Spreading mulch is easy. The most important rule is to avoid making the layer of mulch too thick.
Research shows that a settled mulch that exceeds 4 inches deep restricts the access of oxygen to the soil and to plant root systems. Also, mulch piled excessively high around the stem of a small tree or shrub can actually kill the plant over time.
Usually organic material such as chopped leaves settles down considerably a few weeks after it has been spread. So, to establish a layer about 2 to 4 inches deep, put down 6 to 8 inches of chopped leaves. They will settle down to the desired 2 to 4 inches within a month or so. Wood chips on the other hand don't settle very much, so hold them to the desired 2 to 3 inch layer from the start.
Mulching Do’s and Dont’s
Do not pile mulch up around plant stems or tree trunks. Bark or stem tissues will start to rot if they are surrounded by moisture and cut off from air.
Do not use mulch over landscape fabric if you expect to improve the soil. When the mulch decomposes it can not enter the soil if there is a fabric barrier between them.
Do mulch trees and shrubs out as far as the tips of their branches, the dripline, if possible. Roots normally spread well beyond that radius and will benefit from the mulch.
Do limit the depth of the mulch layer to 3 inches, to be sure that roots and soil are not cut off from air.
Do not spread mulch on dry soil. Mulch after a soft rain has thoroughly soaked the soil, but has drained away so that the soil is not soggy
Seasonal Mulching Cycle
Mulch is most effective when it is in place year round. It is easy to follow a seasonal cycle if you are using yard waste materials.
Fall: Shred falling leaves with a lawnmower, blower/vac or shredder and spread a 4 to 6 inch layer as a winter mulch on all planting beds and around trees, shrubs, and hedges. They will settle to a 2 to 3 inch layer by spring.
Spring: Pull aside mulch temporarily from planting and bulb beds to encourage the soil to warm up more quickly and bulbs to bloom. Then restore the mulch layer around existing plants and mulch newly planted ones. Leave the mulch around the trees, shrubs, and hedges year around.
Summer: Renew the mulch layer to a 2 or 3 inch depth when it begins to decompose in the summer heat. In the North this might mean as much as 6 inches of mulch added twice a year.
However, in the South because of more rapid decomposition in the hotter climate, two added layers of 10 to 12 inches each year may be necessary. Replace it in the fall with newly chopped leaves.