As its name suggests, Himalayan balsam is native to the Himalayas and was introduced to the country in 1839 as a garden plant from which it escaped within decades, quickly becoming one of the most invasive weeds. It’s posing a serious threat to riverbank flora but is spreading further towards road verges and damp woodlands.

It’s pretty pink nectar-rich flowers are said to have contributed to its spread, as it’s attractive to both people and bees. Wherever Himalayan balsam can get a foothold, it outcompetes native flora by overshadowing and smothering it. An annual plant, it grows up to 10 feet within a single year, then dies down in winter leaving riverbanks bare. Exploding seed pods catapult the seeds up to 22 feet away, which then stick to shoes and car tyres, spreading the weed even further.

It is now a legal requirement for landowners to tackle Himalayan balsam on their land, so every summer, Herts and Middlesex Wildlife Trust organises “balsam bashing parties” to try to free its nature reserves from the invader. The Trust has also come up with an idea to approach this problem: In a partnership with Tring-based Puddingstone Distillery, it produces the world’s first Himalayan balsam-induced pink gin. After all, all part of the plant are edible, so we might just eat and drink our way out of this.

Not much bigger than a dog, this non-native deer is a common sight in Hertfordshire’s woodlands, parklands and even gardens. It has quickly gained a stronghold across South East England after being introduced to Woburn Park in Bedfordshire at the start of the 20th century. 

Native to South-east China, muntjacs are notorious browsers, eating the shoots from shrubs as well as woodland herbs and brambles – in high density, they can cause significant damage to the local flora. Like other deer species in the country, muntjacs are thriving due to a lack of natural predators such as lynx, wolfs and bears that would have controlled populations hundreds of years ago.

Escaped from fur farms in the 1950s and 60s, the American mink is one of the biggest threats to our native water vole which is now the fastest-declining mammal in the UK. Minks are fierce predators, feeding on anything that is big enough to catch, including ground-nesting birds and water voles, with devastating impacts on biodiversity. They are good swimmers which can easily enter the water vole’s underground burrows taking their young.

Almost half of the world's bluebells are found in the UK, but our native favourite blue flower is being outcompeted by a Spanish invader. The Spanish bluebell is said to be a garden escapee from the 19th century and is now widespread across the country. Compared to its English cousin, the Spanish bluebell is much more robust and can spread quicker. English bluebells are very delicate and it can take years for them to become established. There are now many hybrids between these two, so an unambiguous identification may prove difficult. Generally, our English native has bell-shaped flowers with a more drooping stem. Spanish bluebells in your garden are not necessarily a reason to worry – unless you live close to woodlands – but it’s important to keep an eye on them so that they don’t spread uncontrollably.

Among the muted plumage of many of our native birds, the ring-necked parakeet is probably the most exotic invader. It is native to central and South Africa and Asia, but its origins in our area are a bit more mystical – rumour has it that birds were released from film studios in Middlesex after filming The African Queen in the 1950s. Fact is that it is now thriving in London and throughout the Home Counties, seemingly preferring the urban jungle. While it has been spreading fast, its threat to native wildlife hasn’t yet been established, but conservation organisations are carefully monitoring numbers and their impact.