These weedkillers are systemic and are absorbed into the weed and must travel to all parts of the plant to work well. It can take up to 4 weeks for the weed to die. The effectiveness of all lawn care products is dependent on the weather and this is why they are best applied when the weeds are growing actively. To work effectively, they should not be applied when rain is imminent, when the soil is dry, or conditions are too cold for the weeds to grow properly.