Warning over NINE invasive plants that spread as quickly as Japanese Knotweed – as expert reveals how to get rid of them
- Invasive non-native plants can harm the environment in devastating fashion
- Ahead of the summer, a gardening expert issued a warning over nine species
- Read below to learn about species which can be as bad as Japanese Knotweed
Finding an invasive species in your garden is probably the worst nightmare of many a gardener.
Japanese knotweed is the most well-known of these, requiring professional assistance to remove.
In the lead-up to summer, Graham Rice of Gardeningetc has warned of nine other plant species which have the potential to be as catastrophic as the aforementioned Japanese knotweed.
Read on to find out what they are, and how to deal with and prevent them.
Japanese knotweed’s unassumingly dainty flowers can go under the radar of inexperienced gardeners
The Himalayan balsam is known for its aggressive expulsion of seeds, making it spread at pace if uncontrolled
Tree of Heaven (Ailanthus altissima)
The tree of heaven is a fast-growing deciduous tree native to China.
It has two ways of invading your garden, by flowers spreading huge quantities of seeds, or by shooting off numerous ‘suckers’ via its roots which can appear as far as 50ft from the parent tree.
You can combat the tree of heaven by uprooting it if you catch it early enough, but if you find one late in live, your best bet is to cut back ruthlessly and use weedkiller.
However, it is unlikely you will need to do that, as the tree is rare in the UK, but much more of an issue in the US.
The tree of heaven can grow to over 80ft tall
Water Fern (Azolla filiculoides)
Water ferns are aquatic plants which spread via spores carried by the water’s current or the wind, introduced in the UK from America in 1880.
It thrives on still and slowly moving bodies of water and can eventually form a layer up to 30cm deep, completely covering the water surface in a thick carpet.
This blocks both light and air, eventually killing aquatic plants below. It also prevents amphibians and invertebrates from reaching the surface and may disrupt movement of animals on the water surface itself.
You can combat it by using a net to drag the plant off the water in summer.
Water ferns can be found in places like canals, ponds and lakes and can multiply and spread quickly
Hollyberry (Cotoneaster)
Hollyberries may look familiar to you, but they should not be confused for their native cousins.
While often admired for its aesthetic value, with leaves turning golden-brown in autumn, the effectiveness of this plant makes it a nuisance which can eradicate others competing for its space.
It is illegal to plant in the countryside or let escape from the garden as it is taking over valuable habitats including limestone grasslands.
To deal with it, you need to cut it back severely and weed kill the exposed wood.
Charlotte Howard, from Capability Charlotte suggested hawthorns, holly, honeysuckles, guelder roses, and firethorns as better native alternatives.
Once a valued garden shrub, the hollyberry cotoneaster has become a pest for gardeners
New Zealand Pygmyweed (Crassula helmsi)
Introduced to UK from Tasmania in 1911, these wiry weeds form a thick entanglement on the banks of ponds and lakes, sometimes totally submerged.
Initially sold as a plant to oxygenate ponds, the New Zealand Pygmyweed can grow for most of the year, without ever dying down, making it a pain for any with it in their garden.
As a result, like water fern, it will outcompete native species, and can also cause flooding.
To control growths you should collect them from the bank with a net or rake, but even repeated removal is rarely a complete success.
Even repeated removal of New Zealand pygmyweed is rarely completely effective
Giant Hogweed (Heracleum mantegazzianum)
Related to the more recognisable cow parsley, giant hogweed can reach heights of up to 4.5 metres, and its leaves can suffocate everything around it.
While you may be tempted to swiftly cut the plant back, you should take care.
Nic Seal, founder and Managing Director of invasive plant removal company Environet warned gardeners.
He said: ‘You want to be extremely careful because of the burns it can cause.
‘It basically reverses the skin’s ability to block out UV light so it gives horrible, horrible burns and blisters.’
You should wear protective clothing and gloves to shield yourself from its spikes and sap.
Giant hogweed can grow to a height of 4.5 metres, and engulf everything around it
Spanish Bluebell (Hyacinthoides hispanica)
This is very similar to the usual, native bluebells you are accustomed to, only bigger and with more flowers per plant.
Howard explained: ‘Not only does it outcompete native bluebells but it hybridises with them.
‘It easy to tell from the native species as the flowers open wider and are spread all round the stem instead of on only one side.
‘It’s a pretty flower and attracts early pollinators.’
You can dig up the bulbs while the plants are in flower to protect our native ones.
The Spanish bluebell is very similar to the native bluebells you are used to, only bigger and with more flowers.
Himalayan Balsam (Impatiens glandulifera)
Also known as the policeman’s helmet, this flower is notorious for its aggressive expulsion of seeds.
Seal said: ‘In a funny sort of way they are more difficult to stop spreading.’
This plant is legislated against by the Wildlife and Countryside Act 1981 as a non-native invasive species, so it is illegal to plant or cause to grow in the wild.
You can pull it out with your hands if you catch them young enough, or once they have died out in the winter, as well as calling upon weedkiller if required.
The key to controlling the Himalayan balsam is catching them before they grow old
Virginia Creeper (Parthenocissus quinquefoli)
This plant is native to the US and Canada is another which falls under the remit of the Wildlife & Countryside Act.
Howard explained the appeal of the climber, saying: ‘Although it is listed on Schedule 9 of the UK Wildlife & Countryside Act as an invasive non-native species, it can be grown in your garden and is a stunning way to cover an ugly house.
‘But beware, it can reach up to 20m and it must not be allowed to escape into the wild!’
To control a creeper like this, cut it down to the root in autumn before digging up what remains underground.
The Virginia creeper’s berries are highly toxic and can be fatal to humans
Rhododendron (Rhododendron ponticu)
Despite being probably the most common plant on this list in the UK by a long way, rhododendrons’ recent spread has left people conscious of the damage it can cause.
Seal explained how they are quite so effective: ‘It is one of the one that has the tendency to exclude the ability for any other plants to grow in and around it.
‘It’s basically plant chemical warfare! They put a chemical in the ground that kills others around them.’
Aggressively cutting the plant back before digging up the roots are an effective but potentially monumental task to control these beasts.
Potentially a surprising addition to the list, rhododendrons can cause great difficulty if you find them in an unwanted space
Why should I bother to fight invasive plants?
There is the aforementioned legislation in the UK against growing, or selling, certain non-native invasive species, as the Royal Horticultural Society explains.
While you are not likely to be prosecuted just for having them on your land, you are required to take all possible steps to remove them.
Seal said: ‘It is very important to keep on top of them because they have the potential to grow very quickly and some have a tendency to cause damage to property.
And it is not just the damage it can cause that is a problem. It is causing more and more neighbour disputes which can go to court.’
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