses to the construction industry of £150m due to a single species - Japanese knotweed, (above, which cost £70m to remove from London’s Olympic site alone). This attractive plant was introduced into Victorian gardens, but has spread wildly around Britain. It can grow more than a metre a month and is able to push through tarmac, concrete and drains.
Rhododendron ponticum (left) is another well-known invasive species that broke free from its garden base in the 19th century. It shades out and poisons woodland trees and other plants, and costs the forestry and conservation industries about £8.6 million every year. Although now seen as a dangerous invader originating from Portugal or Spain, it was native to Britain before the last Ice Age.
Other well-established exotic thugs include giant hogweed (left) and Himalayan balsam (over £3 million annual costs together), and a large number of extremely invasive water plants including water fern, Australian swamp stonecrop, floating pennywort Canadian pond weed and parrots feather. River and water habitats seem particularly vulnerable to most current plant invaders except Rhododendron.
The thin end of the wedge?
Could any of these invasions have been predicted when the plants were first grown in gardens? There is a growing worry that the trickle of non-native invaders could become a flood as our climate changes in the next century. Already invasive species have been identified as one of the main causes of global biodiversity loss alongside habitat destruction and climate change, so spotting a potential invader before it is too late is increasingly important.
In practice, however, only about 2.7% of the 70,000 non-native plant species available to gardeners have established in the wild, and only 3.5% of these are considered to be a problem, so simply banning all species but natives from gardens would be as ridiculous as it would be unenforceable.
A recent Natural England report identified 599 non-native plants that could be potentially invasive, of which 33 aquatic and 59 terrestrial species were thought critical. Garden plants now under suspicion include many Cotoneaster species (pictured), Buddleja, three-cornered garlic, several tree species, exotic honeysuckles and some broad-leaved bamboos. It would be sensible for all gardeners to be aware of potential escapees, especially if they live in the country.
Advice and information
Photography: RPS Group PLC; Image Tube Lines Ltd, GBNNSS, Wye and Usk Foundation