There are many invasive species in various habitats across the UK and, like any other habitat, woodland has several which threaten the native species found there.
Invasive species are so called because they have a negative effect on the habitat they are growing in. Invasive species can be native or non-native.
In woodlands, invasive species can take over huge areas of habitat, out-competing other species as well as influencing diseases present. Here are a few you might find on your woodland walk.
Japanese knotweed (Fallopia japonica)
A non-native species, originally introduced into gardens, this plant has rapidly spread across areas of the countryside in the UK.
It is a perennial, which can grow around two to three metres tall and the roots can be several metres deep. It has rounded leaves with a pointed tip, growing in an alternate formation. Due to its robust nature this plant grows in various habitats including along woodlands, roadside verges, rivers as well as wasteland. It also has the ability to grow through concrete.Japanese knotweed dominates areas and out-competes other species. All the plants of this species in Britain are male sterile clones and so do not produce viable seeds. However, it spreads very quickly via rhizomes at a rate of around 10cm each day. A very small fragment of rhizome can give rise to a whole new plant. Less than one gram is able to spark new growth!
Japanese knotweed can be seen across large areas during the summer but when winter arrives the plant dies back leaving the bare earth vulnerable to erosion, especially if it is growing along rivers.Controlling this species is very difficult and a combination of cutting and spraying is often used, although this still takes several years. Disposal also has to be carried out under strict guidelines to prevent any potential growth in other areas. Control of Japanese knotweed costs millions of pounds each year.
Rhododendron (Rhododendron ponticum)
Native to parts of the Mediterranean and Asia, this evergreen bush can be considered an attractive plant, with its long leaves and purple flowers. It is however a huge problem in many areas across the UK. It’s able to dominate a variety of habitats, especially woodland, and can also grow well in disturbed areas.Once established, rhododendron out-competes other species and shades out woodland ground flora. It also has toxins which reduce foliage growth of other species and which make it unappetising for grazing animals. It spreads via vegetative propagation and seed dispersal, so is difficult to control as each flower produces several thousand seeds each year.It’s not just Rhododendron ponticum that’s a problem. There are many rhododendron hybrids in the UK. For example Rhododendron x superponticum is a highly invasive species.As well as out-competing other species, rhododendron can also aid in the spread of Sudden Oak Death, which in the UK affects our native larch, rather than oak. The effect of climate change is also likely to benefit this species, as it is native to areas which are drier with a higher temperature, so it may become more of a problem in the future.
Spanish bluebells (Hyacinthoides hispanica)
There is nothing more pleasing than seeing a carpet of native bluebells (Hyacinthoides non-scripta) across the woodland floor in early spring, but in fact many of these displays have been invaded by the non-native Spanish bluebell.This species was originally grown in gardens but has escaped and crossbred with the native bluebell. This creates a hybrid which is fertile. Many people do not see this as a problem but it means that the individual qualities of the native UK bluebell are reduced and losing the qualities of a true British flower is very sad indeed.You can tell the two different kinds of bluebells apart by looking at the pollen. The Spanish bluebell has green-blue pollen, whereas the native has white-cream.
Three-cornered garlic (Allium triquetrum)
This plant appears to fit in well with the native flora, with its green, three-angled stem and white flowers but it is non-native to the UK.The plant produces seeds, which are then spread by ants. Growing well on disturbed areas it can be found along woodland edges as well as other habitats such as roadside verges and hedgerows. Even though it is small in size when compared to other invasive species, it produces areas of dense flowers which out-compete native wild flowers, such as violets.Invasive species have unfortunately spread across much of the countryside, often due to irresponsible disposal of garden waste as well as escaping from gardens. Therefore it is important to monitor where these plants are located, and the effect they have on our native flora.