Earthworms and night crawlers are the main cause of bumpy lawns, a common complaint of many homeowners. Worms leave castings or small mounds of soil on the soil surface. These mounds soon dry and harden causing an uneven surface, which can be somewhat bothersome during mowing operations. It is important to remember that the benefits of earthworms far outweigh the nuisance caused by these castings. Earthworms help decompose thatch, improve water and air movement and soil structure for good root growth. Earthworms are indeed a good indicator of a fertile soil.
Extremely bumpy lawns may also indicate other problems such as a thin turf stand due to a poor fertilization program. Following good fertility and watering guidelines will keep a lawn growing vigorously and will make bumps less noticeable.