The time of mulch application depends on what you hope to achieve. Mulches, by providing an insulating barrier between the soil and the air, moderate the soil temperature. Mulched soil in the summer will be cooler than an adjacent unmulched soil; while in the winter, the mulched soil may not freeze as deeply. However, since mulch acts as an insulating layer, mulched soils tend to warm up more slowly in the spring and cool down more slowly in the fall than unmulched soils.
In your vegetable garden or flower garden, it is best to apply mulch after the soil has warmed up in the spring. Cool, wet soils tend to slow seed germination and increase the decay of seeds and seedlings.
If adding additional layers of mulch to existing perennial beds, wait until the soil has warmed completely.
Mulch used to help moderate winter temperatures can be applied late in the fall after the ground has frozen but before the coldest temperatures arrive. Applying mulches before the ground has frozen may attract rodents looking for a warm over-wintering site. Delayed applications of mulch should prevent this problem as, hopefully, the creatures would already have found some other place to nest!
TIP: Our expert gardening advisor, Rachel Klein adds, "Convert autumn fallen leaves into a carbon-rich mulch to be applied in early wintertime."
Mulches used to protect plants over winter should be loose material such as straw, hay, or pine boughs that will help insulate the plants without compacting under the weight of snow and ice. One of the benefits from winter applications of mulch is the reduction in the freezing and thawing of the soil in the late winter and early spring. These repeated cycles of freezing at night and then thawing in the warmth of the sun cause many small or shallow rooted plants to be heaved out of the soil. This leaves their root systems exposed and results in injury or death. Mulching helps prevent the rapid fluctuations in soil temperature and reduces the chances of heaving.
TIP: Rachel says, "Conveniently, you never have to worry about removing old mulch before spreading new. Either turn the old layer of mulch into the soil, or spread the new layer directly on top of the old."
Courtesy of the U.S. Department of Agriculture.