Invasive plants are a major threat to our environment, but gardeners can help prevent the problem from getting any worse.

Plants that have been in the British Isles since the last Ice Age are natives. Apart from thugs such as bracken, gorse and brambles, most rarely cause any problems. The vast majority of the plants introduced from abroad are perfectly harmless when grown in the garden.

A tiny handful of introduced species, the invasive aliens, can damage the environment when released into the wild. Probably the most infamous Scottish invader is Rhododendron ponticum, introduced from Spain in 1782. It was considered ideal for large gardens, but gradually spread by seed, sucker and rootlet and now blankets huge tracts of hillside in the west. It stifles other plant growth, and may produce biochemicals that prevent other plants from germinating.

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Several species are just as bad. Unfortunately, gardeners frequently start these problems by dumping or throwing away unwanted plants. Folk with ponds or aquariums sometimes scoop out handfuls of oxygenating or bog plants and discard them in burns or ponds, where many thrive and may end up choking waterways.

The Scottish and UK governments have prepared lists of these alien plants which are listed in Schedule 9 of the Wildlife and Countryside Acts 1981. However, because gardeners can buy them in many garden centres, they don't realise there's a problem with these aliens. Not everybody is going to check whether it's sensible to buy a plant species.

Plantlife Scotland ( has detailed information about plants to avoid and also lists species that should be included in Schedule 9. They have, in addition, compiled a comprehensive list of safe alternatives. Gardening Without Harmful Invasive Plants is an excellent, well-illustrated leaflet.

On a recent visit to Switzerland, I was delighted to see that, like several mainland European countries, the authorities there simply ban the sale of some plants. Invasive species are divided into three categories and it's illegal to plant, propagate or sell the 24 most dangerous. If, for any reason, a gardener has one of these and it spreads beyond the garden, he or she must pay to have the offending plant removed.

It was also refreshing to see a well-designed and presented exhibition in Bern's Botanical Gardens: a joint initiative with the city's university to educate the public on the dangers of invasive plant species. One of the organisers, Dr Steffen Boch, told me: "We wanted to raise interest of the general public, schools, students and conservation organisations. We have had very positive feedback from visitors and a lot of people have attended guided tours, talks and discussions. A lot of them were not aware that they are cultivating so many invasive plants in their gardens." This initiative is well worth copying here.

One species on the Swiss list of forbidden plants is Rhus typhina, a small American tree or large shrub with striking, (in my opinion hideous), autumn colour. It's fairly widely grown here, suckers madly and needs severe pruning to be kept under control. Surely we should learn from other countries and make it hard, or preferably impossible, to grow species like this here.

The Swiss also have a blacklist, containing plants that are known to be problematic, but not forbidden. Many, but not all, of these species feature on our schedule 9. Confusingly, the English schedule 9 is different to and more extensive than our own. Whatever list or lists a species features on, I can't help feeling it ought to be treated cautiously everywhere.

Bog, pond and aquatic plants feature highly on all these lists, so pond-owners should be specially careful when buying new plants or tidying up the pond. One of the worst plants is Crassula helmsii (Australian swamp stone crop or New Zealand pigmyweed). It's sometimes wrongly labelled Tillaea recurva or Tillaea aquatica when sold. A tiny fragment of this plant will regrow and quickly form a dense mat. The plant is frost-hardy and will dominate a waterway within five years.

Oxygenators such as Elodea nuttallii (Nuttal's pond weed) are horribly invasive and the pond weed, Ludwigia grandiflora (water primrose), is every bit as bad. It's an offence under the English Wildlife and Countryside Act 1981 to release this supposedly attractive water garden plant into the wild. It devastates habitats by out-competing native plants and clogging up waterways. Environmentalists are trying to clean up several sites throughout the UK at the moment.

Given the absence of guidance from garden centres, gardeners need to be vigilant, checking whether a plant is listed or considered a problem by Plantlife Scotland. It would be much better to buy a safe alternative. If you have or really want a problematic species, be sure to safely compost surplus vegetation and never release it into the wild. n

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