The effects of weed killer are not instantaneous. It will take some time for the weed killer to work before it rains. Weed killers work by preventing plants from absorbing sunlight, water and nutrients—in other words, they starve them out! So it’s not a quick fix but any weeds that come up after the initial treatment will die if you give them a follow-up treatment. keep an eye on the forecast and apply weed killer as soon as the rain is expected for best results.