Q. I purchased a house with mature trees that have mulch 12 to 18 inches deep at the base of each of them. The mulch has been there for at least two years. I’m afraid that the trees have been damaged. Would it be a good idea to remove the mulch?
A. You are absolutely right to be concerned about so much mulch around the base of your trees. Excessive mulch holds moisture against the bark, which can eventually cause it to rot. Also, the roots under this mulch have been buried too deeply, depriving them of oxygen. Another problem with excessive mulch is that it creates a safe haven for small animals such as mice to feed on the bark unseen until you notice dieback in the top of the tree. By then, it may be too late to save it.
The cambium - the thin layer of light green wood just under the bark - is responsible for transporting water and nutrients from the roots to the leaves. It also transports the products of photosynthesis from the leaves to the trunk and root system for storage. When the cambium is damaged or destroyed by the bark rotting or by animal feeding damage, this transport system is disrupted. If damage is confined to one side of the plant, one side of it may die.
If the cambium is destroyed around the circumference of the trunk - a condition known as girdling - the plant is doomed. Thick rings of mulch can also become hydrophobic – water repellent – when they dry out completely during long periods of dry weather like we experienced this summer. Rain that falls after thick mulch rings have completely dried out may be lost to runoff rather than benefiting the affected tree.
Although mature trees have thicker bark and do not experience severe damage as quickly as young trees with very thin bark, you should remove the excessive mulch as soon as possible. You will probably find that the trees have produced roots in the mulch. These are known as adventitious roots.
They are not true roots since they are produced by stem tissue rather than root tissue, and are not capable of supporting the trees as their real root systems decline. Those adventitious roots will die after you remove the excess mulch, but that should not cause additional damage to the trees. Restoring the balance of air and moisture in the soil will do more to help them now than anything else.
Leave about two inches of mulch. This is sufficient to obtain the benefits of mulching without endangering the health of woody plants. It may be matted together, so be sure to rake out the remaining mulch to break it up. Keep it a few inches away from physical contact with the trunk. If possible, extend the mulch out to the ends of the branches (drip line). The fine, hair-like feeder roots responsible for absorbing water and nutrients extend past the drip line of most trees.
Organic mulches benefit all plants by conserving soil moisture, moderating soil temperatures and controlling weeds that compete for water and nutrients. As an added bonus, they add organic matter to the soil as they decompose. Mulching around the base of woody plants also keeps lawn mowers and weed whackers at a safe distance from their trunks. Young trees and shrubs often succumb to lawn mower or weed whacker “blight.”