Weed control is an obnoxious and costly part of home lawn maintenance. To be truly effective, good weed prevention/elimination needs to be done on a bi-annual basis-Fall and Spring. When you factor in the environmental costs in using highly toxic chemical components being delivered into your ground soil, it becomes a question of "is it worth the trouble?"
There is an alternative that is unquestionably effective and totally safe for the environment. Corn gluten meal, a natural by-product of corn crops, kills a wide variety of weeds in your lawn and garden. Crabgrass, spurge, dandelions, bent grass and other common weeds are stopped cold.. This coarse grained weed control eliminates weeds at their source: the roots. It is a pre-emergent that applied in Fall and again in early Spring, effectively kills weed seeds before they germinate. This time saving step-application of a pre-emergent, will help eliminate the need for weeding almost completely.
In the Fall, after completing your routine clean up of the lawn and garden, you simply apply corn gluten meal straight onto your lawns or bare soils. Water it in lightly and you're done until Spring. Corn gluten meal also acts as a fertilizer as it breaks down into the soil. Application rates are 20 pounds per 1000 square feet, so a 50 pound bag should take care of Fall and Spring treatments.
You can find corn gluten meal at some larger stores like Home Depot or Lowes, but you can also find it at Planet Natural online too. This is a best practices in the garden buy, and one I highly recommend adding to your lists of must have products.