Weed killer is most effective when it is applied to damp, uncut grass, according to Gertens. Grass should not be watered for 48 hours after the application of herbicide.
Because most herbicides are contact-based, surface area and contact time are integral for full effect. Depending on the kind of herbicide applied, the amount of time it takes to have an effect can vary.
Pre-emergent herbicides are designed to kill weeds before they sprout, while post-emergent herbicides are designed to kill weeds that are already visible, according to SFGate. Pre-emergent herbicides are most effective early in the season, before weeds begin to germinate.