Taking a look around Westchester County, NY, you may notice that mulching has become a fairly standard practice in landscaping, but there are some serious downsides to this practice if done improperly - namely killing the trees it is supposed to be protecting.
How Mulch Kills Trees
Raised mounds of mulch, up to between 9 and 12 inches or even higher around the base a tree, are known as mulch volcanoes. Mulch volcanoes stress and kill trees in many ways. Ordinarily, mulching is used to protect fragile parts of a plant, provide insulation against cold and excess moisture, and guard the ground against drying out. Mulch can also reduce the maintenance involved in weeding.
However, a mulch volcano covers parts of the tree that would ordinarily be exposed to air. The resulting excess moisture in the soil can predispose the roots of a tree to rot and leave it vulnerable to attack by fungus. A thick insulating layer of mulch, particularly inorganic mulch that can take years to decay, also reduces the amount of oxygen in the soil. Over time, the tree compensates for the lack of aeration by growing more adventitious roots. These roots end up wrapping around the deeper tap root, essentially girdling it, causing the tree to slowly strangle itself.
Another way over-mulching affects tree health is through the heat created when the mulch decays. This can lead to internal damage in young trees and bark that is soft and underdeveloped. The hardening of bark as a tree matures is essential for protection against parasites and weather conditions.
Why Mulch Volcanoes Are Thought A Good Thing
Many homeowners’ yards tend to have over-mulched trees; they believe mulching improves the growth of a tree and that it is good for plants in general. Looking at trees around businesses and homes, you would think that a professional landscaper would know better. Of course, mulch has its value, but when misapplied, plants suffer and die. Perhaps the appeal stems from a belief that if a little is a good thing, a lot must be even better. In moderation and with the correct mulch, mulching can be a tool for retaining soil moisture, moderating soil temperatures, and suppressing weeds. However adding more mulch does not have any additional value. Mulch volcanoes may also present a degree of visual appeal, but the practice ignores the needs of the tree.
Remaining mulch should be removed from the base of the tree each time new mulch is added. If left, mulch that has not broken down can form a compacted, insulating layer.
A one or two inch layer can be used to suppress weeds and retain soil moisture if necessary. In this case, only easily degradable organic mulch should be used. Inorganic mulch is a definite no no, as are artificially colored or treated mulches. Likewise, mulch that smells of vinegar indicates it has not been stored properly thus may contain harmful acids and should not be used.