Ignoring recommended seeding rates
Don't overdo or cut corners. Too much grass seed causes undue competition for resources such as light, water and nutrients, and grass seedlings struggle as a result. Too little seed leaves lawns thin or bare.
What happens if you over seed?
If you put down too much grass seed, you will encourage competition that will cause your grass seedlings to struggle after germination because there will be excessive competition for sunlight, soil nutrients, and water. You'll know if you seeded too heavy when the grass grows in very thick patches.
How often should you over seed?
You should plan to overseed at least every 3 to 4 years, but, if you aerate and overseed every year, you'll see real nice results -- a lush, green lawn that fends off weeds, insects and diseases.
