Many people think that plants, while beautiful to look at, are defenseless and passive organisms. However, the many alien species – or “monster plants” – around us show that we should never underestimate plants and the fascinating battles that take place beneath our feet.

Many of the first alien plants – those introduced by humans outside their natural range – to arrive in Britain were brought by plant hunters in the Victorian era.

Plants have adapted to their environment over thousands of years, so you’d expect alien plants to struggle in new places. However, some survive a little too well and are so devastating to the local flora that you need a permit to grow them.

One of these monster plants is Himalayan balsam (Impatiens glandulifera). Invasive plants often grow faster and larger because they don’t have to compete with their native enemies. This means they can grow taller than their native counterparts and literally outshine other species.

Himalayan balsam is native to the western Himalayas and was brought to Britain in the 19th century by plant hunters who liked its pink, orchid-like flowers and exploding seed pods, which can hurl seeds up to seven metres.

This plant escaped from gardens and spread along the British river network, where it formed dense swathes along river banks. Himalayan balsam also successfully invades forests. It is the tallest annual herb in Britain, growing up to four metres tall. However, in its native range Himalayan balsam only grows half that height. In summer, British conservationists engage in ‘balsam bashing’ to stop its spread.

When we see plants in the wild, we may not think about what is happening beneath them. For example, many plants form relationships with fungi and bacteria that can provide them with nutrients that might not otherwise be available to them.

When an alien plant species arrives in a new environment, the soil community may be different from its native environment. Some of these plants respond by altering the soil to promote their own growth.

Himalayan balsam does this by reducing the number of mycorrhizal fungi – the fungal species that form mutually beneficial relationships with other plants below and above ground. It changes the bacterial community in the soil and some of the fungal community above ground. When Himalayan balsam grows in soil that it infested the previous year, it grows faster and larger, which scientists have linked to these fungal and bacterial changes. And even if you remove Himalayan balsam, native plants often don’t grow as well in that soil.

Another example of monster plants in the UK is the rhododendron (Rhododendron ponticum), an invasive plant from southern Europe and south-west Asia that grows up to eight metres tall and thrives in woodlands, shading out native plants. Rhododendron is host to two deadly pathogens, Phytophthora ramorum and P. kernoviae, which infect native trees in the UK.

Japanese knotweed (Reyneutria japonica), a monster from Asia that reproduces by cloning, can regrow from a fragment weighing just 0.3 grams (about the weight of a pinch of salt). This plant can be so damaging to buildings that British homeowners must declare in their legal documents if there is any evidence of the plant on their property when selling their home.

The giant hogweed (Heracleum mantegazianum) is native to Central Asia and can grow up to five meters tall. Each plant produces about 20,000 seeds. As a defense mechanism, the giant hogweed secretes poisonous sap. This sap causes severe skin blisters and burns that can last for months or even years.

Monsters all over the world

As people traveled more and more around the world, the species they brought with them also increased. Monster plants are a problem on every continent on Earth. Invasive plant species can even be found in Antarctica.

Pines were introduced to southern Africa as timber following European colonisation in the 1650s. South Africa is already drought-prone, but a single pine tree uses 20% more water than native plant species. In some rivers, pine invasion upstream can reduce natural flow by as much as 45%, meaning less water for people and ecosystems.

Kudzu (Pueraria montana) is a climbing plant native to Japan and China that was introduced to the United States in the 19th century to prevent soil erosion. It can grow up to half a meter a day and smothers grasses and other plants, including trees, with its creeping vines that can grow up to 30 meters long. In her novel The Color Purple, Alice Walker wrote, “Racism is like this local creeping kudzu vine that devours entire forests and abandoned houses.”

Gorse (Ulex europaeus) is native to continental Europe and Britain, but was introduced outside its original range mainly in the 19th century as a nostalgic ornamental and hedging plant in several European-colonised countries. This woody, thorny shrub can grow up to seven metres tall in its original range, compared to just two and a half metres in its native habitat. Gorse increases the risk of fire in Chile, reduces the growth of tree seedlings in New Zealand and acidifies the soil, stopping the growth of rare plants in British Columbia, Canada.

Mesquite (Prosopis juliflora), also known as the “devil tree” in infested areas such as India and parts of Africa, is native to Mexico and parts of South America. This species has been introduced outside its natural range as firewood and livestock feed, but it grows quickly to heights of up to 12 meters. It has large thorns and low branches that form impenetrable thickets and overgrow pastures and wetlands. Livestock that eat large quantities of its seed pods become poisoned and often lose their teeth due to the high sugar content of the pods. In dry, arid regions with water shortages, the root system of mesquite absorbs water more quickly than native plant species.

For all these different reasons, invasive alien species are one of the biggest causes of native species loss worldwide. However, eradicating these plants is a difficult and expensive task. For many species, the best we can do is reduce their size and distribution.

The most effective strategy, of course, is to prevent alien species from arriving in the first place. Today, many of the alien plants growing wild in the UK have escaped the horticultural trade, so choose native plants for your garden and for gifting.

And if you like something a little more exotic, botanical gardens offer a wonderful opportunity to enjoy the extraordinary plant diversity without damaging the local habitat.

This article is republished from The Conversation under a Creative Commons license. Read the original article.