There are organic herbicide options that will kill grass but not flowers, including white vinegar or a dish soap solution. Commercial selective herbicides can also target grass over other plants, but read the instructions carefully to avoid damaging your garden. Herbicides that kill grass but not flowers usually include ingredients like clethodim and sethoxydim. Otherwise, your best bet will be to weed by hand.

No, it's not really possible to completely prevent weed growth. Weeds are incredibly resilient—all they usually need is a tiny bit of dirt to sprout, so their eventual appearance is pretty much inevitable. However, you can definitely keep them at bay by getting rid of weeds as soon as you notice them. The longer you let them sit and grow roots, the more they'll spread.

The most effective way to eliminate quack grass without killing the grass nearby is to overseed the area, keep the seed wet until it grows, mow often, and regularly fertilize the area to promote healthy grass that can choke out the invasive weed.

Mowing a lawn will, unfortunately, not get rid of clover, and it may even encourage it to flourish. Let your lawn grow to a healthy height of over 3 inches and then set your mower blades no lower than this level. Higher grass will cut out the light that clover loves and discourage it from spreading.