RE previous post; there are certain annual weeds that can spring up In your lawn in late winter, bloom and set seed, and then they die. The seeds lie dormant until the next year, when they sprout and the whole cycle starts again. This would be something like henbit. I've found to get rid of hen bit, you just mow it down before it flowers and sets seed. No more henbit next year. I'm sure there are annual weeds that pop up in the summer you could manage the same way. Perennial weeds would need to be handled differently. Some weeds have shallow roots and are easy to remove, some have deep taproots, like dandelions, and some grow by creeping rhizomes or little bulbs. So before attempting to remove any weeds, whether by spray or mechanical methods, it's very important to ID the weeds. Then look up the weed to find out the best way to remove it.