Glyphosate effects on Canada thistle ( Cirsium arvense ) roots, root buds, and shoots

Summary Glyphosate † was sprayed at 0009–1·12 kg a.i. ha −1 on the foliage of large potted glasshouse‐grown Canada thistle [ Cirsium arvense (L.) Scop.], which had extensive, well‐developed roots. Increasing the glyphosate rate progressively reduced the total number of visible adventitious root buds plus emerged secondary shoots per plant proportionately more than root biomass, 10 days after treatment. Cortical tissue of thickened propagative roots became soft, water‐soaked, darkened, and some regions decomposed, exposing strands of vascular tissue. Lateral roots completely decomposed. When thickened roots were segmented to stimulate secondary shoot emergence from root buds 10 days after foliar treatment, Fewer secondary shoots emerged than expected from the number of visible adventitious root buds present on both control and herbicide‐treated plants. Increasing the rate of glyphosate also reduced the regrowth potential of root buds proportionately more than root biomass. Regrowth potential was measured as the number of emerged secondary shoots 35 days after segmenting unearthed roots from plants that had been sprayed 10 days earlier. When foliar‐applied at 0·28 kg ha −1 , glyphosate decreased the regrowth potential of root buds to zero in 2 and 3 days, as measured by secondary shoot dry weight and number, respectively, even though root fresh weight was unchanged 3 days after foliar treatment. These dose‐response and time‐course experiments demonstrate that glyphosate did not reduce root biomass as much as it decreased root bud numbers and secondary shoot regrowth potential from root buds.