The best way to get rid of weeds in your lawn is to catch them in the spring before they reseed. Annual weeds end up in your yard via birds, animals and wind. Perennial weeds have been there a long time; the only way to eradicate them is by pulling or killing off the roots with a pre-emergent herbicide. When you hear the term broadleaf weed, it refers to any plant in your grass that's undesirable; not all broadleaf weeds have broad leaves.
Crabgrass, an annual grassy weed, sprouts from seed every year. You'll recognize it by the light-green color, thick clump formation and long offshoots. As it dies in the fall, it leaves behind seeds that germinate the following spring. The ideal time to kill crabgrass occurs in early spring before germination. Apply a pre-emergent herbicide to kill crabgrass and other broadleaf weeds. After seven days, spot-spray any remaining weeds. If you'd prefer not to use chemicals, pour white vinegar on the crabgrass. There's some controversy over the effectiveness of vinegar, but many gardeners insist it works.
You'll recognize dandelions by the small yellow flower that shoots up from the green leafy base. When they go to seed, they appear fluffy, round and white. The seeds are easily scattered with a light breeze. Once the dandelions reach this stage, you'll need a post-emergent herbicide. If there are just a few dandelions in your grass, spot-spray them rather than treating the whole lawn. A less toxic way to get rid of dandelions is to dig them up with the long tap root intact. A weed tool with a long shaft and a two-pronged end works effectively at prying the root up without breaking it. Push the tool into the soil next to the base of the dandelion and use a prying motion, being careful not to sever the root.
The perennial ragweed returns every year if not eradicated in early spring. Ragweed lies inconspicuously in the lawn every spring, waiting to wreak havoc on allergy sufferers. Ragweed derives its name by the ragged edges of its leaves. It also resembles the feathery foliage of Powis Castle, the hybrid artemisia plant. Like other weeds, to successfully eliminate it, catch it before it reseeds. Ragweed's shallow root system makes it easy to pull, which eliminates the need for chemicals.