Put those leaf blowers down, and ease up on your rakes! Fallen leaves are among the most under-appreciated garden resources. When we remove them from our garden beds we take away the most natural form of compost and mulch, and eliminate untold potential for biodiversity in our gardens.
Why is leaf litter so important? The decomposition of fallen leaves returns nutrients to the soil, and is part of the vital ecological cycle that sustains both garden and woodland environments. The organisms that break down the leaves— including fungi, insects, and other arthropods— form a complex and vibrant food web, and are an important food source for birds. Leaf litter also insulates the ground, helping to protect the roots of shrubs and trees from freezing weather.
Leave the leaves in place in your woodland garden and around your shrubs, and they will become next spring's mulch. As they decompose over the growing season, they will build up into a rich layer of organic soil and naturally suppress weeds throughout the year. Your perennials will emerge just fine in the spring, so long as the leaf build up is not unnaturally high. And you will support a rich community of beetles, ants, moths, snails, and the many birds that eat them.
Some gardeners might ask, why not shred the leaves and add them back onto garden beds, instead of leaving them as is? While shredded leaves look more similar to traditional mulch, what is gained in formal aesthetic quality is vastly outweighed by the ecological damage done by the shredding. When you shred the leaves or remove them altogether, you are also killing many beautiful creatures that may be living in your garden bed.