Answer: Most herbicides biodegrade in the presence of sunlight and air so there may not be any residue, especially since it has rained so much. However, some grass and weed killers contain a pre emergence product which will keep seeds from germinating. If you still have the product, read the label. It should say how long to wait before planting seeds. If not, see how long it says to wait before reapplying the product. This will give you an idea of how long it typically lasts in the soil. If you can't find the information on the label you might just want to scrape off the top few inches of soil and replace it with fresh topsoil, then sow your seeds and cover them with a thin layer of compost or peat moss to help keep them moist. The seeds should germinate in 7-10 days and eventually fill in the dead spots.
Good luck with your lawn!
Q&A Library Searching Tips