Mulching spruce trees
How much mulch is too much? I had 6' to 10' trees moved in last Sept. I was only able to get half of them mulched. I used chopped soybean straw and put it in about a foot deep. I have been told that this is too much; however, those trees look great. Some of the trees that didn't get any mulch look tough and have lost a lot of needles. Also is there any need to fertilize Colorado Blue or Blackhill spruce trees? if so what and how?
According to the experts, you have too much mulch on your trees. They may look good now, but there may be problems ahead unless you adjust the mulch. A mountain of mulch, piled high against the tree trunk, does not kill a tree immediately but results in a slow death. Over-mulching is a waste of mulch and money. It is a leading cause of death of cherry trees, ash, birch, linden, spruce, and many other landscape plants.
Over-mulching can kill trees and shrubs by oxygen starvation. Suffocation of tree roots is the most common cause of tree and shrub death from over-mulching. Repeated applications can contribute to a waterlogged soil/root zone by slowing soil water loss via evaporation. With water occupying most soil pore space, air content is minimal and diffusion of oxygen is essentially blocked. Roots need oxygen for respiration. When soil oxygen levels drop below 10%, root growth declines to the point of death of the plant. Oxygen deprivation symptoms may take several years to appear, depending on the plant and soil type.
Other problems that may occur are diseases caused by fungus or bacteria that may reside in the mulch, and the roots may grow upward toward the mulch, which will cause further problems. Appropriate mulching is a depth of 3" and none on the root ball or closer to the trunk than a 6" radius.
Generally you do not need to fertilize your spruce trees, especially if they are in a lawn that gets fertilized regularly. Here is a good link on that topic: Fertilizing Evergreens (Conifers).