Invasive Plants

There are many plants that will run riot in your garden if left to their own devices, but a little investigative work will help you control any potential intrusion. When they arrive of their own volition, however, what may initially seem innocuous can quickly take over, leaving you with an unexpected battle that could cost more than the loss of your favourite perennials. Invasive plants in the UK – some of which are non-native – often dominate the landscape, overwhelming native species and upsetting local eco-systems; some are also toxic to human and/or animals, and the combination of both factors render a few of them illegal to grow, even on your own property.

We’ve done a little research into the top five invasive plants in the UK and how to recognise them:

Giant Hogweed

Originating in Central Asia, Giant Hogweed was first introduced into the UK in the 19th century as an ornamental plant, and belongs to the same family as parsley, carrot, parsnip, cumin and coriander, but unlike its cousins, its inedible. Whilst it’s often mistaken for Common Hogweed, it is much bigger and can cause severe burns if skin comes into contact with the sap, which can be found on all parts of the plant. Irritation is not always apparent immediately; often it will occur once the skin has been exposed to the sun, and then, severe blistering may appear, and medical attention will be necessary. But it doesn’t end there; once contact with the plant has been made, the skin can remain sensitive to sunlight – phytophotodermatitis – for up to seven years and continue to blister.

The plant can grow up to fifteen feet tall and can produce up 50,000 seeds, which are spread by the wind, birds or water – Giant Hogweed is aften found along riverbanks – and can remain dormant for up to 100 years, so tackling it before it flowers is essential in eradicating the plant. Given the invasiveness of this species and the danger it poses to both humans and dogs, it is illegal to plant it or allow it to grow and affect neighbouring properties. You are responsible for preventing its spread if it appears on your property, but that doesn’t necessarily mean removing it. That said, with such potential for damage, it would be wise to consider the option, but research is needed to ensure no harm to yourself or the environment and correct disposal.

Often mistaken for Common Hogweed, Cow parsley, Gunnera and Rhubarb, it’s best to familiarise yourself with Giant Hogweed to ensure correct identification:

Stems: Green with purple blotches and fine, white, bristly hairs. They’re hollow and ridged with a thick circle of hairs at the base of each leaf stalk.

Leaves: Can be up to 1.5m wide and 3m long, divided into smaller leaflets. They look very similar to the leaves of Rhubarb – sharp and jagged – and the underside is also hairy.

Flowers: They appear in several umbrella-like clusters – called umbels – that form bigger umbrellas that face upwards, and each of these can reach up to 60cm in diameter. Each flower is small and white, possibly with a hint of pink, and they appear throughout June and July.

Seeds: Each plant can produce up to 50,000 flat, oval seeds that are almost 1cm long. They’re easily dispersed and can remain dormant for up to 100 years.

Japanese Knotweed

Another plant brought to the UK as a decorative plant in the 19th century, Japanese Knotweed is not known as harmful, more for its aggressive invasion. Unlike Giant Hogweed, although the plant will flower between August and September, it rarely produces seeds, but it has strong, fast-growing roots that can produce an entirely new plant from the smallest fraction of root that may be left behind. Growing up to 10cm a day, the plant can grow to 3 or 4 metres in height within 10 weeks.

Again, having Japanese Knotweed on your property is not illegal, but ‘causing’ Japanese Knotweed to grow is an offence, and when it comes to the buying and selling of property, the presence of it can have serious consequences as the removal of it is essential and costly. Most mortgage lenders will not offer funds until they receive assurances – a management plan from an eradication company should suffice – that it will be safely removed, and the responsibility falls to the property seller – it’s worth noting that the presence of the plant can devalue a property by up to 20%. Professional removal is recommended as the smallest fraction of root left behind will start the whole cycle again, and as it’s classed ‘controlled waste’, disposing of it is only possible at licensed landfill sites. This particular weed costs the UK economy around £166m a year.

Japanese Knotweed can often be mistaken for Russian Vine, Himalayan Honeysuckle, Chameleon Plant and Red Dragon, so seek expert advice if you are unsure:

Stems: In spring, ground-level crimson-pink buds produce reddish-purple fleshy shoots that grow rapidly to produce dense stands of tall bamboo-like canes that can grow to 7ft; the canes have characteristic purple flecks, and although they die during the winter, the canes can remain for several months.

Leaves: In the shape of hearts or shovels with pointed tips, the leaves can grow to 20cm in length and are light green with red of purple flecks; new leaves will have dark red veins and are rolled up, growing in a zig-zag pattern along the stems.

Flowers: The creamy-white flowers form like tassels in August and September and can reach up to 15cm (6in).

Seeds: Also heart-shaped, they feature small wings, but it’s extremely rare for the seeds to germinate; in the main, new plants grow from nodes or pieces of green stem in soil or water.

Himalayan Balsam

Yet another non-native plant introduced to the UK in the 19th century, Himalayan Balsam is related to the Busy Lizzie and can often be found along riverbanks, in ditches and on waste ground. Like Japanese Knotweed, it’s not a danger to health, but it is extremely vigorous and spreads quickly, often smothering native plants which may have ecological benefits to insects. Because the flowers produce large quantities of nectar, it’s extremely attractive to bees, and aside from the lack of attention to other plants, it enables the weed to produce an abundance of seedpods that burst in an explosive manner, dispersing the seeds. As the plant is often found near water, the seeds – up to 800 per plant – are able to travel further afield, and can survive for up to 2 years.
As it’s an annual, the weed dies back in the winter, and because it’s often found along riverbanks, it can cause erosion; when it reappears each year, it’s reach expands due to its dense root system, colonising areas rapidly, successfully restricting the growth of native species. It can grow up 3m in height, flowering June to October, with the seed pods exploding from late July right through to October. Once again, it is an offence to cause this plant to grow in the wild, so it’s advisable to take steps to eradicate it if found on your property, and ideally before the plants flower and seed to avoid starting the cycle again.

      

Stems: Fairly thin and jointed, the stems are mostly reddish in colour, but some may be green or a mixture of the two; they’re also hollow and hairless.

Leaves: Oval in shape, they’re long and green, with a slightly serrated and reddish edge.

Flowers: They’re delicate and can vary in colour between white, pink and purple. With five petals, they have a hooded appearance.

Seeds: Seed pods explode dispersing the creamy, teardrop-shaped seeds in all directions, up to 7m away from the plant.

Green Alkanet

Green Alkanet has been in the UK since the 1700s but is native to Western Europe, and although technically a perennial, due to its invasive nature, it’s generally classed as a weed. There is no legislation to prevent its spread, but given its tenacity, it will quickly dominate, growing in clumps, and as it can grow to 1m in height, it can easily shade other plants. Like Himalayan Balsam, it is irresistible to bees, and not only does it self-seed, close to the parent plant, seeds can be transported on animal fur or clothing; failing that, it can regenerate from its root, which can cause problems when digging the plant up. Glyphosate-based weedkillers work well with repeated use, but if you prefer not to use chemicals, be sure to dig very deep to ensure you get every bit of the root to avoid the plant’s return.

It is often found growing wild in damp and shady places, but is happy to grow wherever it can, and whilst its flowers are similar to Forget-Me-Nots – it’s part of the same family – the blue colour is more intense, and they have a white centre; appearing as early as March, the blooms may continue through to June. The stems and leaves of more mature plants are covered with bristly hairs that are irritating to the skin and may cause rashes on sensitive skin.

Along with Forget-Me-Mots, Green Alkanet is related to Borage and Comfrey and is often mistaken for all three.

Stems: Green and round, the stems can grow to 1m in height and are covered in white hairs that cause irritation to skin.

Leaves: Spade-like, they’re slim and taper to a point, generally growing to around 20cm in length but can reach 40cm. Just like the stems, the leaves are covered in fine white hairs that cause irritation to skin.

Flowers: Small and delicate, they’re blue with white centres and are around 8-10mm in size. Insects are forced to squeeze inside the narrow tube or use tongues to reach the nectar as the stamens are hidden.

Seeds: Generally fall close to the parent plant but can be transported elsewhere on clothing or animal fur.

Rhododendron Ponticum

Native to the Mediterranean and Asia, Rhododendron Ponticum was also introduced to the UK in the 19th century and was primarily planted in parkland to provide cover for game birds, but it has thrived, to the detriment of other, native plants. It can grow to a height of 8m and almost as wide, blocking out sunlight for other plants, and where low-lying branches touch the ground, they can root, increasing the spread, leaving little room for anything else. It has caused great damage to woodland, particularly in milder and wetter western areas of the UK, preferring the moist climate and acid soil, and as such, is also listed as being illegal to plant in the wild or allow to spread from your own property; that said, it is not illegal to buy or plant in your garden, but it is highly recommended that you avoid doing so. Many other varieties of Rhododendron are available that’ll have less impact on the wild: aside from its growth, Rhododendron Ponticum hosts the Phytophthora fungus which has caused the death of thousands of Oak and Larch trees in the UK.

  

Its flowers appear in spring in large trusses containing 10-15 individual, funnel-shaped blooms, each growing to around 5cm and ranging from a pale mauve to a reddish-purple, and each plant can produce up to a million seeds a year, which are spread by the wind or water as far as 100m away, making it extremely difficult to eradicate it. Digging its root system up is the most effective way of removing this invasive plant, but it’s a huge, demanding task, and as the plant contains toxins that may be harmful to humans and livestock, care should be taken when handling cut foliage and disposing of it, and that should be done responsibly.

Stems: Brown/Copper in colour, the stems are woody, eventually forming tree-like trunks, but the pubescent twigs can grow laterally if low-lying.

Leaves: Large and dark green, they’re glossy and evergreen, lance-shaped and potentially up to 18cm in length. They’re paler in colour underneath and are toxic to animals.

Flowers: Each has 5 petals that taper to a point, and can be mauve or purple, growing in large trusses.

Seeds: Small, reddish-brown and shaped like tubers; each plant can produce up to million tiny seeds a year.

As always, this is just a guide, and should you have any real cause for concern, take appropriate precautions and seek further advice.