Home What are the problems with using Weed and Feed products? Combined products are usually not a good mix – the best time to treat weeds is NOT usually the best time to fertilize • Spreading weed killer over the entire lawn is usually overkill • Some lawn weed killers can stress and damage turf if applied during warm weather • Using an herbicide over the entire lawn can harm nearby trees • Herbicides can be tracked into the house by pets or on shoes