Ecology

Ecus is using innovative techniques to control the invasive species Crassula helmsii, otherwise known as New Zealand pigmyweed or Australian stonecrop, as part of a pond restoration project in the North York Moors National Park 

Ecus has been working to control an invasive plant species as part of a complex project restoring two ponds in the North York Moors. Schedule 9 of the UK Wildlife and Countryside Act 1981 lists this plant as one that must not be caused to grow in the wild. Invasive species can have serious impacts on our ecosystems and native biodiversity by outcompeting other species or spreading disease. 

North York Moors National Park approached Ecus as they were aware of the issues around the spread of New Zealand pigmyweed across the two ponds within the National Park. These moors and the accompanying nature are widely recognised for their benefits to human health, wellbeing, and development. The North York Moors National Park recognised that they needed assistance to control this invasive species from two pond areas and prevent it spreading – there was a particular risk of the invasive species spreading to a nearby waterway. 

According to Department for Environment, Food and Rural Affairs (DEFRA) tackling invasive species costs the UK roughly 1.8 billion pounds per year.  Invasive species can cause extinctions of native plants and animals, reducing biodiversity, competing with native organisms for limited resources, and altering habitats.     

New Zealand pigmyweed was present in two separate pond areas in the North York Moors. There was a risk that this could spread, especially to the river Seph nearby, where expensive remedial action had already been taken in previous years against other invasive species. 

New Zealand Pigmyweed, also known by its scientific name Crassula helmsii, is listed as an invasive non-native species (INNS) under Schedule 9 of the Wildlife and Countryside Act 1981. This means that it is an offence to ‘plant or otherwise cause to grow a plant in the wild’. In 2014, along with 4 other plants, it was banned from sale in the UK.  

This INNS had taken hold in two ponds near to a water course – the river Seph. If the crassula were to propagate along the water way it would likely outcompete the native flora and could have a negative impact on fish and invertebrate communities. 

This is because New Zealand pigmyweed is a highly adaptable plant which can tolerate many different growing conditions. It can grow below the water’s surface or at the margins of water. It is even semiterrestrial – meaning it can also grow on land if the ground is damp enough. New Zealand pigmyweed is a tenacious plant and it grows throughout the year with no dormant period. It has a terrestrial form and it can grow submerged or emergent from water. 

It has a high capacity for regeneration as it readily breaks into small fragments. It also has little winter die-back and can form dense stands of 100% cover, which has negative environmental, aesthetic and economic impacts. 

The purpose of section 14 of the Wildlife and Countryside Act 1981 (‘the Act’) is to prevent the release into the wild of certain plants and animals which may cause ecological, environmental, or socio-economic harm, so it prohibits the introduction into the wild of any animal of a kind which is not ordinarily resident in and is not a regular visitor to Great Britain in a wild state, or any species of animal or plant listed in Schedule 9 of the Act. Schedule 9 lists non-native species already established in the wild, which pose a conservation threat to native biodiversity and habitats, so further releases should be regulated. 

The team looked to place a 2 metric ton sheeting to provide coverage on each pond to smother the invasive species by blocking sunlight for photosynthesis. The material used was Carbufol, a 1mm thick HDPE sheeting which comes on 5m x 200m rolls weighing in at around 1 metric ton each. Any areas not covered by the sheeting could then be manually spot treated with an appropriately selected herbicide, mitigating the risk of any pesticide reaching the nearby river Seph. 

The pond was measured, and the appropriate amount of sheeting was rolled out in the field adjacent to the pond using a 10m telehandler and an 8t excavator. The sheets were then welded together using a Leister wedge welder. 

Due to access issues on the first pond, the team had to use four welded sheets to achieve full coverage, each sheet weighing between ¼ and ½ a tonne. The sheets were then drawn across the pond using the telehandler and excavator. Once the sheeting was in place, it was stretched out by hand to cover the pond surface and vegetation including crassula to ensure adequate control. 

The sheeting was then staked around the perimeter and along the joins, with rope fed through staples at the base of the stakes. The stakes were then driven further into the ground to pull the rope tight to ensure a secure, tight fit. 

The team were able to problem-solve quickly, being able to draw on decades of expertise in habitat management. Even despite some delays due to issues with third party suppliers the work programme was followed and delivered on time through our team’s adaptability.