How do weed killers work? There are different types of weed killers: a) contact weed killers, b) systemic weed killers, c) selective weed killers, d) non-selective weed killers, e) residual weed killers, and (f) non-residual weed killers. The contact weed killers kill only the part of the plant that the chemical touches, therefore the weed killer must contact most of the plant’s surface. Systemic weed killers kill the weeds by applying throughout the plant’s system. The effect of this kind of weed killer may not show for a week or more after application. The selective weed killers kill certain plants without killing others. These kinds weed killers can control specific weeds when used at the proper time and rate. Non-selective weed killers
