The answer to 'when to mulch' is that the best time to apply mulch in your landscaping is typically in the spring or fall. Applying a 2-4 inch layer of organic mulch around plants, trees, and shrubs helps retain soil moisture, suppress weeds, and insulate plant roots from temperature extremes. Mulching in the spring helps protect plants as they emerge and in the fall it helps insulate the soil and roots before winter.
It's important to avoid piling mulch directly against the trunks of trees and shrubs, as this can lead to rot and pest problems. Instead, leave a few inches of space between the mulch and the plant stems. Reapply mulch annually as the existing layer breaks down over time.