By Eric Jones and Graig Reicks et.al
Yellow toadflax is a perennial weed that infests pasture and rangeland across South Dakota. Compared with other perennial weeds (i.e., Canada thistle and leafy spurge), the appropriate time to spray yellow toadflax occurs in late summer between August and September.
Last year, yellow toadflax flowering was delayed due to environmental conditions (https://extension.sdstate.edu/yellow-toadflax-has-not-flowered-some-parts-south-dakota). This year, with an abundance of heat and moisture, plants flowered on time (early August) and now have started to produce seeds. However, there are still plants that are flowering that could be treated in the infested areas (Figure 1).
Growth Stages
Yellow toadflax flowers are bright yellow and showy (Figure 2), this is the growth stage when herbicides are most effective. Shortly after flowering and pollination, the plants begin to produce seed pods that house seeds and herbicide applications are not as effective (Figure 3).
Source : sdstate.edu