Great for filling in bare/thin spots, improve grass variety and enhance your lawn.  Overseeding can help improve the health and look of your yard while reducing the amount of fertilizer, water and pesticides required on older lawns.  

Iowa State University recommends seeding between late August to Late September in Iowa.  Planting in late summer/Early fall is best, as it allows the seed to germinate and grow quickly in warm soil, and will give your grass 2 cooler seasons to establish before encountering weather related problems such as heavy rains, drought and high temperatures.   Seeding in Spring is okay, but you may be at higher risk for weather related problems.