Invasive Species Week is taking place from 12-18 May to highlight the impact of plants and animals which threaten native species and upset the balance of our ecosystem. As well as their effect on our natural world, invasive species have consequences on us too – they cost the economy nearly £2 billion a year, interfere with recreational activities like walking, boating and fishing, increase the risk of flooding, and, in certain instances can harm our health. During spring, Herts and Middlesex Wildlife Trust and the charity’s army of volunteers set to work on minimising the effects of these unwanted species on their reserves.

Here are four examples of invasive species control work being carried out by the Trust in Hertfordshire:

Himalayan Balsam at Tewinbury Nature Reserve

Himalayan Balsam poses the biggest threat from any invasive species on the Trust’s nature reserves. Originally introduced by Victorian plant collectors, the plant grows quick and tall, overshadowing other plants and dominating vegetation along riverbanks to the detriment of slower growing native plants – it makes riverbanks unstable too as its roots are very shallow compared to the plants that should be thriving in that habitat. The seeds of Himalayan Balsam are present in the flower heads, which, if left, can explode to set seed in a wide area and remain viable for up to 2-3 years. The seeds can also wash down waterways and infect new areas easily. At this time of year, the Trust and their volunteers actively pull-up the plants before they flower – and with shallow roots they can make a rapid impact on plant populations. This action is called ‘Balsam bashing’ and is particularly pertinent at Tewinbury Nature Reserve, between Welwyn Garden City and Hatfield, a Site of Special Scientific Interest (SSSI), with an extensive reedbed. The reserve has high concentrations of Himalayan Balsam but getting rid of it is not straight-forward – the site is home to many nesting birds and it is vital that they are not disturbed during breeding season so it is difficult to ensure no plants set seed. Controlling its impact is therefore the target. The Trust’s volunteer work parties also tackle Himalyan Balsam on their nature reserves at Archers Green in Tewin, where they are helped by volunteers from the site’s fly fishing club, Amwell near Ware, and Rye Meads in Hoddesdon.

Small Balsam at Hertford Heath Nature Reserve

Small Balsam is a plant of the same family as Himalayan Balsam but in contrast has yellow flowers, is much smaller and grows in woodland habitats. It is found at the Trust’s Hertford Heath Nature Reserve, just outside the county town, and is managed in the same way as Himalayan Balsam. Although much less problematic for the Trust than its larger cousin, it shares the same habit of domination and thereby impacts on biodiversity.

Goats Rue at Waterford Heath Nature Reserve

Goats Rue is another attractive looking hazard. At the Trust’s Waterford Heath Nature Reserve, near Hertford, its seed was introduced in topsoil that was added to the site after quarrying. A perennial plant with a bigger root system, this plant cannot be easily eradicated and the Trust’s volunteers have to dig it out by hand in small areas to try and contain it, and, in larger areas, cut it to attempt slowing its growth. The Grizzled Skipper, a species of butterfly, is impacted by the plant as it can overshadow and halt the growth of the low-growing wild strawberry plants, which the butterflies feed upon.

Hemlock at Amwell Nature Reserve

Hemlock is a poisonous plant, which features in many dark, historic tales. Although a native plant, Hemlock is an invasive species whose might is gathering pace due to our changing climate. Warming temperatures are seeing the plant spread widely on road verges and across the countryside. The plant is prevalent at the Trust’s Amwell Nature Reserve, near Ware, where it had grown in concentration to a height of two to three metres, spreading across open grassland. For the last two to three years, the charity’s team have used small mattocks (a mini pickaxe-type tool) to tackle the roots of the plant and have been successful in virtually clearing the Hollycross compartment (also known as the dragonfly trail) of the reserve. This has allowed natural grassland species to recover, which should, in turn, benefitted pollinators and other insects, restoring the biodiversity of the site and enabling livestock used for conservation grazing to do so safely.

Kathryn Dunnett is a Reserves Officer at Herts and Middlesex Wildlife Trust and manages volunteer work parties on the Trust’s reserves. Kathryn said,

“With warmer winters and our changing climate, we are seeing a shift in the types of invasive species we are dealing with, with the more prevalent and dominant species becoming even more so, creating way more work to keep on top of. For example, we are having to clear vegetation from the pathways on our reserves probably twice as much as we would have had to 20 years ago.

“It’s not just climate change that’s impacting on the upsurge in invasive species though, pollution is another factor. With more nitrogen in the air, from sources such as air traffic and combustion processes from engines, that is going into our soil and increasing nutrients, favouring the fast-growing and more dominant plants and negatively impacting on smaller, slower growing plants, such as delicate grasses. The plants subsequently being put under threat are vitally important to native invertebrates, resulting in detrimental effects on biodiversity overall.

“The work we are doing on our reserves is therefore vital to protect these prime wildlife sites. We are extremely thankful to all of our wonderful volunteers, who attend our work parties, and who work tirelessly to control invasive species so that our local wildlife can recover and thrive. We could not achieve what we are doing without them.”

For more information about the work of the Trust see our latest news stories and to find out more about volunteering click here.