Despite what you might think, the best time to seed your lawn in the fall, if possible. Of course, lawn seed can be sown at other times of the year. But fall is the ideal time for seeding to rejuvenate an existing lawn or to start a new one. In most parts of the United States, an existing lawn should be reseeded in late August or early September. This gives the new grass seed time to grow during the cool fall days. Fall is also the best time to get rid of lawn weed as the weeds roots are storing up nutrients for winter and more easily absorb herbicide.