A small number of plants such as Japanese knotweed, giant hogweed and bamboo are highly invasive in the UK. Invasive plant species can damage buildings and structures, so they need to be considered and noted whilst undertaking building surveys. In the document below @Callum Halliday provides a review of some of the main invasive plant species which can be encountered and how they can be identified and eradicated.
Japanese Knotweed
What is Japanese Knotweed?
Japanese knotweed is a non-native species and was introduced into the UK in the early 19th Century and is now the country’s most invasive weed and is typically found on brownfield sites. It grows rapidly and can cover large areas to the exclusion of other plant species. It has been known to grow through building components such as foundations and flooring, causing significant problems for owners and occupiers.
Why is Japanese Knotweed Considered an Invasive Species?
Japanese knotweed is a highly invasive plant species and is difficult to control due to its extensive underground rhizome system spreading horizontally to great depth, which enables the plant to survive when all above ground parts of the plant are removed. It grows to a height of roughly 3m and has canes resembling bamboo which grow densely in the summer and die back during winter. If the root system is not killed or removed in its entirety, it will continue to grow, infesting more land and exacerbating the problems it causes.
Identifying Japanese Knotweed
Early identification of Japanese knotweed is essential to prevent its rapid spread and any potential legal ramifications. The appearance of the plant changes throughout the seasons, starting with new shoots in spring, growing to its full height by July, flowering in August, before dying back in November.
Legislation
If a developer or land owner is found to have caused the plant to have spread off site, they could find themselves at the wrong end of criminal proceedings under either the Wildlife and Countryside Act 1981, or the Environmental Protection Act 1990. Offences under these Acts, whether intentional or not, can result in significant fines and, in extreme cases, custodial sentences.
Eradication Methods:
- Herbicide treatment - typically applied either as a foliar spray or by stem injection. Discussion on the effectiveness of stem injection centres around whether it simply kills the above ground part of the plant, leaving the rhizome system alive but in a state of temporary dormancy. However, application by foliar spray is tried and tested, and if done correctly can kill knotweed. Note that this is not a one-off treatment and can take up to three years.
- Physical removal - if the ground within an area of knotweed-infested land is likely to be disturbed, then herbicide treatment alone is unlikely to be sufficient and a physical removal method will be preferable. This comes with potential implications, as the removal of contaminated soil carries the risk of spreading the plant; disposal must be under controlled conditions and transport off site can be very costly.
Giant Hogweed
What is Giant Hogweed?
Giant hogweed is a poisonous non-native species to the UK and was first introduced as an ornamental plant in the late 19th Century before escaping into the wild. Once established, it can dominate other plants and damage the natural habitats of insects and other animals and can also create very unpleasant skin conditions caused by its sap. During the winter the plant dies down, leaving bare patches which can result in soil erosion.
Why is Giant Hogweed Considered an Invasive Species?
The control of giant hogweed is difficult as, unlike knotweed, plant propagation is by seed, with a large plant producing up to 50,000 seeds. It is often found growing along railway land, footpaths, roads, rivers and wasteland and its seeds can be transported along waterways spreading to areas further downstream. Carrot fly can inhabit infestation making it even more undesirable.
Identifying Giant Hogweed
As its name suggests, giant hogweed is characterised by its size, often growing to 3-5m. The plant has a purple to red stem and furry spotted leaf stalks with large leaf and flower growth.
Legislation
Under the Wildlife and Countryside Act 1981, it is an offence to plant or otherwise cause giant hogweed to grow in the wild. Offences under this Act, whether intentional or not, can result in significant fines and, in extreme cases, custodial sentences.
Eradication Methods:
- Herbicide treatment – treating the active plant growth with glyphosate is effective if undertaken regularly; this is because seeds can remain dormant for up to seven years. It is essential that spraying is undertaken early into the growing season, before the flower heads have time to develop and produce new seeds.
- Physical removal – the alternative is to dig up the plants and dispose of them as controlled waste, although care is required as during removal some of the plant’s seeds will fall to the ground. It is essential to not distribute dropped seeds and also to minimise the risk of exposure to sap during removal to avoid potential health implications.
Bamboo
What is Bamboo?
Bamboo is a non-native species to the UK and was introduced into the UK in the early 19th Century and has very similar characteristics to Japanese knotweed, although, unlike Japanese knotweed, bamboo remains largely unregulated, with no legal obligations for owners or occupiers; this leaves neighbours and potential buyers vulnerable to large removal costs due to its destructive effects.
Identifying Bamboo
There are over 1,200 species of bamboo although they are generally categorised into two groups, ‘running’ or ‘clumping’ - dependant on their characteristics.
- Running bamboo - spread widely, sending out rhizomes which sometimes range far from the parent plant from which stems arise from.
- Clumping bamboo - have a very short root structure, are genetically incapable of expanding more than a few inches a year and will generally form discrete circular clumps.
Is Bamboo Considered an Invasive Species?
Like Japanese Knotweed, Bamboo is spread through a rhizome. Clumping bamboo remains as a large root ball, whereas running bamboo creates a long network of rhizomes that can spread up to 30ft, under fences and through existing structures. ‘Running’ bamboo are therefore the most problematic as they can damage widespread areas. Bamboo, however, is not officially classed as an invasive species under Wildlife and Countryside Act 1981.
Legislation
The major challenge regarding bamboo lies in the absence of regulations requiring surveyors to identify or flag the presence of bamboo as awareness around the issues caused by ‘running’ bamboo is fairly recent. Surveyors and professional bodies are, however, becoming increasingly aware of the potential issues caused and are becoming more vigilant towards its potential issues when giving guidance and advice towards clients.
Eradication Methods:
- Herbicide treatment – typically applied as a foliar spray between July and September and can be enhanced by stem injection throughout the winter. Note that this is not a one-off treatment and can take up to three years.
- Physical removal – the alternative is to manually remove the plant in its entirety ensuring that all rhizomes are dug up; this can be difficult on mature plants as the rhizome system can become so extensive. Be sure to monitor the area for any regrowth, as although weakened by the removal of the main plant, rhizomes still have the ability to produce new roots and shoots if not eradicated.