Mulching is one of the best lines of defense against dropping temperatures. A good mulch layer will protect your perennials through the winter by helping to keep the ground from experiencing a constant freeze and thaw. Additionally, mulch will protect the soil from winter erosion.
There is a reason for mulching twice a year. In the spring we mulch to assist keeping the soil moist during the hot, dry summer. But in the winter, the mulch provides insulation for the soil and plant roots. Leaving mulch out of your fall clean up can certainly lead to root injury in your plants.
How much mulch should you apply? A two to three inch layer is optimal, and will provide the perfect amount of insulation for the cooler days ahead. Over mulching won't give more protection, in fact it can often cause more harm to your plants. Never place mulch too tightly or closely to the stems and trunks of your plants. This can lead to disease in some native plants.
Any newly planted fall trees and plants should be mulched immediately. This will keep your soil warmer and more moist and will definitely help the roots grow for a longer period of time.
Remember, a good mulch cover will keep your soil ten degrees warmer than the air temperature, and this could be the difference in the survival of your plants. Mulch...it's not worth forgetting!