Is Japanese Knotweed Harmful? Here's What You Need to Know

Japanese knotweed (fallopia japonica) is a resilient and invasive plant known for its aggressive growth and ability to cause significant problems for homeowners and ecosystems alike.

Understanding Japanese knotweed is essential to managing its spread, especially as it’s known for rapidly taking over gardens, damaging property foundations, and disrupting local ecosystems.

In this guide, we’ll explore what Japanese knotweed is, its history, how it is viewed differently in the West and in Japan, how to identify it, and effective methods for controlling it. We’ll cover both chemical and non-chemical ways to eradicate it, as well as the essential tools for tackling this persistent plant.

Whether you're a gardener, property owner, or nature enthusiast, this article will provide you with the knowledge needed to handle Japanese knotweed responsibly and effectively.

What Is Japanese Knotweed?

Japanese knotweed is a fast-growing plant is recognized by its tall, hollow stems, large heart-shaped leaves, and small white flowers that bloom in late summer.

Its bamboo-like stems reach up to 10 feet in height and can turn reddish-brown in late summer. The leaves, measuring 4-6 inches long, are smooth-edged and grow alternately along the stem. The plant’s dense clusters of creamy-white flowers make it more noticeable in late summer and early autumn.

Although Japanese Knotweed can damage property, it is not harmful to humans. In rare instances, some people have experienced mild skin irritation after contact, but there is no confirmed evidence linking these reactions specifically to Japanese knotweed.

Brief History of Japanese Knotweed

Japanese knotweed originated in East Asia, specifically in Japan, China, and Korea, where it naturally thrives along volcanic soils and riverbanks. Traditionally valued in Japan for its resilience, it was used in herbal medicine and even as a seasonal food source.

In the mid-19th century, German botanist Philipp Franz von Siebold introduced it to Europe, particularly the UK. Prized for its ornamental, bamboo-like appearance and rapid growth, it was initially planted in gardens and used to control soil erosion along riverbanks and railway embankments.

However, by the early 20th century, Japanese knotweed's invasive nature became apparent in Europe and North America, where it lacked natural predators and diseases to curb its spread. It soon overran gardens, roadsides, and riverbanks, and its extensive underground rhizome system made it nearly impossible to eradicate, as even small fragments could regrow.

Today, Japanese knotweed is classified as an invasive species in many countries, including the UK, Canada, and the US, posing significant ecological, structural, and economic challenges. Strict regulations govern control and disposal.

Despite its invasiveness, Japanese knotweed has some practical uses; it provides habitat for wildlife, is used in traditional medicine, and is foraged as a vegetable with a lemony, rhubarb-like flavor.

Perception of Knotweed in Japan Vs. West

Japanese knotweed is perceived quite differently in the West and Japan, largely due to its ecological roles and cultural significance in each region.

In Europe and North America, Japanese knotweed is regarded as a highly invasive and destructive plant. Introduced as an ornamental species in the 19th century, it quickly earned a notorious reputation for its aggressive growth and resilience.

Without natural predators or diseases to keep it in check, knotweed spreads rapidly, overtaking gardens, riverbanks, and roadsides. Its strong rhizome system can penetrate concrete and compromise building foundations, causing significant structural damage.

As a result, Japanese knotweed is heavily regulated, with strict laws governing its control and disposal. Its presence can decrease property values, and it’s considered a costly nuisance rather than a beneficial plant.

In contrast, Japanese knotweed is viewed more positively in Japan, where it is native and integrated into the natural ecosystem. Known locally as itadori (meaning "remove pain"), it coexists with other species without posing the same invasive threat.

Knotweed is valued for its resilience and traditional uses; it has long been used in herbal remedies for inflammation and digestive issues, and its young shoots are foraged as a seasonal vegetable with a rhubarb-like flavor.

How To Get Rid Of Japanese Knotweed

We will now look at some techniques for removing Japanese knotweed, looking at both permanent and temporary removal, and removal with and without using chemicals.

Permanent Removal

Applying herbicides containing glyphosate over multiple growing seasons is the most effective method for eliminating Japanese knotweed. Glyphosate inhibits EPSP synthase, an enzyme essential for plant protein synthesis. These chemicals penetrate the extensive root system, preventing regrowth. This approach requires patience and consistent application over 2-3 years for complete eradication.

Another permanent solution is excavating the entire root system, though this method is labor-intensive and costly. It is crucial to remove all rhizomes—thick, horizontal stems that store nutrients—as even small fragments can regrow. Careful disposal of excavated soil is necessary to prevent the plant from spreading to new areas.

Ongoing research into natural predators and pathogens shows promise for managing Japanese knotweed. While still experimental, these biological control methods offer potential for long-term management without the use of chemicals.

Temporary Removal

Regularly cutting the stems and covering the area with heavy-duty weed suppressant fabric can temporarily control Japanese knotweed. This method requires continuous maintenance to prevent the plant from regrowing.

Regular mowing can keep the plant in check but will not eradicate it. This technique is useful for managing small infestations and requires consistent effort to be effective.

Removal Without Chemicals

There are several techniques that can be used to remove Japanese knotweed without having to resort to the use of chemicals. These methods include manual removal, smothering, boiling water, and grazing.

Manual removal involves digging out the plant and its rhizomes by hand, which is labor-intensive and may require repeated efforts for complete eradication.

Smothering uses thick mulch or plastic sheeting to block sunlight, suppressing growth temporarily but needing regular upkeep.

Pouring boiling water on young shoots can help kill above-ground growth temporarily, but it rarely affects the deep roots, providing only temporary control. Grazing by livestock like goats can help manage the spread, though it won’t eradicate the plant fully. Each method requires persistence and maintenance for effectiveness.

Removal Using Chemicals

Eradicating Japanese knotweed with chemicals requires systemic herbicides, primarily glyphosate-based, that penetrate the plant’s extensive root system.

Common methods include foliar spraying herbicide on leaves, stem injection directly into mature stems for targeted application, cutting and applying herbicide on cut stems, and soil drenching to treat roots in severe cases.

Foliar spraying is most effective during the active growth season, while stem injection, and cut-and-apply methods reduce impact on surrounding vegetation.

Multiple applications over 2–3 years are necessary for full eradication. Chemical methods require persistence, caution, and adherence to local regulations due to environmental impact.

Tools For The Job

If you would prefer not to go down the chemical route and want to manually remove Japanese knotweed from your property, then you will need some heavy-duty tools.

Manual removal involves tools designed to cut, dig, and extract the plant’s tough stems and deep-rooted rhizomes.

Pruning Shears

Pruning shears are essential for the initial stages of manually removing Japanese knotweed. They allow for trimming back the tall, bamboo-like stems close to the ground, making the plant's base and root system more accessible.

Start by cutting the thick stems as close to the ground as possible to reduce height and ease the digging process. Regular trimming of new shoots is necessary, as knotweed regrows quickly; this weakens the plant by reducing its ability to photosynthesize and store nutrients.

After cutting, dispose of the stems in heavy-duty bags according to local regulations to prevent further spread. While pruning shears are a crucial first step, repeated cutting and additional tools like mattocks are needed to fully address the root system for effective removal.

A few examples of some pruning shears you might like to purchase:

These hand-forged pruners from Japanese manufacturer Tobisho are made entirely of high carbon steel, offering excellent sharpness, durability, and usability. The red and yellow handle grips improve grip, visibility in low light, and make the shears easier to locate if misplaced.

Another good option are these heavy-duty pruning and branch cutting shears from Doukan. Featuring an innovative design, these shears include a standard pruning blade at the front for regular cuts and a wider opening at the rear for thicker branches, eliminating the need to switch tools.

The “clam-sharpened” blades offer a rounded edge that balances sharpness and durability, reducing the risk of chips or cracks.

Handcrafted in Miki, Hyogo by skilled blacksmiths with decades of experience, these Doukan shears reflect the town’s renowned blacksmithing tradition and high standards for agricultural and gardening tools.

Handcrafted in Miki, Hyogo, by skilled blacksmiths, these shears reflect the town's renowned blacksmithing tradition and high standards for agricultural and gardening tools.

Mattocks

Mattocks are essential tools for removing Japanese knotweed due to their ability to handle the plant’s deep and resilient root system (rhizomes). With a pick on one side and a hoe-like blade on the other, mattocks are ideal for breaking tough ground and removing stubborn roots.

To use a mattock for knotweed removal, start by using the pick side to break through compacted or rocky soil, creating access to the root system. Then, use the hoe side to slice through the thick, woody rhizomes, separating them from the soil.

The hoe side also helps pry and lift large sections of roots out of the ground. Given knotweed's persistence, this process may need to be repeated multiple times as new shoots emerge. Mattocks, combined with other tools like pruning shears and shovels, are highly effective for thorough knotweed removal.

This single-handed pick mattock from Azui is a powerful, versatile, and lightweight tool. It features a hammer-forged carbon steel head, hand-finished by skilled Japanese blacksmiths, ensuring strength, durability, and a perfect balance of sharpness and weight.

Ideal for light field work, garden weeding, breaking dense soil, cutting roots, and even handling rocks, bricks, or concrete mixed with soil, this tool stands out for its efficiency. Its small, light design minimizes fatigue during extended use, and its high-quality materials ensure long-lasting durability.

Additional Tools

In addition to pruning shears and mattocks, shovels, spades, and gloves are crucial for manually removing Japanese knotweed.

Shovels are effective for digging out smaller roots and loosening soil around the plant's base. Their broad, curved blades help scoop up sections of soil and roots, ensuring thorough removal once larger rhizomes are exposed.

Spades, with their flat, sharp-edged blades, are perfect for precise cutting around roots and digging straight down along the root system's perimeter. This allows for cleaner, controlled removal and easy slicing through smaller rhizomes.

Heavy-duty gloves protect hands from tough and sharp stems and roots, preventing cuts and blisters while providing a strong grip on tools and roots. Together, these tools enhance the efficiency and effectiveness of knotweed removal.

Japanese Knotweed: A Threat or a Treasure?

Japanese knotweed is an invasive plant in the West but a valuable resource in Japan. Its rapid growth and strong rhizomes pose environmental, structural, and economic challenges, damaging property and outcompeting native plants. Early detection and management are key.

This guide covers its origins, cultural perceptions, and control methods, including chemical treatments and non-chemical techniques like manual removal and smothering. Tools like pruning shears and mattocks can improve removal efforts.

While considered a threat in many areas, knotweed’s resilience and cultural importance in Japan show its potential benefits. Understanding it helps manage its impact effectively.

Have you found effective ways to control knotweed? Let us know in the comments.