Most Greater Boston lawns need weekly lawn mowing from the middle of April through June, when cool-season grass is growing fastest and a week of rain can put on three inches. Things slow down in July and August, and every other week is usually plenty unless you're watering heavily. Growth picks back up in September and runs into late October or early November depending on the first hard frost. The signs you've waited too long are easy to spot: the lawn looks pale or straw-colored right after a cut, there are clumps of wet clippings sitting on top, or you're seeing seed heads. Cutting off more than a third of the blade at once stresses the plant and opens the door for weeds and disease, which is why sticking to a schedule beats catching up later.