Time to kill the beast – Importance of taxa, concentration and timing during application of glyphosate to knotweeds

Abstract

Knotweeds (Fallopia spp., syn. Reynoutria spp.) are among the most invasive plants globally, mainly due to their ability to regenerate from rhizomes and their extremely high biomass production. Spraying with glyphosate is a common control method, yet little is known about its effectiveness on underground rhizomes. In addition, there are concerns about the negative environmental impact of glyphosate. Therefore, it is essential to use appropriate dosages and application times to avoid overuse. Based on a pot trial and field experiments, we assessed the effectiveness of glyphosate concentration, application time, and influence of glyphosate on rhizomes from different soil depths to determine their effect on the aboveground and belowground parts of knotweed plants of different taxa. The study demonstrates that sampling rhizomes is a more consistently accurate indicator of knotweed regeneration rate than sampling shoots. Regeneration of shoots and rhizomes was affected differently by glyphosate spraying depending on the application time. The effect on rhizomes was much greater with early season spraying than late season spraying, which primarily reduced shoot biomass. However, no differences were found between rhizome vitality at different soil depths. F. sachalinensis was sufficiently controlled by early season foliar spray with 5% glyphosate (3.65 kg a.i. ha −1 ) in contrast to F. japonica and F. ×bohemica. For rapid and targeted control, early season foliar spray with 8% glyphosate (5.85 kg a.i. ha −1 ) is needed and, in the case of the hybrid, for a minimum of two consecutive seasons.

Efficacy and rates of diquat for reducing invasive knotweeds and potential for sequential use with imazapyr

“…However, an alternative explanation might lie in the potential for better, more complete coverage of the systemic herbicide applied in mid‐summer, simply as a function of reduced canopy area (Boyd et al, 2017). Our data do not allow us to distinguish these two alternatives, but does suggest that optimizing long‐term control of knotweed using sequential contact and systemic herbicide treatments warrants additional testing; ideally, these would include examination of the interactive effects on above and belowground systems (Ito & Ito, 2021; Kadlecová et al, 2022).…”

“…It is challenging to speculate on the outcome of sequential application of a contact and systemic herbicide applied over several years; dramatic reduction (albeit without full eradication) of aboveground growth seems likely, and remaining stems and leaves might exhibit signs of stress (e.g., light utilization efficiency, Jones et al, 2018). However, eradication of knotweed ultimately demands reduction of rhizome activity and viability (Bashtanova et al, 2009), which have been shown to vary with treatment timing (Kadlecová et al, 2022). We recommend that future studies of sequential applications should include assessment of changes in belowground and aboveground growth (Ito & Ito, 2021; Kadlecová et al, 2022); ideally these would be implemented as long‐term field‐based designs that can help inform management (Jones et al, 2018; Kadlecová et al, 2022).…”

“…However, eradication of knotweed ultimately demands reduction of rhizome activity and viability (Bashtanova et al, 2009), which have been shown to vary with treatment timing (Kadlecová et al, 2022). We recommend that future studies of sequential applications should include assessment of changes in belowground and aboveground growth (Ito & Ito, 2021; Kadlecová et al, 2022); ideally these would be implemented as long‐term field‐based designs that can help inform management (Jones et al, 2018; Kadlecová et al, 2022).…”

“…For example, if there is little difference in efficacy of imazapyr applied in early summer versus autumn, treatment is simpler when canopy cover is low in early summer (Figure 4). In addition to potentially improving long‐term control, other benefits to sequential treatments include reducing the volume of herbicides needed, and reducing canopy area and height to make application easier (Boyd et al, 2017; Kadlecová et al, 2022). Although tank mixing can be a popular approach, general trends toward antagonism when herbicides are combined suggest that this is unlikely to be effective (Zhang et al, 1995).…”

“…First, it is hypothesized that early-season treatment using a contact herbicide (one that preserves apical dominance) will force regrowth within the season, depleting the rhizome while still allowing plant recovery and sufficient biomass to absorb a subsequent systemic herbicide (Bashtanova et al, 2009;Jones et al, 2018). Early-season treatment can facilitate subsequent herbicide treatments simply by reducing canopy area (Boyd et al, 2017), as well as volumes of herbicides needed (Kadlecov a et al, 2022). Early season treatment also limits flowering and seed production.…”