4 minute read

What do we mean by exotic plants?

When we talk about exotic plants, it simply means those that are not native. Historically, here in the UK there has been a drive to promote the planting of native trees and shrubs, provided the provenance is known, because of the risks of pests and diseases. The danger is lower when growing seed that has been harvested from trees grown here. The young plants tend to be more resilient and they display a better ability to adapt. Imported seeds, plants and trees carry a risk of introducing alien pests and diseases into the UK, against which our native species have no natural defence. There is, however, a problem with the long-term biosecurity of our UK trees. Did you know there are now only around 30 truly native species, plus subspecies and hybrids that have naturalised themselves here? The term ‘native’ is generally defined as trees that established themselves in Britain around 10,000 years ago. This was the end of the last ice age, at a time when the English Channel was gradually forming. Plant hunters began collecting specimens from around the world during Roman times. The activity saw plants introduced into Britain and this gradually opened the window for pests and diseases too. The Ages of Discovery and Enlightenment saw more and more explorers setting off around the world and bringing back unusual plants, in addition to teas, food, spices and animals. Many of the living species ended up in private collections or in botanical gardens. This extended the range of plants within the UK, and we have come to love and admire many of these introduced species. The glorious Liquidambar, or sweet gum, for example, is native to Asia, parts of the Mediterranean and North America. Many acer trees originate not only from Japan but also from China and Korea. The tulip tree, Liriodendron tulipifera, hails from North America, and many of the flowering cherries originate from Japan, China and the USA. Even apple trees have an exotic history! The original ancestor of many modern apple cultivars can be linked back to an ancient apple: Malus sieversii. This wild apple is native to the mountains of Kazakhstan in Asia, where they can still be found today. It’s not surprising that UK trees have succumbed to many devastating problems. One of the first to be widely recognised was Dutch elm disease, which was said to have been accidentally imported from

Canada in the late 1960s. It is thought to have killed over 30 million elms. Ash dieback was first reported around ten years ago and this disease is expected to kill up to 95 per cent of Fraxinus excelsior. In the last 30 years there have been more than 20 serious problems caused by imported pests and diseases. Many diseases are caused by fungus pathogens, such as Phythopthora ramorum, which is affecting larch trees, and Phythopthora pluvialis, which affects western hemlock, certain types of pine trees and the Douglas fir. The latter was originally found in Oregon in the USA. The Oak Processionary Moth, native to southern Europe, is now found in London and restrictions on movement of oak trees have recently been put in place by Defra (Department for Environment, Food and Rural Affairs). This raises a question about exotic plants. Should we avoid planting them, or welcome species that are not native? Many people believe that the tipping point has now been reached. UK species are declining so rapidly that we need to extend the range of plants. This means planting non-natives that will safeguard the future tree canopy of this green and pleasant island, especially now that we know the true value of trees in respect of tackling climate change. There is a lot to be said for strength in diversity. Planting exotic species together with natives will help to plan for the future, knowing that the climate is changing. It is estimated that summer temperatures will rise by up to ten degrees centigrade by the end of the century. Extreme weather events, such as flooding, drought and high winds, are increasingly likely. UK wildlife generally prefers native species of tree and shrub, but looking to the future is now essential. Our woodlands, garden planting and the wider landscape requires future-proofing and cannot adapt fast enough without our help. The right plant in the right place, for both now and the future, has never been so important.

Advertisement

This article is from: