Quick answer
It's best to wait until grass is mostly dry. Wet grass cuts unevenly, clumps, and heavy mowers can rut soft clay soils. Light dew is usually fine; soggy ground is not.
Problems With Wet Mowing
- Torn, uneven cut
- Clippings clump and smother grass
- Ruts and compaction in clay soil
- Slipping hazard on slopes
When It's OK
If the lawn has been rained on for days and is getting too tall, a careful mow with a sharp blade at a higher setting is better than letting it get out of hand.