Native to East Asia, Japanese knotweed is a perennial herbaceous plant, which has become a notorious invasive species in many parts of the world, including Europe, North America, and New Zealand.
But, says FRICS, Paul Santo, it's not the killer of house sales – or demolisher of houses. In recent years the key players in the house-buying and selling process – estate agents, lenders and surveyors – have developed a clearer understanding of the actual problems Japanese knotweed can admittedly cause and, importantly, recognise how properly accredited specialists can effectively manage those problems through to successful sales and purchases.
Read on to find out how to identify Japanese knotweed and what to do if you've got it...
What is Japanese knotweed?
Reynoutria japonica (synonym: Fallopia japonica) is a fast-growing, herbaceous perennial plant that belongs to the Polygonaceae family.
It typically grows in dense, bamboo-like thickets, reaching heights of up to 3 metres (10 feet).
What does Japanese knotweed look like?
1. Japanese knotweed leaves
- Look for large, vibrant green leaves that are often described as heart- or shovel-shaped and have a distinct tip at the end, and grow in an alternate pattern up the stem.
- The leaves turn a reddish-brown colour in autumn before the plant dies back in winter.
2. Japanese knotweed stems
- Its stems are hollow with intermittent bulges (known as nodes), which look like bamboo stems.
3. Japanese knotweed flowers
- During the spring and summer, it produces clusters of small, creamy-white flowers,
Need more help identifying it? The GB Non-native Species Secretariat (NNSS), which has responsibility for helping to coordinate the approach to invasive non-native species in Great Britain, has produced a series of ID sheets to help with identification of non-native species.
Why is Japanese knotweed a problem?
Japanese knotweed is a threat to native ecosystems, infrastructure, and property values.
Its aggressive growth habit allows it to outcompete native vegetation, reducing biodiversity and disrupting natural habitats. Moreover, its extensive root system can penetrate through cracked or weakened concrete and asphalt, as well as unstable building foundations, causing structural damage to vulnerable roads, buildings, and flood defences.
How does Japanese knotweed spread?
One of the most alarming characteristics of Japanese knotweed is its rapid growth rate. In optimal conditions, it can grow up to 8 inches (20cm) per day, quickly forming dense thickets that smother native plants and outcompete them for resources.
Additionally, its rhizomes (underground stems) can extend several metres horizontally and vertically, allowing the plant to spread rapidly and establish new colonies.
Controlling Japanese knotweed infestations often requires a combination of mechanical, chemical, and cultural methods.
How to Get Rid of Japanese knotweed
Managing Japanese knotweed requires a multifaceted approach due to its resilience and persistence. Methods for control include:
- Chemical Control: Herbicides containing glyphosate or systemic herbicides are often used to control it. However, multiple applications over several years may be necessary for effective eradication.
- Mechanical Control: Cutting or mowing can help reduce the above ground foliage, but it must be done repeatedly to exhaust the plant's energy reserves. However, this method alone is unlikely to eradicate the plant.
- Barrier Methods: Installing barriers, such as root barriers or impermeable membranes, can help contain the spread by preventing its rhizomes from spreading into adjacent areas.
- Biological Control: Introducing natural enemies, such as insects or pathogens, that specifically target Japanese knotweed may help suppress its growth. However, biological control methods require careful consideration to avoid unintended harm to native plant species.
Facing knotweed when buying or selling? An expert's advice
Japanese knotweed can be a concern for property transactions. To shed light on this issue, we spoke with Philip Santo, a Fellow of the Royal Institution of Chartered Surveyors (RICS) and a leading expert on knotweed. Instrumental in updating RICS' professional standards for dealing with this invasive plant, he has the following advice:
- Limited structural threat: Knotweed does not pose a threat to buildings with substantial foundations. It won't grow through solid concrete, but it can exploit cracks and gaps.
- Environmental concern: Left untreated, knotweed can spread aggressively, especially in neglected areas.
- Manageable with guarantees: Specialised firms can effectively control knotweed. Opt for reputable companies with guarantees, ideally members of the Property Care Association (PCA) or the Invasive Non-Native Specialist Association (INNSA).
Philip Santo's advice if you're buying a property with knotweed:
- Mortgage considerations: Most lenders require a knotweed inspection from a reputable PCA member firm, including a treatment strategy, report, and guarantee.
- Remediation plan: Consider having a qualified company develop a plan to manage the knotweed. This plan can often be transferred to the new owner.
Philip Santo's advice if you're selling a property with knotweed:
- Transparency is key: Do not hide the presence of knotweed. Disclose it on the TA6 vendor information form ("Is Japanese knotweed present on the property?").
- Negotiation tips: You are not obligated to treat the knotweed yourself. Be prepared to negotiate the cost of treatment into the sale price or transfer an existing plan to the buyer.
- Misrepresentation: Failing to disclose known knotweed is misrepresentation and could result in legal action from the buyer seeking compensation for the property's devaluation. Claims often run to tens of thousands of pounds.
- Unsure About knotweed? ALWAYS err on the side of caution and answer "don't know" on the TA6 form. The form acknowledges knotweed might be present but invisible.
New Builds or extensions with knotweed:
- Excavation Required: If knotweed is present where new construction is proposed, excavation is typically required instead of herbicide treatment.
What plants look like Japanese knotweed?
Several plants share similar characteristics with Japanese knotweed, leading to potential confusion. Some of these include:
- Giant Knotweed (Fallopia sachalinensis): Giant knotweed is closely related and exhibits similar growth habits. However, it typically has larger leaves and longer stems.
- Himalayan Balsam (Impatiens glandulifera): This invasive plant is often mistaken for Japanese knotweed due to its rapid growth and ability to form dense stands along waterways.
- Bamboo: Certain species of bamboo, especially those with similar hollow stems and segmented appearance, may be confused with Japanese knotweed, particularly when young shoots emerge in spring.