Seas of Green – UPDATE
Last September we reported on the installation of black plastic sheeting on a couple of ponds in Bilsdale with the aim of shading out the non-native invasive plant species – New Zealand pigmyweed Crassula helmsii – which was accidentally flourishing there. The idea was to give the plant a taste of its own medicine by depriving it of light.
By two months the pigmyweed was becoming etiolated – pale and weakened due to the loss of sunlight – indicting the sheeting was effecting growth.
The sheeting was left on the two ponds through the winter and spring, and a second survey was carried out this July. The sheeting has killed off 100% of the pigmyweed that was covered, however pigmyweed plants remain around the edges of the ponds, where it was difficult to install the sheeting due to the surrounding vegetation and irregular shape of the pond edges.
The National Park Authority’s southern Apprentice Team will be spraying off the pigmyweed round the edge with a herbicide. The sheeting will remain on the ponds until at least late autumn to try to finish off this invasive species once and for all in this location, allowing the biodiversity of the ponds to recover.
Other non-native invasive plant species
New Zealand pigmyweed is one of the most common non-native invasive plant species found in England, along with Common rhododendron Rhododendron ponticum, Himalayan balsam Impatiens glandulifera, Japanese knotweed Fallopia japonica, and Giant hogweed Heracleum mantegazzianum. They were all introduced into the UK as garden plants. All of these species are present in the North York Moors to some extent, and work continues to control these particular plant species, without natural competition and predators, that can have such a detrimental effect on the area’s habitats and water quality.
We’re grant aiding the removal of rhododendron from important Plantation on Ancient Woodland Sties (PAWS), to help let in the light and give native ground flora a better chance of survival. Rhododendron can harbour the Phytopthora ramorum pathogen which is a great threat to forest species such as larch.
Himalayan balsam can be pulled out/cut down by hand but this needs to be done before the seeds are setting (August/September) because one shake of a plant can release 1000s of seeds that can travel up to seven metres potentially creating 1000s of new plants. Repeatedly removing the plants from a location before they can seed over a number of years will eventually mean this annual plant no longer regenerates there.
Japanese knotweed is trickier to tackle because it needs to be treated by careful herbicide injection. Repeated treatment can kill the rhizome which is so effective at spreading. The accidental breaking up of live rhizomes can spread the plant expediently. Careful disposal is vital.
We’re currently making best use of four years of funding from Yorkshire Water to tackle Himalayan balsam and Japanese knotweed along the banks of the Rivers Esk, Rye, Seph and Seven, through whole catchments and sub-catchments starting at the top. Both species are particularly menacing to river habitats as they out compete evergreen native species and die back in the winter leaving banks bare and prone to erosion increasing the sediment loads in the water.
Giant hogweed isn’t quite so common as the other plants in this area. It can be dangerous to deal with because its sap can burn skin so it needs to be treated with care. It can be cut down or tackled with herbicides, but like all non-native invasive species repeat control will be necessary to achieve eradication at a site.
There are lots of initiatives now across the country to address the threat of these out of place species, it can sometimes seem overwhelming but concerted repeated local efforts can have an effect.