Be sure to apply postemergence herbicides in adequate volumes of water per acre to ensure coverage of weed foliage. The rate of application also interacts with coverage and volume. Preemergence herbicides, - timing and preparation for herbicide applications . Probably the most common reasons for preemergence herbicide 'failure' is that the chemicals are applied to fields that haven't been cleaned up, or are applied too late - after weeds have germinated. If your application is late, first eliminate weeds prior to application or include a postemergence herbicide in the tank.
Problems also occur in early spring when applying herbicide to what appears to be a weed-free field.
Most preemergence herbicides only control weeds from seed and have little effect on the emergence of quackgrass, Canada thistle, or nutsedge from below-ground structures. Preemergence herbicides - climatic factors . Drought following the application of preemergence herbicides can result in failure, since all preemergence herbicides require some moisture for activation. A worst-case scenario for preemergence herbicides occurs when sufficient moisture prior to application allows weeds to germinate, but lack of rainfall after application prevents the herbicide from controlling the weeds. The reverse situation - excessive rainfall, is much less a problem since the solubility of most preemergence herbicides is extremely low.