Author: Mary Gibby

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Friday, June 30th, 2006 at 12:23 pm
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Homes and Gardens

Alien Invaders

Native plants are under threat, says Professor Mary Gibby

With interest in gardens and garden makeovers riding high, it is understandable that Scots are buying increasingly unusual plants and becoming more inventive in the way they present their displays. As countless magazine and TV makeovers have demonstrated, we don’t need acres to make bold horticultural statements. But, what happens when cultivated species stray into the wild? Could it even be a good thing to encourage some exotic colour in the countryside?

It’s a common enough sight: bright clusters of rhododendron or pockets of cotoneaster punctuating rows of wildflowers along our roadsides and pathways. Often they will have self-seeded. Sometimes, they will have been fly-tipped. Occasionally, a well-meaning gardener will have planted them. Almost unfailingly, these wild alien invasives are damaging our natural habitat and the wildlife on which it depends. For, rather than being a pleasing distraction, they come second only to property development in the causes of plant loss in this country. It is against this backdrop that gardeners are being urged to support the Horticultural Code of Practice for Scotland.
In an era of burgeoning rules and legislation, it is understandable that people have questioned the need for yet more paperwork and debated whether this is just another example of bureaucracy gone mad. Nothing could be further from the truth: stopping the spread of non-native plants is a key factor in conserving Scotland’s indigenous diversity.
To put this in context, consider the water hyacinth – a South American native now found in 50 countries. As it spreads and develops into floating mats of interlocked plants it causes rapid changes in the area it inhabits. As well as absorbing large quantities of nutrients which kill phytoplankton and may result in reduced fish stocks, it also overcrowds and kills other plants. As these plants die and decompose, we witness further oxygen deficiency and fish death. So extreme can this be that it has caused some fishing communities in West Africa to be abandoned.
Rhododendrons have been responsible for the destruction of many Scottish habitats. In suitable conditions, they will out-compete most native plants and grow to considerable heights. As little light can penetrate their thick leaves, smaller native plants struggle to grow in their shade. Their root system is also toxic to other plants.
Japanese knotwood can regenerate from very small pieces. The estimated cost of eradicating it from Britain is £1.56 billion.
Reversing the damage is not easy and mature crops of any invasive species are particularly difficult to tackle. For example, Japanese knotweed can regenerate from very small pieces, around the size of a fingernail. It causes problems in towns and cities where, as well as out-competing native plants, it causes structural damage. The estimated cost of eradicating Japanese knotweed from Britain is £1.56 billion.
The history behind the current situation is a long one. Many of the plants we currently regard as invasive were introduced with the best of intentions as exotic new species to enhance gardens in the 18th and 19th centuries. But, by identifying them and outlining the very real dangers they bring, we can also help garden centres to be aware to be more aware of what they are stocking and to ensure gardeners understand what they may be buying.
Significantly, the new code was produced with considerable input and support from trade representatives and environmental groups. This is important as, far from being yet more legislative red tape, it is intended to be a source of advice which can empower gardeners and garden centres to play an active role in tackling the threat to the environment posed by high-risk invasive species. It also recognises the fact that, while we already understand the problems associated with these species, we need to be aware of potential damage in the future, through the introduction of new invasive plants.
Of course, not all non-native species are harmful. In fact, many have an essential place in the garden, in agriculture and in forestry. But, it is important to use non-native plants sensibly and to be aware of the environmental and economic consequences of allowing aliens into the wild.
So, how do we act responsibly? Conservation projects are underway at many different levels. In scientific institutes, environmental organisations and in likeminded bodies, research into native plants and their conservation are ongoing. For example, efforts are underway to turn the tide against the spread of the seemingly innocuous cultivated, non-indigenous, bluebells which threaten the native variety.
But each of us, whether tending the smallest of plots or servicing major horticultural outlets, can play a part in ensuring we contain problem plants. We can all start to compost unwanted plants or use local authority garden waste uplifts – and we can stop planting outwith our own garden spaces. So many professionals and interested members of the public already share a common commitment. By working together we can all play our part in safeguarding Scotland’s precious natural heritage – and containing our alien invaders to the plots in which they belong.
Professor Mary Gibby is director of science at the Royal Botanic Garden Edinburgh and acted as an adviser on The Horticultural Code of Practice for Scotland.

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