November 9, 2023

What Actually Is Japanese Knotweed?

Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.) or Fallopia Japonica is an aggressive and resilient invasive plant species that has steadily gained a formidable reputation. With roots that can burrow as deep as two meters and stretch up to seven meters wide, this plant has proved to be a tough adversary for property owners and environmental specialists alike.

1. Understanding Japanese Knotweed

1.1 Origin and Introduction

Japanese knotweed, a member of the buckwheat family, hails from Eastern Asia, specifically Japan, China, and Korea. It was brought to the U.S and the UK in the late 19th century as an ornamental plant for gardens. It was also used for erosion control due to its rapid growth and robust root system. This plant has now found a home in 42 states across the U.S and in eight Canadian provinces, and it’s one of the most invasive plants in the UK.

1.2 Characteristics and Adaptability

The plant grows between 10 to 15 feet tall and is notorious for its bamboo-like stems, hence its other name, Japanese bamboo. It exhibits an upright, shrubby, herbaceous, and woody-appearing perennial growth pattern. The leaves are broadly ovate, 3 to 6 inches long, and 2 to 4 inches wide, alternating on the stem. The plant develops small winged fruits and branched sprays of small greenish-white flowers from August to September. These flowers function unisexually due to vestigial complementary male and female organs.

The Japanese knotweed has an exceptional tolerance to various environmental factors. It can survive in deep shade, high temperatures, high soil salinity, and drought. Its versatile nature has enabled it to invade low-lying areas, disturbed areas, and old home and farmsteads.

2. Japanese Knotweed: The Unseen Damage

2.1 Impact on Native Plant Communities

Japanese knotweed’s rapid growth forms dense thickets that crowd out and overshadow native vegetation. This aggressive growth pattern reduces species diversity, alters natural ecosystems, and negatively impacts wildlife habitat. The ground beneath the knotweed thickets, usually barren, becomes highly susceptible to erosion, particularly in riparian areas.

2.2 Threat to Buildings and Infrastructure

Japanese knotweed has earned a reputation for causing significant structural damage to properties. It can exploit weaknesses in buildings and paving, leading to costly repairs. In severe cases, this plant can cause subsidence, especially if a property is built on shrinkable clay soils. The most extreme instances have seen Japanese knotweed grow through the cracks in concrete and walls of houses.

3. Identification of Japanese Knotweed

The Japanese knotweed exhibits unique characteristics that make it relatively easy to identify. It possesses an upright growth habit with smooth, stout, and hollow stems. It has a signature ‘zig-zag’ leaf pattern with ‘tipped’ shovel-shaped leaves that turn a luscious green in summer. The stem is swollen where the leaves attach, surrounded by a membranous sheath. The plant’s shoots emerge from a network of spreading rhizomes, which can reach lengths of 65 feet or more.

4. The Spread and Distribution of Japanese Knotweed

Japanese knotweed’s primary propagation methods are through seeds (transported by wind, water, animals, humans, or as a soil contaminant), stem fragments, and shoots sprouting from its rhizomes. These seeds and fragments can be easily carried and deposited in new locations, leading to the rapid spread of this invasive species. It has now invaded every continent except Antarctica, making it one of the most invasive plants in the world.

5. The Economic Impact of Japanese Knotweed

5.1 Property Value Reduction

The presence of Japanese knotweed can significantly reduce property value. Restrictions on amenity use, potential structural impacts, legal considerations, and the cost of remediation all contribute to this devaluation. According to some reports, properties infested with Japanese knotweed can suffer a value loss of up to 15%.

5.2 Difficulty in Acquiring Mortgages

The presence of Japanese knotweed can also make it challenging to secure a mortgage. Many lending institutions are wary of properties infested with this plant due to the potential costs of eradication and the risk of structural damage.

6. Environmental Impact of Japanese Knotweed

Though the financial and property damage caused by Japanese knotweed often takes the spotlight, its environmental impact is equally devastating. The plant grows in dense stands, blocking sunlight for native plants below. Additionally, it consumes large amounts of minerals and nutrients from the soil, leaving fewer resources for other species. This leads to a reduction in biodiversity, making Japanese knotweed a significant environmental concern.

7. Controlling Japanese Knotweed

7.1 Mechanical Control

Mechanical control involves physically removing the plant. Young plants can be pulled by hand, depending on soil conditions and root development. However, if any part of the root system remains, re-sprouting can occur. For larger infestations, grubbing with specialised tools to remove all the roots after cutting back the standing vegetation can be an effective measure.

7.2 Chemical Control

Chemical control involves the application of herbicides like glyphosate and triclopyr to freshly cut stems or foliage. However, before using chemicals, it’s advisable to check with local environmental or natural resources management agencies to ensure legality and safety.

8. Japanese Knotweed: A Persistent Nuisance

Despite the various control measures, Japanese knotweed is notoriously difficult to eradicate. Once established, it is extremely persistent. Its rhizomes are resilient, and even a tiny fragment can lead to a new outbreak. Therefore, it’s critical to dispose of removed plants properly and avoid disturbing contaminated ground.

9. The Journey of Japanese Knotweed in the UK

Japanese knotweed was introduced to the UK in the 19th century and was initially celebrated for its ornamental appeal. However, by the end of the 19th century and the beginning of the 20th, the dangers of the plant were becoming evident. Despite warnings, it wasn’t until 1981 that the British government enacted legislation to control its sale and spread.

10. The Current Status of Japanese Knotweed

Today, Japanese knotweed’s status is increasing and invasive. Its contamination requires special measures for treatment, extraction, and disposal. The plant places severe restrictions on the free use of property and continues to devalue properties. Its impact on the environment is also a significant concern, with its fast growth and aggressive spread posing a substantial threat to native plant species.

11. Public Action Against Japanese Knotweed

Public participation in the fight against Japanese knotweed is crucial. Reporting sightings, assisting in mechanical removal, and avoiding planting Japanese knotweed for decorative purposes are essential steps. Property owners are encouraged to remove Japanese knotweed, and proper excavation practices are recommended to decrease the chances of new infestations.

12. Conclusion: The Battle Against Japanese Knotweed Continues

The fight against Japanese knotweed is ongoing and challenging. Though deeply entrenched in many regions, comprehensive management strategies and public awareness can help control this invasive plant. It’s crucial to remember that early detection and immediate action are key to preventing the spread and mitigating the impact of Japanese knotweed.

The relentless growth and invasive nature of Japanese knotweed mean that the battle against this plant is far from over. However, armed with knowledge and commitment, the tides can turn in this fight, protecting properties and preserving natural ecosystems from the clutches of this aggressive plant.