Benefits

• A large worm population indicates healthy turf grass, according to the Utah State University Cooperative Extension. Earthworms share a symbiotic relationship with lawn grass. The worms aerate the soil around the grass's roots. Their energetic tunneling also helps water seep down to the grass plant's root system. Unfortunately, as the worms tunnel they push dirt and excrement to the surface, which forms brown mounds. The earthworms depend on the grass to provide thatch and decomposing plant material to consume. The worm's voracious appetite rapidly eliminates and controls thatch buildup in the lawn. They eat only dead plant material and pose no danger to green blades of grass.
  
  

Maintenance

• Maintaining a healthy lawn can help the grass even out and be less prone to suffering from a lumpy appearance. Regular fertilizing will help the grass thicken. Consider seeding over the tops of the earthworm mounds to encourage lawn growth. A filled-out lawn will hide the lumps easier than a sparse lawn. In late summer, core aerifying the lawn and power raking it will help even it out. Remove any thatch from the lawn and reseed the area.
  
  

Effort and Outlook

• In areas with only a few lumps, consider using a light roller to flatten the bumps and make them less noticeable. Iowa State University advises avoiding a heavy roller because it will compact the soil. Grass does not grow well in heavily compacted soil conditions. Raking the mounds will also help break them down and disperse debris. Despite a homeowner's best efforts, a healthy lawn will always suffer from some lumps caused by earthworms. Never use a pesticide to kill the worms because their overall benefits to the lawn far outweigh their mounds.
  
  

Other Causes

• In some regions of the country, a lumpy yard is not caused by earthworms but by weather. During winter months, ongoing freezing and thawing of the soil can create a lumpy appearance. The freezing and thawing will move the soil up and down in a heaving motion, which creates an uneven appearance in the turf. Grass can also become thin as it ages. The aging grass makes the lawn appear lumpy and uneven. Overseeding in the spring can help combat the damage from climatic conditions and lawn age.