Most Greater Boston properties get mulched once a year, usually April into June once the beds have been cleaned out and the perennials are up enough to see. Fall mulching works too and gives beds some insulation heading into winter. You know it's time when the color has gone gray, the mulch has broken down thin enough that you can see soil through it, weeds are coming up through the beds faster than you can pull them, or the edge between the lawn and the bed has gotten fuzzy and the grass is creeping in. Beds on a slope or near a downspout lose mulch faster and may need a light topdressing more often. Skipping a year isn't the end of the world, but bare bed soil dries out fast in July and the weed pressure gets worse every season you let it go.