Microbiologist Dr. Uwe from Neutrog has to say...

"There is a raft of reasons why you should apply mulch in Winter - the most obvious are weed suppression and of course regulating soil temperature by placing an insulating blanket of organic matter down. However, a good organic mulch such as Whoflungdung will also encourage earthworms to come to the surface, thereby aerating your soil and moving nutrients deeper in the soil profile - your plants will get that benefit as soon as it warms up. If you have sandy soils, applying Whoflungdung in Winter will give your soil microbes time to break it down and get some organic material into the soil, which will improve your water and nutrient-holding capacity, which is often lacking in sandy conditions. If you’re on a block with a slope, then mulch applied in Winter can help prevent erosion from heavy rain, as the rain needs to percolate through the mulch - an added benefit is that you end up with less runoff and more water absorbed into the soil".