Invasive Plants

Keeping track of plant introductions and invasive plant spread, and knowing how to identify these species is important to conserve our ecosystems – this is where the Welsh National Herbarium comes in…

Have you heard of the term “invasive” species?

As international travel has become easier and more accessible over time, we’ve increasingly moved plants, animals, fungi and algae around the world. Sometimes this happens intentionally – for example, importing pretty plants to be grown in gardens – and sometimes accidentally, such as marine species picked up in the ballast water of cargo ships. You can learn more about invasive species and marine invaders here.

Non-native vs invasive species…

The number of species introduced from their native range (where they originally occur) to new locations has been increasing for years. Species that have been transported to an area they couldn’t have colonised by themselves are called “non-native species”.

Although most of them are harmless, 10-15% of non-native species end up thriving in their introduced habitats – they spread and cause negative impacts to our native biodiversity and ecosystems. These are called “invasive species”, and they can have serious knock-on effects for our health, the climate and our economy.

Why are plants important?

We often forget that we interact with plants every day. The array of habitats found in Britain are made up of a diversity of uniquely adapted species, and plants form the basis of many of these habitats. Plants act as food sources for a huge diversity of organisms (including us!); they also influence our physical environment, for example reducing flood risk by stabilizing riverbanks and storing water. Some plant species are special for being so rare and only growing in the British Isles. However, our flora is changing; we now have more non-native than native plant species in Britain!

Invasive plants – how do they get here and what’s the problem?

There are 36 plant species that are of special concern in Britain, but there are also more species that are considered invasive and have special legislation around how they can be handled to control their spread. Mainly these species have ended up in Britain through horticulture, but less commonly, invasive plants turn up by hitch-hiking in imported materials or in the ballast of ships.

Horticulture

Many species have been imported as ornamental plants because they are pretty and were thought to make for good garden interest – this was particularly common in the Victorian period, before we knew much about the risks of introducing species to new lands!

Japanese knotweed/Clymog Japan is a prime example. This species was introduced to Britain in the 1800’s, where it was propagated and became a popular exotic garden plant. However, records of garden escapes were recorded from as early as 1907 in Glamorganshire and to this day, Japanese knotweed is particularly problematic in South Wales. Evidence indicates that every plant of Japanese knotweed (Reynoutria japonica var. japonica) growing in Britain originated from a single female plant introduced all those years ago! You can find this plant growing along roadsides, riverbanks, railway lines, woodlands and grasslands, where it spreads by deep underground rhizomes (underground stems that produce new shoots) and seriously reduces biodiversity. Around £1.7 million is spent in the UK every year on managing Japanese knotweed and mitigating against its damage!

Japanese knotweed/Clymog Japan growing in South Wales.

Specimen of Japanese knotweed in the Welsh National Herbarium (Amgueddfa Cymru), collected from St Fagans.

Himalayan balsam/Jac y Neidiwr (Impatiens glandulifera) was also introduced as an ornamental plant from the Himalayas in 1839. It is found today growing along riverbanks, in woodlands and along roads. It has explosive seed pods which allow it to spread across large distances. Although the flowers are beautiful, it outcompetes native species, reducing biodiversity. It also has shallow root systems which can worsen riverbank erosion, which in turn can reduce water quality and increase the risk of flooding. A study by Cardiff student, Amy Wyatt, found that since 1865, Himalayan balsam has evolved to better adapt to its introduced range.

A specimen of Himalayan balsam/Jac y Neidiwr.

Contamination of imported materials

Pirri-pirri burr (Acaena novae-zelandiae) found its way to Britain from south-eastern Australia and New Zealand as a hitch-hiker on imported wool. It has hooked seeds which can easily attach themselves to fur and clothing. Like many invasive plants, this species can outcompete our native flora and form dense stands which reduce biodiversity.

Example of Pirri-pirri burr flower, showing hooks/spines that allow seeds to spread by attaching to fur and clothing. 

A specimen of Pirri-pirri burr.

Ragweed (Ambrosia artemisiifolia) is native to North America and is thought to have arrived in Britain in bird seed, horticultural soil and soil used to ballast ships. It has highly allergenic pollen and can cause hay fever.

Heath Star Moss (Campylopus introflexus) is an invasive moss that is thought to have originated in the Southern hemisphere and to have been introduced with produce transported by ships. It is also known as Tank Moss, for its suspected spread via World War II tanks. An expert coloniser of acidic bare ground, such as that found on coal tips, it is now common in Wales where it threatens these wildlife-rich habitats.

Heath star Moss growing in Tylorstown.

Recently, a new fungus (Bryoscyphus granulosus) has been found infecting Heath Star Moss and closely related mosses. The fungus causes the moss to die back and form dead patches that expand outwards, like a fairy ring. Specimens of this fungus have been donated to the museum recently by former curator George Greiff. These include the ‘holotype’, the original specimen used to describe the fungus as new to science. This fungus parasite has significant ecological importance, perhaps with the potential to control an invasive moss that can cause substantial harm.

How we’re working on invasive plants at Amgueddfa Cymru

The Welsh National Herbarium held at National Museum Cardiff contains over three quarters of a million plant specimens, representing around 300 years of collecting and covering virtually all species found in Wales. The herbarium is a treasure trove of information that acts as a reference point for identifying plants, where species are found, and how non-native and invasive plants are spreading across the British Isles. You can learn more about what an herbarium is and how we preserve plant specimens here. The Botany team work hard to look after and develop these collections of specimens for the benefit of us all, and to support ongoing research into the changing diversity of our ecosystems. We form a small part of the Natural Sciences team at Amgueddfa Cymru, made up of curators who look after the natural history collections. Some of us specialize in species taxonomy (naming and classifying species) and ecology; some of us research non-native and invasive species so we can understand their spread and impacts.

What can you do to help?

There are many ways to help stop the spread of invasive plants. You can find out more here. You can help prevent future invaders by keeping an eye on the plants in your garden and alerting others to species that may jump the garden wall. There are also ways to stop the spread of invasive species in and around water – learn more here.