If you need to mulch a garden in fall that is so large that you don't have enough leaf mulch (from your fall raking) or other material for the job, one alternative is to use cover crops. Since, when cover crops serve in this capacity, they are essentially functioning as mulches, even though they are live plants, they are sometimes termed "living mulches." Cover crops are an alternative to mulching with bark, leaves, or other conventional mulches in order to protect your soil for the winter.
Cover crops have another nickname, besides "living mulches." They are sometimes referred to as "green manure crops." They derive the latter nickname from the fact that, once tilled into the garden, they fertilize the soil -- as would manure.
Some gardeners choose to use cover crops in both their capacities: as "living mulches" for winter protection and as "green manure crops" that will be tilled under in spring. If you do so, of course, next spring, after tilling the cover crop under, you'll have to find another mulch to apply to the garden to suppress weeds.