No Turtle Necks
One important rule of green thumb is not to pile mulch up around the necks or stems of your plants. A turtle neck is a cone of mulch surrounding and touching the bark of trees, shrubs, and perennials. This can trap too much moisture on the bark and encourage disease problems. Keep the mulch 4 inches from tree trunks and 1 to 2 inches away from the stems of shrubs and perennials. You can feather the mulch so that a very light layer is sitting on top of the soil near the stems of plants with the thickness of the mulch getting deeper as you work your way away from the plants.
How Deep to Mulch
Although even a one inch layer of mulch will offer benefits to your plants the insulating properties of the mulch are best if it is put down at least 2 to 4 inches deep. A lightweight mulch of shredded leaves can be piled up to six inches deep or more. The shredded leaves will break down quickly. A leafy layer will often shrink to half of its original depth by the time spring arrives. In very cold winter areas a mulch that is 6-12 inch deep can help marginally hardy plants survive. A loose or more chunky mulch of corn cobs, straw, or wood chips creates even better insulation.
Removing Mulch in Spring
A thick blanket of mulch acts as insulation from the cold. When the sun comes out in the spring, it is possible for the soil to heat up quickly. This can cause the plants to break dormancy too soon. For this reason you should pull a thick layer of mulch away from plants in early spring just as their buds begin to swell. The cooler soil temperatures caused by the removal of the mulch will stop the plants from leafing before winter is really over.