Invasive Species Week 2019 – are you aware?

Japanese Knotweed | 21st May 2019

The NNSS (Non-Native Species Secretariat) invasive species awareness week

The NNSS have chosen this week as Invasive Species Week 2019 to increase general awareness of these culprits and the problems that they cause.

The definition of a non-native species is that of a plant or animal which has been transported from its native range to a new region with the assistance of humans. An invasive non-native species is a non-native species that has a negative impact on the environment, economy or our health and way of life.

It is important for us all to be able to spot invasive species problems before they become major infestations – and take action sooner rather than later. This will help us preserve the ecology of the British landscape and protect our vulnerable native species.

Taking a look and the main offenders in the plant world, at the top of the list we have Japanese knotweed.

Japanese knotweed – Reynoutria japonica (synonym Fallopia japonica)

Introduced in the Victorian era Japanese knotweed was regarded as an attractive ornamental plant. However, it has now spread across most of the UK. The reason for this spreading success is that the rhizomes below the ground are highly vigorous. New plants can grow from just a centimetre of root fragment, making this plant so hard to manage. Japanese knotweed is regarded as a blight to residential properties due to the long time frames required to treat it. Thus preventing the “quite enjoyment” home owners would expect from their gardens. In expensive areas such as London, this can significantly affect the value of the property.

Left alone Japanese knotweed forms dense stands which are extremely hard to control and costs the GB economy £166 million a year. Whilst Fallopia japonica cannot produce seed, Japanese knotweed is capable of creating hybrid plants when crossed with Russian vine, Giant knotweed or Dwarf knotweed. These hybrids, often referred to as Bohemian knotweed, have the capability of producing potentially fertile seeds.

Fully grown Japanese Knotweed can reach three metres, so at this height it’s hard to miss. However, being perennial it changes with the seasons, starting with new knotweed shoots in spring, growing to its full height by July. It flowers in late summer before dying back in November leaving brittle leafless knotweed canes in winter.

Remedy: Japanese knotweed needs to be professionally managed in order that mortgage companies will lend against impacted properties. You can send us a photo if you suspect you have Japanese knotweed and we will be able to help you, offering our treatment programmes should the plant be confirmed as Japanese knotweed.

The images below will give you some help in identifying Japanese Knotweed;

Supple young stems turn Bamboo-like with age – Shield shaped leaves – Late summer flowers (white clusters) – Bare winter stems

The NNSS identification sheet for Japanese knotweed can be found by clicking the link.

Giant Hogweed – Hercaleum mantegazzianum

Not only is Giant Hogweed a towering thug of a plant, growing up to 2-3 m tall, with leaves often at least 1 m wide, it has also earned the label of being the most toxic plant in the UK.  Its sap is to be avoided at all costs as it can cause massive blisters and ulcers, long-term scarring and photosensitivity. The internet is full of photos of disfigurements caused by contact with Giant Hogweed. In 1981 legislation was passed to make it illegal to spread or plant Giant Hogweed in the wild, and it is now recognised by the UK’s environmental protection agencies as a number two most undesirable weed after Japanese knotweed.

Blisters will form within 48 hours of contact, and black or purplish scars may be left on the skin for several years. Contact with the plant can result in hospitalisations, with children suffering third-degree burns to their skin. This severe reaction is caused by the presence of linear derivates of furanocoumarin in the plant’s leave, seeds, flowers, roots and stems. These chemicals enter the nucleus of human epithelial cells, form bonds with the DNA and cause cells to die.

Originally introduced as an ornamental in Victorian times it quickly escaped into the wild and is often found along river sides as the seeds are dispersed and carried downstream. One Giant Hogweed plant can produce up to 80,000 seeds per flower head.

Remedy: Giant Hogweed needs to be professionally managed due to its dangerous nature. Now is the ideal time for us to treat Giant Hogweed to prevent it generating and dispersing seed. Typically a plant needs to grow 3-5 years before it flowers. Upon successful completion of the seed creation process the parent will die. However if the plant is prevented from seeding by removal of the flowerhead it can live up to 12 years .Prior to flowering a huge cabbage-like bud forms, this needs to be carefully removed and safely disposed of. The plant can then be sprayed with herbicide to kill the parent.

If you suspect a plant could be Giant Hogweed then send us several photos showing the plant detail and the context in which it is growing. We will ben be able to advise you the best course of action.

Giant Hogweed emerges in early spring as tightly folded leaves. Grows up to 10 feet in a season. Releases thousands of seeds before the flower spike dies off during winter

The NNSS identification sheet for Giant hogweed can be found by clicking on the link.

Himalayan balsam – Impatiens glandulifera

Himalayan balsam is another Victorian introduction to ornamental gardens. It can now be found growing wild across much of UK, where it crowds out native plants including tansy which is the only food plant of the highly endangered tansy beetle. Once widespread the tansy beetle was reduced to a single population. Help was at hand with the Tansy Beetle Action Group controlling Himalayan balsam to restore sites across the beetle’s range. This action is proving as success as the Tansy beetles are now found in two areas and numbers increased by 60% between 2015 and 2016.

Himalayan balsam are annual plants growing from seed scattered the previous year or years. Looking a little like the Busy Lizzie to which it is related, Himalayan balsam grows tall stems quickly which produce a succession of pretty snapdragon shaped flowers. Once pollinated, these flowers produce seedpods that have a trigger bursting mechanism for seed disbursal. If you touch a ripe pod it explodes instantly scattering seed over a wide area.

Of most concern is where Himalayan balsam grows on river banks, and huge areas of Himalayan balsam smother all native species and when they die back in winter leave bare earth exposed to bank erosion by the winter rains.

Remedy: The key way of managing and reducing the populations of Himalayan Baslam is to reduce the amount of seed available to grow year on year. This means that there is a time between germination and it creating flower buds that the Himalayan Baslam can be pulled up by hand and composted. While there will still be a seed bank in the soil, this will diminish over the next few year is this pulling is repeated religiously every year.

The challenge is that there is so much Himalayan Baslam in so many locations across the UK it would require a national effort over a number of years to have an impact.

The NNSS identification sheet for Himalayan Baslam can be found by clicking the link.

Buddleia – Buddleja davidii

Varieties of Buddleia are sold across the country by Garden Centres and Nurseries, in a range of different shades and habits, as the Butterfly Bush. Generally an attractive medium to large perennial shrub with long arching branches. Lilac/purple (sometimes white) flowers occur in dense pyramidal shaped panicles, which produce large quantities of nectar – which attract butterflies. The majority of these are well bred and have charming habits, but the original wild species, Buddleja davidii, is a complete opportunist and freely seed everywhere, notably on rough ground and in crevices in buildings.

Buddleia can escape from anywhere and seedlings can germinate between pavers and in pointing in walls as well as cracks in hard surfaces and on any soil they find. Once established their roots can force their way into construction weaknesses creating significant cracks in masonry. Due to this characteristic Buddleia is a particular threat to listed buildings The estimated annual costs due to Buddleia damage to UK properties, both historical and private, is £960k.

Potentially a single flower spike can produce as many as 40,000 seeds so a full sized bush could generate as many as 3 million seeds in a year. These lightweight, winged seeds are easily dispersed by the wind and are often caught in the slipstream of lorries and railway trains that can transport them over miles. The seeds can remain viable in the soil for 3 to 5 years. They require little in the way of soil or moisture to germinate and hence they can easily establish in so many places.

Remedy: While some rhizomous plants take years to manage, particular offending Buddleia plants can be tackled directly with plant and root removal. Measures to prevent seed dispersal require deadheading if the shrub is to be kept. In areas where there is a need to control the spread of Buddleia this can be done with a range of herbicides by competent qualified personnel.

The NNSS information portal for Buddleia davidii can be found by clicking the link.

American skunk-cabbage – Lysichiton americanus

It is quite incredible how many gardens of stately homes have swathes of American Skunk-Cabbage growing happily along the banks of lakes, ponds and boggy creeks. Once you know what the plant looks like you will not be able to avoid it.  In similar ways to the other invasive non-native species mentioned above this plant is very successful in our climate. And as a result it multiplies dramatically extinguishing native flora and fauna.

It makes its presence known by emitting a strong skunk-like odour in wet woodlands where it has become established, crowds out the natives. But again it is capable of escaping the well-ordered garden, and only today has been discovered growing for first time in Derby, see the GOV.UK page: Invasive non-native plant species found in Derbyshire brook.

In 2016, American Skunk Cabbage was banned from sale in the UK. Now gardeners are being urged to make sure that they “Be Plant Wise”, dispose of plants correctly and ensure they do not discard this species in the natural environment.

Remedy: The best approach is prevention. Whilst it is still legal to have American Skunk-Cabbage growing on your land it is listed among the plants under The Wildlife and Countryside Act 1981 as an invasive species. It is therefore illegal to plant them, or allow them to escape into the wild.

American Skunk-Cabbage is hard to control once it has becomes established as the rhizomes grow deep. The plants can be dug up and allowed to die by drying out, or selected herbicide can be used, however, spraying near water requires a licence and needs to be done by a professional company.

The NNSS identification sheet for American Skunk-Cabbage can be found by clicking the link.

Rhododendron – Rhododendron ponticum

The Rhododendron was first introduced into Britain in 1763 from Spain when a gardener Conrad Loddiges who worked at a large estate in Hackney, grew the plant from imported seed. Since when this shrub has spread all over Britain sold as cultivated plants to wealthy landowners. It has been used as a rootstock for grafting more attractive varieties of Rhododendron and this has allowed further spread.

Forming a large evergreen shrub or tree it can grow up to 5 to 10m high at maturity. Its leaves are ‘leathery’ generally elliptic measuring 10-20cm by 2-6 cm and are dark green above with a pale green underside. Flowers appear in rounded heads are dull violet to purple in colour. Later in the season these flower heads are capable of producing around 7,000 seeds.

The NNSS identifications sheet for Rhododendron ponticum can be found by clicking the link.

  • Rhododendron are strictly confined to acid soils, its habitats include moorland, woods, screes, rocky banks, derelict gardens and stream sides.
  • Rhododendron has been found to cause dramatic declines in lower plant and fungi diversity in highly ecologically important Atlantic oak-woods in Scotland (Long and Williams, 2007).

Remedy: Prevention is the best policy. If areas are abandoned and Rhododendron is allowed to take a hold its march can be inexorable. It blocks out the light for all plants that grow beneath it and the leaf litter that forms below the bushes also prevent other plants from growing. Excavation and removal is the best method of control.

“Post rhododendron, a site is inhospitable due to soil/litter modification by rhododendron plus soil biota impoverishment caused by exclusion of plants and establishment of a monoculture, so that restoration of the original plant community will be distressingly slow. To the under informed or misconception driven observer such a site will have the appearance of a place that has been poisoned.” (RHODODENDRON POISONS THE SOIL, DOESN’T IT? Chinese whispers become conservation lore James Merryweather)

Bamboo – Bambusoideae

Bamboos planted for ornamental use in gardens have been identified as a future potential non-native invasive due its similarities with Japanese knotweed. Neither produce seed, but rely on spreading rhizomes out through the ground and producing new plants at a distance from the parent. Once established Bamboo can be very difficult to remove. The rhizomes are very tough and can exploit weaknesses in walls and hard surfaces, worsening previous damage.

Planted as screens at the edges of gardens can leave the possibility that Bamboo rhizomes will invade your neighbours’ garden, and they may not be very happy about this. Additionally if you are planning to excavate an area in order to build it will be necessary to make sure all rhizome fragments are removed first.

Remedy: Care when planting Bamboo is key to future management. Choose your bamboo carefully in the first place. Bamboos are either classed as running or clump-forming bamboos. The following are running bamboos and are best avoided in small gardens: Arundinaria, Bashania, Chimonobambusa, Clavinodum, Hibanobambusa, Indocalamus, Phyllostachys Pleioblastus, Pseudosasa, Sasa, Sasaella, Sasamorpha, Semiarundinaria, Sinobambusa and Yushania

For a well behaved Bamboo chose a variety from the clump forming group. If you plant a running variety in your garden is will grow rhizomes out in all directions and pop up new shoots a long way from the parent. Bamboos like to run along close to the surface and under paving slabs so keep a close watch on what is going on. The best way of planning bamboo in your garden is to contain it within a root barrier like CuTex Copper Composite root barrier.

CuTex root barrier can be installed once bamboo has become established and you want to limit its range. This can be to encircle the plant area or protect boundaries or walls. PBA Solutions are happy to discuss the options open to you for achieving this. We can also offer a dig-out option for major stands of bamboo that require removal.

In Conclusion

Invasive species escaping into the wild create ecological threats to our native species. It is our responsibility to manage non-native plants to prevent them from becoming problems for the natural environment. However these species can also become a problem for home and landowners, creating situations that require the help and support of specialist companies.

This article covers several of the well known non-native invasive plant species; however, there are many others. Cotoneaster and Montbretia are common in the front gardens of many homes, particularly in coastal areas. Whilst Gunnera tinctoria is a dramatic architectural plant with huge leaves which likes to establish itself along waterways.

If you are affected by any of the above plants and would like to talk through the implications and the remedies please call the PBA Solutions Team on 01202 816134 or email us at

We offer a FREE ID My Weed service to UK residents. Simply click the link and complete the form, or email us your photos and we will confirm if you have a non-native invasive species.

Further Information

The Wildlife and Countryside Act 1981 (WCA) is the principal legislation dealing with non-native species. The WCA has been amended in relation to England and Wales by various pieces of legislation, including the Wildlife and Countryside Act 1981 (Variation of Schedule 9) (England and Wales) Order 2010, the Natural Environment and Rural Communities Act 2006 and the Countryside and Rights of Way Act 2000.

Fiona Aucott    May 2019


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