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

Abstract

AbstractKnotweeds (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.