Japanese knotweed (Fallopia japonica) is a hardy, invasive plant that grows rapidly and can cause significant structural damage to buildings. Originally brought to the UK in the early 19th century as an ornamental plant, it quickly became known for its vigorous root system, which can penetrate concrete, asphalt, and walls.
Because of its strength and durability, Japanese knotweed can lead to costly repairs for property owners and serious concerns for anyone considering buying or selling a property where the plant is present. Homeowners need to be aware of how Japanese knotweed grows, what ways there are to combat its spread, and ultimately, how to prevent it from damaging the value of their property.
What Is Japanese Knotweed?
Japanese knotweed is one of four invasive species of knotweed in the UK, along with Dwarf, Giant, and Bohemian knotweed. Japanese knotweed is particularly survivable and can grow in most soil conditions throughout the UK, making it the most common of knotweed species. The plant grows directly from its roots and is capable of spreading from fragments of the plant as little as 1cm in diameter.
Properties along river banks and streams will commonly encounter Japanese knotweed, whilst the plant can also be transported to gardens via hardcore and rubble being used to lay down building foundations, garden paving, and patios, or for building embankments and multi-level gardens. If Japanese knotweed is present at a property, the owner is legally obliged to prevent it from spreading or else, face prosecution for allowing the plant to spread into the wild.
Japanese knotweed’s presence on a property often leads to a reduction in its market value, with estimates showing that it can decrease the price of a house by up to 15%. Because the plant’s roots can extend underground for seven metres or more, the reduction in value is due not only to the potential damage the plant can cause to the structure but also to the perceived risk associated with owning a property affected by knotweed.
Mortgage lenders are cautious about financing properties where Japanese knotweed is present, and some may even refuse a mortgage application unless an effective management plan is established. This will impact the ability of some buyers to purchase properties with Japanese knotweed infestations, and will also deter cash buyers since they are placed in a similar scenario when reselling the property in the future.
Managing Japanese Knotweed
Effectively treating Japanese knotweed requires professional involvement and it is strongly recommended by the government that property owners do not attempt to contain or remove infestations themselves. There are several ways to approach removing an infestation, but each has unique considerations for the property owner which should be carefully looked at before opting for any course of action.
Herbicide treatments are common and involve applying chemicals to the knotweed over multiple growing seasons, which can take several years to complete, but it is a relatively invasive action. However, the possibility of a re-growth of the weed remains, and it may involve having to open up access to other parts of the property and grounds to fully eradicate any infestation, which can be inconvenient for property owners over prolonged periods.
Excavation is a faster but more expensive approach, where the affected soil is entirely removed, however, whilst this is usually very effective, it may require significant restoration works afterwards. Excavation also includes the option of installing geomembranes, which contain the spread of the plant underground and are commonly employed against very severe outbreaks. Whilst excavation works very well to permanently remove an infestation, costs can rise sharply if it requires dismantling various parts of the property, such as concrete floors and walls, to access the roots of the plant.
After treatment, obtaining a warranty or insurance agreement is essential, as it reassures future buyers and mortgage lenders that the knotweed issue is managed and won’t resurface. These guarantees typically last five to ten years and ensure that treatment will be re-administered if a new or existing infestation re-emerges on the property.
Buying A Property With Japanese Knotweed
If you are considering buying a property with Japanese knotweed, it’s essential to understand the potential challenges involved. Before committing to a purchase, request a professional survey focused specifically on identifying Japanese knotweed if you are already aware of its presence at the property or a neighbouring one.
A TA6 property information form is usually completed by the seller to disclose any known knotweed presence. Since some cases are unknown to the seller, investing in an expert survey can prevent costly surprises later. Buyers should not be deterred from buying a property if there is Japanese knotweed present, however, the more aware the buyer is about the level of infestation, then the better position they may find themselves in to negotiate for a reduction in the price of the property.
Whilst some mortgage providers are reluctant to lend on knotweed-infected properties, many lenders will consider the application if there is a detailed, guaranteed treatment plan in place. Japanese knotweed can also impact a home insurance policy, as not all policies cover damage or regrowth after initial treatment. Buyers may wish to look for policies that provide knotweed-specific coverage or, warranty on treatment plans that protect against reinfestation, especially if the property is in an area where the plant is common.
Selling A House With Japanese Knotweed
The presence of Japanese knotweed may seem like a significant obstacle, but transparency and proactive management are essential to making the sale of a property with an infestation happen. Sellers are legally obligated to disclose Japanese knotweed presence on their property. Failure to disclose could result in legal consequences, including the buyer potentially rescinding the contract or pursuing compensation.
For many sellers, the best approach is to address the problem before listing their property. There are two common options for treatment: herbicide and excavation. Sellers can obtain a management plan and treatment guarantee to reassure potential buyers and their mortgage providers that the knotweed will not affect future property value or usability, which should not impact any possible sale.
Enlisting expert advice from estate agents that specialise in selling homes with Japanese knotweed from the moment a seller considers putting their property on the market, is a great first step to avoiding any issues with the sale. The selected estate agents can provide advice on market positioning, the potential impact on property value, and strategies for working with buyers to alleviate their concerns about any infestation. This should make the property more appealing as potential buyers will see that the knotweed issue is under control and managed by experts.
While Japanese knotweed presents challenges, informed buyers and sellers can manage these effectively. Buyers should seek out properties with management plans and guarantees, while sellers should invest in treatment and communicate transparently to avoid legal or financial issues. Addressing the presence of Japanese knotweed upfront and with proper expertise can ease the process and help retain or achieve fair market value for affected properties.
