Clover is an annual or perennial weed (depending on the species) that grows easily in moist areas, and can invade most lawns--particularly cool season varieties. This shallow rooted weed is found throughout California and Arizona, and actually all over the US. Clover spreads primarily by seed, and thrives in nitrogen depleted soil. Sometimes clover is actually made part of the seed mix, so if you overseed, be sure to purchase a quality product. Applying a fertilizer application can help keep it from reappearing, as will maintaining a healthy, dense, and well groomed lawn.
Clover can easily be controlled by hand pulling. It can also be controlled with a postemergent herbicide. The best herbicide to use depends on the variety of turfgrass you purchased for your lawn--either warm or cool season. Ask your local garden store for a recommendation and be sure to follow directions on the label. Don't forget to keep children and pets away. And remember not to kill the 4 leaf clover!
You can "Ask Mr. Wise Grass" your weed control questions as well.