Mulch 2 to 3 inches thick helps maintain moisture around the root zone.
Over-mulching your trees will cause them to decline long-term as the tree cannot breathe at the natural soil level. Also, mulch tends to knit together and can form a barrier keeping rain water from soaking in to the root zone.
This type of over-mulching problem is more prevalent when using hardwood mulch products as they do not decay as quickly as pinestraw. But even when using pinestraw, it may be helpful to remove some of the decaying matter before applying new material.
This problem can also be detrimental to your shrubs in the landscape. Over- mulching shrubs will cause them to surface root into the decaying mulch material and when we get our hot, dry summers, these plants tend to struggle and sometimes die. Again, this problem is more prevalent when using hardwood mulch, but pinestraw over time can build up an unnatural mulch layer that may need to be raked off before applying new material.
