Japanese knotweed (Fallopia japonica): an analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension

Abstract

Fallopia japonica (Japanese knotweed) is a well-known invasive alien species in the UK and elsewhere in Europe and North America. The plant is known to have a negative impact on local biodiversity, flood risk and ecosystem services; but in the UK it is also considered to pose a significant risk to the structural integrity of buildings that are within seven m of the above ground portions of the plant. This has led to the presence of the plant on residential properties regularly being used to refuse mortgage applications. Despite the significant socioeconomic impacts of such automatic mortgage option restriction, little research has been conducted to investigate this issue. The 'seven-m rule' is derived from widely adopted government guidance in the UK. This study considered if there is evidence to support this phenomenon in the literature, reports the findings of a survey of invasive species control contractors and property surveyors to determine if field observations support these assertions, and reports a case study of 68 properties, located on three streets in northern England where F. japonica was recorded. Additionally, given the importance of proximity, the seven-m rule is also tested based on data collected during the excavation based removal of F. japonica from 81 sites. No support was found to suggest that F. japonica causes significant damage to built structures, even when it is growing in close proximity to them and certainly no more damage than other plant species that are not subject to such stringent lending policies. It was found that the seven-m rule is not a statistically robust tool for estimating likely rhizome extension. F. japonica rhizome rarely extends more than four m from above ground plants and is typically found within two m for small stands and 2.5 m for large stands. Based on these findings, the practice of automatically restricting mortgage options for home buyers when F. japonica is present, is not commensurate with the risk.

Keywords: Fallopia japonica; Impacts; Invasive species; Japanese knotweed; Rhizome; Structural damage.

Figure 1. Distributions maps showing F. japonica records and soil shrink-swell potential.

(A) Records from the Botanical Society of Britain and Ireland live database based on presence/absence data in each hectad. Almost all hectads report fewer than 100 records. Map was produced using records collected mainly by members of the Botanical Society of Britain and Ireland (BSBI) (2018). (B) British Geological Society map showing areas at risk of shrink-swell action. Reproduced with the permission of the British Geological Survey ^©UKRI. All rights reserved (British Geological Survey (BGS), 2018).

Figure 2. Photographs illustrating F. japonica appearance and structure.

(A) F. Japonica growing within the case study area. (B) Specimen of F. japonica leaves, stem and inflorescence. (C) F. Japonica crown, associated with the plant from panel A. (D) Specimen of F. japonica mature rhizome with immature rhizomes emerging. Photos by M. Fennell.

Figure 3. Photographs illustrating examples of the rating scheme that was applied.

(A) Example of non-plant-based wear and tear to hard standing. (B) Rating ‘0’—B. davidii growing in a raised landscaping area, having no discernible impact on undamaged adjacent built structures. (C) Rating ‘1’—F. japonica emerging from existing cracks in paving at the base of a wall, causing no discernible impact away from baseline damage. (D) Rating ‘2’—F. japonica emerging from existing gaps in worn paving, while the gap has not been widened some mortar has been pushed aside. (E) Rating ‘3’—B. davidii growing out of a crack in worn concrete hardstanding, with additional cracks forming in the area. F. japonica visible in the background emerging from similar cracks in the hardstanding, also exacerbating existing damage but to a lesser extent. (F) Rating ‘3’—B. davidii growing out of cracks in worn brickwork, with additional cracks forming in the area. (G) Rating ‘4’—B. davidii growing out of cracks in worn brickwork. It has found its way between two structures and is facilitating the dilapidation of the wall and pushing out brickwork. (H) Rating ‘6’—B. davidii growing behind a small retaining wall and pushing some brickwork over. (I) The remains of a tree stump, which have destabilised the base of what remains of a dilapidated wall. Photos by M. Fennell.

Figure 4. Comparison of horizontal rhizome extent between small (four m² or less) and large (greater than four m²) stands of F. japonica.

The box represents the lower 25 percentile, the median value and the upper 25% percentile and the whiskers represent the range of the data. The circle represents an outlier value (greater than two standard deviations away from the median value). Mann–Whitney U: U = 412; p < 0.05 (p = 0.01802). N = 21 (small) and 60 (large).

Figure 5. Comparison of vertical rhizome extent between small (four m² or less) and large (greater than four m²) stands of F. japonica.

The box represents the lower 25 percentile, the median value and the upper 25% percentile and the whiskers represent the range of the data. The circle represents an outlier value (greater than two standard deviations away from the median value). Mann–Whitney U: U = 260; p < 0.0001 (p = 6.105^e−5). N = 21 (small) and 60 (large).