Managing Japanese Knotweed on Development Sites

It is a particular nuisance to developers, who must apply considerable time and financial support to resolve the problem. If allowed, Japanese knotweed will mercilessly invade a development site rapidly, contaminating the land and causing a massive legal and financial headache. The key to managing a site contaminated with Japanese knotweed is to act quickly and appropriately. Following this clear advice should avoid excessive costs, project delays and further harmful infestations.

Let’s look at some specific areas of concern for developers and the appropriate actions that should be taken.

Why Is Japanese Knotweed Such a Problem for Developers?

Developers are in the business of excavating, disrupting, clearing and moving land and debris, so an outbreak of Japanese knotweed can soon get out of control, spreading to all corners of a development site if it goes unnoticed.

• Identification – if the developer fails to spot Japanese knotweed early in the project, any amount of rhizome particle existing on site will quickly multiply into large plant infestations, damaging roads, buildings and other infrastructure. This is particularly difficult since Japanese knotweed can lie dormant for up to 20 years.
• Project Delays – if Japanese knotweed is identified at any stage of the development, the project will undoubtedly be delayed, which will impact each stage of the project going forward.
• Cost – Japanese knotweed is costly to treat and remove effectively. Treatment may span many months and even years, escalating costs constantly. While developers facing Japanese knotweed on their sites can qualify for tax incentives (Land Remediation Tax Relief), certain conditions must be met for them to be successful.
• Rapid Growth – developers must address Japanese knotweed quickly and effectively to avoid a large-scale infestation and the impact resulting.
• Damage and Devaluation – Japanese knotweed is highly invasive and will grow through tarmac, walls, and other structures. Just the existence of Japanese knotweed will devalue a building or pocket of land.

You might think that developing land with a Japanese knotweed outbreak isn’t an option, but in many cases, it is entirely possible. However, there are certain conditions that developers must abide by.

If the development footprint is 7m or more away from the Japanese knotweed, a management plan must be defined and implemented to treat and remove the plant.

If the footprint is within 7 metres, all knotweed, rhizome growth and contaminated soil must be excavated and removed prior to development.

Developers – What You Can Do

Japanese knotweed is not an easy plant to control for a number of reasons. It can propagate from a tiny speck of contaminated soil, it is extremely robust and can withstand harsh conditions and herbicides, and it has an extensive underground rhizome system that allows it to grow all year round. To make things worse it can lie dormant for up to 20 years, making it extremely hard to detect and treat.

If you’re managing a development project, there are steps you can take at the outset to lower the risks associated with developing land that is contaminated by Japanese knotweed.

• Identify early – ensure staff can correctly identify Japanese knotweed and know what to do if they see it. This will curb the risk of cross-contamination from boots and equipment.
• Do not accept topsoil until you have inspected it for Japanese knotweed rhizome.
• Designate a member of staff to oversee the management of Japanese knotweed.
• Dispose of waste properly at licensed landfill sites or incinerating facilities.
• Inspect and pressure wash contaminated vehicles that have been operating in the area.
• Excavate and bury waste to a depth of at least 5 metres, and not within 7 metres of the plant growth area.

The Important Legal Stuff

As you can imagine, with a destructive species like Japanese knotweed, there are significant legal consequences if you don’t act responsibly. The following information is specific to developers.

Unlike private house owners, developers are not required to declare Japanese knotweed, but it’s sensible to survey the site thoroughly to avoid future problems mentioned previously. Time is of the essence if knotweed is on site.

It’s a good idea to log and record instances of Japanese knotweed. A professional survey and management plan will incorporate this and should also include images for reference. Having this type of log will make it easier to transfer properties to the new owners.

Having a Japanese Knotweed Management Plan is essential to developers. Selling properties without this will be difficult, as buyers will struggle to obtain finance without clear proof that the problem is being treated and is under control.

Japanese knotweed will cause long term problems if it is not treated and eradicated at the outset. For developers, this will mean damaging their reputation, as well as their land.

Japanese Knotweed Resources and Guidance

If you’re looking for detailed industry-wide guidance, there are several publications/associations to refer to.

The Invasive Non-Native Specialists Association (INNSA) has published a Code of Practice offering guidance to developers and hauliers about Japanese knotweed. The guidance represents the highest standards of best practice in the industry.

The Property Care Association (PCA) has issued guidance and information for professionals. Companies and contractors that hold PCA certification are bound by their standards, offering reassurance that the highest industry benchmarks are being met. PCA-certified companies and contractors are required to apply best practice methods to treat Japanese knotweed invasions.

The Royal Institution of Chartered Surveyors (RICS) offer guidance on Japanese knotweed. Members of RICS are required to keep their industry knowledge up to date and relevant.

Japanese knotweed poses a significant challenge to developers due to its invasive, damaging rhizomes. If you have a project in mind, call The Knotweed Specialists for some initial advice. We are a family-run business with plenty of knotweed experience, and we’re certified members of the Property Care Association (PCA).

Managing Buried Japanese Knotweed Long Term

Burying Japanese knotweed is one method of disposing of the waste. This method is particularly relevant to developers, who will potentially have a large amount of hazardous knotweed waste and a large area of land available to excavate.

There are strict rules when it comes to burying Japanese knotweed waste, and it’s wise to follow them carefully or risk future infestations and even prosecution from the new landowners.

• Waste must be buried on the site it came from.
• All Japanese knotweed waste, including ash and soils containing potential waste, must be buried.
• The waste must be buried at a depth of at least 5 metres if it is not sealed with a geotextile membrane or 2 metres if it is sealed with a geotextile membrane.
• It is advisable to apply a non-persistent herbicide such as glyphosate at least once.
• Record the location of the burial site on relevant documentation e.g. deeds.
• Choose a suitable location not planned for development.
• Inform the Environment Agency at least one week prior to burial.

Contact an experienced professional like The Knotweed Specialists for prompt, expert advice. We can survey, assess, and provide a detailed management plan to treat, remove, and dispose of the infestation. We can help get your project back on track.