If you are mowing frequently enough, it is best to leave the clippings on the lawn to recycle the nitrogen in them. This can be as much as 1 to 2 pounds of nitrogen per 1000 square feet per year- the equivalent of 1 to 2 fertilizations per year). Mow often enough so that no more than 1/3 of the height of the grass blades is removed at one time. The small amount of clippings generated from frequent mowing will not contribute to thatch.