Please don’t mow the Japanese knotweed!

We welcome Martin et al.’s (2020) significant contributions toward advancing understanding of Reynoutria japonica var. japonica (Japanese knotweed) clonal growth strategies and resource allocation in response to environmental heterogeneity; understanding knotweed ecophysiology is essential to inform and enhance large-scale invasive knotweed management. However, we strongly disagree that mowing should be recommended for the landscape management of invasive knotweeds on the grounds of limited efficacy, practicality and environmental and economic sustainability. To achieve the successful control and long-term management of invasive rhizome-forming plants, we should do more with less, as the evidence guides us (Jones et al. 2018).

the plant to recover from many physical, herbicide-based and integrated control treatments, even when applied over relatively long time periods (>3 years; Jones et al. 2018).
While we acknowledge that mowing is a widely applied vegetation management method for infrastructure maintenance, cutting as a management method for established invasive knotweeds has been reported as ineffective in the medium to long-term at a range of spatial scales throughout the academic and grey literature in Europe and North America (Seiger 1997;Brabec and Pyšek 2000;Child and Wade 2000;Green 2003;CEH 2004;Soll 2004;Gover et al. 2005;Kabat et al. 2006;Rennocks 2007;Skibo 2007;Bashtanova et al. 2009;Macfarlane 2011;Delbart et al. 2012), though native species diversity may increase during active management (Adler 1993;Hartwig and Kiviat 2009).
Historically, where cutting was proposed as a management method in the UK and North America, it was suggested that Japanese knotweed stems were cut down to ground level at least every 2 to 3 weeks through the growing season to deplete belowground rhizomes (Child and Wade 2000;McHugh 2006;EA 2013). Yet, to our knowledge, there are no examples of successful long-term invasive knotweed management using this treatment programme, despite the application of approximately 20 cuts per year. While the 3 cuts per year proposed by Martin et al. is more economically sustainable than 20 applications, if the more intensive programme has not demonstrated efficacy at the field scale, it would seem unlikely that 3 cuts per year will deplete the rhizome sufficiently to achieve short-term control, let alone effective long-term management. Hujerová et al. (2013) and Van Evert et al. (2020) reported that aboveground cutting of taproot-forming Rumex spp. (Docks; also members of the Polygonaceae) three times per year did not eliminate these species from grassland, or result in plant death. This is despite Docks being smaller and less vigorous than any of the invasive knotweeds. Consequently, on the grounds of treatment efficacy alone, we do not recommend mowing as an effective management method for Japanese knotweed.
Aside from limited efficacy, consideration of practicality and the risk of further spread of Japanese knotweed in the environment should inform the application of mowing. Accessing large swathes of invaded riparian or roadside habitat with heavy equipment is frequently problematic, and it is crucial to ensure that stem and rhizome fragments created by cutting methods do not result in wider dispersal of knotweed into the environment (Sieger 1997; Child and Wade 2000;Soll 2004;McHugh 2006;Skibo 2007;Bashtanova et al. 2009;Macfarlane 2011;Delbart et al. 2012;EA 2013, Jones 2015. Minimising dispersal at the landscape scale is unfeasible, considering that leaf (Brabec 1997), stem (De Waal 2001 and rhizome fragments (weighing as little as 0.06 g; McFarlane 2011) may give rise to new plants. Further, Scott (1988), Beerling (1990) and Beerling et al. (1994) highlight direct lateral expansion of rhizome in response to cutting, exacerbating local spread. Causing the dispersal and/or exacerbating the spread of invasive knotweeds in the UK may be in contravention of national biodiversity legislation (e.g. The Wildlife and Countryside Act 1981, UK).

Parsimony as a principle for invasive plant management
There are strong environmental, ecological and economic arguments for the management of invasive alien plants (IAPs) to minimise their negative environmental and economic impacts (Pergl et al. 2020). However, limited empirical evidence underpinning the ecology and management of rhizome-forming invasive plants can lead to the application of ineffective and labour-intensive physical control treatments, and/or unnecessary/excessive herbicide use. This undermines the sustainability of long-term control programmes for these species, resulting in further spread and dispersal in the environment with no discernible management benefit (i.e. 'the cure is worse than the disease'; Kettenring and Adams 2011;Jones et al. 2018;Jones and Eastwood 2019).
In short, to achieve the successful control and long-term management of invasive rhizome-forming plants, we should do more with less, as the evidence guides us (Jones et al. 2018). While we welcome Martin et al.'s significant contributions toward advancing understanding of Japanese knotweed belowground, in particular clonal growth strategies and resource allocation in response to environmental heterogeneity, we strongly disagree that mowing should be recommended for the landscape management of invasive knotweeds on the grounds of limited efficacy, practicality and environmental and economic sustainability.

Acknowledgements
This work is part-funded by the European Social Fund (ESF) through the European Union's Convergence programme administered by the Welsh Government with Swansea University and Complete Weed Control Ltd.

references
Adler C (1993) Growth and dispersal strategies and associations of the neophyte Polygonum cuspidatum with special regard to mowing.