The window opens once the ground has firmed up and closes when the lawn starts actively growing — around here that's usually late March through late April, depending on the thaw. Timing matters more than people realize. Rake too early, while the soil is still soggy, and you compact the ground and tear the crowns off perennials that haven't broken dormancy yet. Wait too long and you're mowing over debris and mulching weeds that already took hold. The practical marker: when you can walk the lawn without leaving footprints and perennials are pushing an inch or two, it's time. Book early — the calendar fills in a matter of weeks.