Overview
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.
The Client
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.
The Challenge
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.
If the New Zealand pigmyweed were to spread, it will likely outcompete native species due to several of its characteristics, namely, its ability to survive on earth and water, and its ability to grow into a full coverage mat. It forms a dense mat covering which creates shade over other plants, which prevents light getting to other plants for vital photosynthesis.
There are several important points the Ecus team had to consider. Strict decontamination and biosecurity protocols have to be followed to prevent carrying the invasive species over to a new, previously unaffected area.
The depth of the ponds posed difficulties as they were not deep enough for a wader but too deep for operatives to safely enter to carry out the work. The damp ground also made these complex sites to work on as machinery wasn’t appropriate or suitable.
Why is New Zealand Pigmyweed a problem in the UK?
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 Solution
Ecus combined methods of treatment to maximise the chances of success of the project while minimising further unintentional consequences to the surrounding environment. The use of high-density polyethylene sheeting to smother the New Zealand pigmyweed is effective in making the New Zealand Pigmyweed die off as the plant can no longer photosynthesize. This helps to avoid the use of large amounts of herbicide in close proximity to a water course.
After initial steps were taken, including an ecology survey to establish presence or absence of protected species such as Great Crested Newts (GCNs), work began. There was the potential to use an excavator with a flail head, but this was not feasible due to the depth of the water. The team was able to quickly adapt and manually flail the peripheral vegetation.
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.
Client Benefits
- Prevention of damage to surrounding habitats and ecosystems
- Preservation of local native species
- The nearby river Seph has suffered with invasive species such as Himalayan balsam which has been the subject of ongoing control programmes
- Preventing impacts from the propagation of further invasive species protects previous investments as the catchment has also benefitted from a Landscape Partnership scheme (Ryevitalise) made possible with The National Lottery Heritage Fund, with thanks to National Lottery players, to improve habitat connectivity and improve water quality by reducing sediment inputs
- Peace of mind that the two areas affected were treated with the most appropriate measures available in terms of efficacy in controlling the invasive species and mitigating environmental impact
Through the Ryevitalise Landscape Partnership we have been seeking to address invasive non-native species in the western part of the Rye catchment. The National Park Authority were aware of the presence of Crassula on the two ponds in Bilsdale and had tried previous control efforts without success. Contractors were sought to assist with control in this location and Ecus provided a clear methodology of their planned approach and justification of the benefits this would bring. Communication throughout was prompt and efficient, providing supplementary information when required and giving adequate prior notice of site visits and works commencement while keeping landowners fully updated. Despite some delays due to issues with third party suppliers the work programme was followed and delivered on time with the sites left in a safe condition and no remediation required. The installation of HDPE sheeting is a more robust material (compared to those previously used), combined with the novel approach of welding the seems to create one continuous sheet which provides a more effective control option to block out sunlight. Several months after installation the sheeting is still securely in place, we eagerly await the follow up inspection next summer to review efficacy and subsequently remove the sheeting.
James Caldwell Ryevitalise Senior Catchment Restoration Officer
