Invasive Aquatics

Aquatic Invasive Species

Several of the invasive species that cause problems are found either in or around water systems. These plants can cause bank erosion, and can take over from native species, but even those that grow on land can spread quickly via waterways. Knighton Countryside Management carry out extensive riparian work, and we are well equipped to work in or near water. Our pesticide accreditations from BASIS reflect our ability to apply herbicides correctly, and we only use properly qualified operators for herbicide work.

There are numerous invasive or damaging plants that grow in or around water. The most common issues can be found below, but please don’t hesitate to contact us if you are looking to control a species not mentioned. Many of the plants are on Schedule 9 of the Wildlife and Countryside Act 1981, which lists plants which may not be planted or otherwise caused to grow in the wild.

There are four basic methods for controlling invasive non-native plant species, and Knighton Countryside can offer any combination of these methods to deliver invasive species control around the country.

  • Mechanical: pulling, cutting, raking, or other methods to uproot or cut weeds.
  • Chemical: controlled application of herbicides under approval from the Environment Agency
  • Biological/Natural: Use of pests and diseases of the target weed to weaken it
  • Environmental: make the environment less suitable for unwanted species

Floating Pennywort: Hydrocotyle ranunculoides
Imported from its Native North America, this can spread at a rate of 20cm per day. Its floats freely or roots in the river bed and forms a thick ‘mat’ which has the knock on effect of blocking out light,, hampering air breathing insects and lowering water temperatures.
Terrestrial growth can be treated with herbicides, and will probably require more than one treatment. Large masses of dying plants need to be removed. It can be helpful to cut the infestation down before treating. Mechanical or hand pulling is more effecting than cutting because it removes roots as well, but even this will probably need following up with a herbicide application, and will need to be constantly monitored and repeated regularly to control the invasion effectively.

Water Primrose: Ludwigia peploides
This highly invasive plant has arrived recently from South America. It is causing severe problems in Europe and is just beginning to encroach in the UK. It grows on river banks and floats on the surface of the water, and has bright yellow flowers and distinctive seed pods.
Terrestrial growth can by treated with an application of herbicide via knapsack sprayer and long lance, but this must only be done with approval from the Environment Agency. For sites with good access, mechanical dredging is an effective solution, and Knighton have the required equipment on hand. A downside to this is the potential damage to other habitats, and Knighton’s staff are knowledgeable enough to understand the delicate balance between effective clearance and irreparable habitat damage. Hand pulling and clearing is more labour intensive but very effective.

Giant Hogweed: Heracleum mantegazzianum
This striking plant generally grows near water courses. It can grow up to 20ft tall, and its white flower head is about the size of a dustbin lid. It was introduced as an ornamental plant in the late 19th century, and has spread via waterways and wind dispersal. It can cause problems such as riverbank erosion and the resulting increased flood risk, but the biggest concern is its sap. This is highly phototoxic and can cause severe burns. Because of this disposal must be carried out with great care.
Hogweed can be controlled by regular applications of a herbicide such as glyphosate. Knighton’s staff hold the required PA1 & PA6 qualifications to carry out knapsack or weedwipe application, and we are able to apply for the necessary approvals from the Environment Agency for applying chemical near water. Hogweed can be controlled by brushcutting or digging out, but these methods must be used with great caution because of the plant’s photo toxicity. This may be necessary in environmentally sensitive areas where it would be inappropriate to use a herbicide. The arisings must then be disposed of under licence as controlled waste. All control methods should be followed by replanting of the areas with more appropriate species to discourage the hogweed from returning.

Himalayan Balsam: Impatiens glandulifera
This pretty plant with its pink & purple flowers is now widespread along river banks and in damp woodland, having been imported from Himalayas in the early 19th century as a garden plant. It grows in dense stands along river banks, and can impede water flow, making areas more vulnerable to flooding. When it dies back in the winter it leaves bare stretches of riverbank which are then prone to erosion. It spreads along watercourses and its seed heads explode, flinging the seeds for up to seven metres.
Control measures include application of herbicide, either by weed wipe or knapsack sprayer. If the surrounding area needs regenerating then it should be planted with suitable species as soon as possible. Himalayan Balsam can also be controlled mechanically with regular brush-cutting or flailing, or is can be hand pulled and the arisings appropriately disposed of. This is a time consuming and labour intensive process but it has excellent results, and the advantage that there is no need for chemical application.

Both the GB Non Native Species Secretariat and The Angling Trust are excellent sources of further information.