A gardening "fad" that can be detrimental to longevity of plantings is soil or mulch "cones" around trees. Recently a caller to the Master Gardener help desk explained that a landscaper had created raised beds, resembling volcanoes, extending two or more feet up tree trunks. The landscaper assured her that lightweight potting soil would not damage trees.
Although striking, a deep mass of either soil or mulch in contact with a tree trunk creates a damp environment promoting rot and inviting wood boring beetles. Elevating soil levels more than one or 2 inches per year under most tree species also seriously affects aeration of roots.
Mulch around trees is recommended for at least the first 3-4 years, but the maximum recommended in contact with the trunk is two inches. Deeper mulch may be applied if a funnel-shaped "crater" is created to keep mulch and soil from touching the trunk. Landowners must use common sense in landscaping. By the time a tree is in obvious distress, it may be too late, and a mature tree is expensive to replace.
Joan Driver, Smith County Master Gardener
Texas AgriLife Extension Service
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