Alex Cripps – Conservation Graduate Trainee
As well as controlling Himalayan balsam along the River Rye and River Seph in the west of the North York Moors, the National Park Authority is continuing to tackle invasive plant species in the River Esk catchment in the north of the Park too. We concentrate our efforts along rivers and watercourses because this is where invasive plants are most problematic. Increased sedimentation in the Esk caused by invasive plant species is particularly threatening for the important protected species found there like the freshwater pearl mussel.
Himalayan balsam control work has been going well over the last few years in the upper Esk catchment so this year work effort has moved downstream and is now being concentrated in the Sleights and Ruswarp area near Whitby.
There has been a lot of help from landowners and members of the public letting us know where the balsam is growing. With a big concerted effort from our Mussel Volunteers and local contractors, coordinated by Simon Hirst our River Esk Project Officer, a huge amount of balsam has been pulled up this season. It is really important to hand pull or strim the balsam early, before the seed pods develop, as the seed pods explode catapulting hundreds of seeds up to 8m from the parent plant.
Seeds are transported by waterways and can stay viable in the soil for a few years, so it is going to be important to continue monitoring sites every year. It usually takes atleast 2 – 3 years to eradicate Himalayan balsam at a site. The plant can also be controlled by spraying it with pesticide (using a pesticide approved aquatic use when working near water) but this method is only used for areas with a dense coverage of balsam to ensure we’re not affecting other plant species.
Japanese knotweed is another non-native invasive plant found in the Esk catchment. Like Himalayan balsam it too creates large monocultures, shading out other plant species. Japanese knotweed does not produce seeds but instead it spreads vegetatively and can regrow from fragments of stems or rhizomes. It’s incredibly tenacious. So it cant be strimmed or mowed because that will just spread the plant and this makes it incredibly hard to eradicate.
The best method of control is by stem injection, whereby stems are injected with a pesticide. This is obviously very labour intensive but it is a very effective method and does not affect non-target plants. Spraying Japanese knotweed using a knapsack sprayer is also another method our contractors use, but this can only be done on a dry, calm day.