Click below to listen to my Garden Bite radio show: Why is my lawn bumpy?
A listener asked me why their lawn was bumpy. It’s likely earthworms. The good news is that the presence of earthworms in the home lawn is really an indicator of a healthy soil environment. Earthworms aid in the breakdown of thatch and other organic matter and create tunnels, which promote water infiltration, oxygen movement, microbial activity, and deeper grass rooting. Their castings are rich in nutrients.
Although the bumpiness caused by earthworm mounds can be annoying, you might consider all the benefits before trying to get rid of them. That said, if you’re like my listener and fear breaking an ankle, there are some options.
Core cultivation of the lawn and spreading the plugs may level out a severely bumpy surface.
You could use a heavy roller over the surface but that’s going to cause compaction and you’ll need to core cultivate again.
You could consider watering your lawn less, as earthworms like it moist. Another option is to use a power dethatching rake. Adjust the power rake so that the teeth operate low enough to shave off the tops of the worm mounds, but not so low that the crowns and roots of the grass plants are pulled up. It is best to do this early in the spring, before the lawn has begun greening up, so a little late now.
Top dressing, adding a thin layer of compost or soil from a nearby garden bed, can provide some relief. Spread ½ to 1 inch of material on the lawn and rake it into the grass canopy; repeat every 1-2 weeks, until surface is acceptably level.
Earthworms and Nightcrawlers in the Home Lawn [Colorado State University]