Safely kill weeds on a newly seeded lawn by using a postemergence herbicide on newly grown weeds. Apply the herbicide once the seeded lawn has grown enough to require mowing four times.
Apply the postemergence herbicide at 1/2 the recommended rate listed in the product's directions. This diluted mixture slows down the growth of weeds but is less likely to harm the new grass. Spring and fall are the best times of year to apply a postemergence herbicide, as the temperatures are cooler and allow the herbicide to work best. Aim for temperatures between 65 and 85 degrees Fahrenheit.
The soil should be moist in the area where you are applying the herbicide. Apply either after a light-to-moderate rain or use a lawn sprinkler. Do not over-water. Apply the herbicide when there is little to no wind. Wind can carry the herbicide to nearby plants and damage them. Also, do not mow the newly seeded lawn immediately before or after application of the herbicide. Longer grass blades give the herbicide more surface to cling to, therefore increasing its effectiveness. Should the weed growth persist and a reapplication of the herbicide become necessary, do so 10 to 14 days after the first application.