tree tips

Too Mulch of a Good Thing

Many people we come in contact with every day have heard the benefits of mulch, but missed the details of application.  Some are of the mind set, “If a little mulch is good, then a volcano pile against the trunk 12 to 16 inches deep should be better.”  Nothing could be farther from the truth.  Trees and plants thrive on balance and moderation.  Too much water can kill as easily as too little.  Over fertilizing can be more detrimental and harder to correct than none at all.  Mulch is no exception. 

Mulch is any material placed on the ground to cover and protect the soil.  Mulches can be organic such as compost, straw, leaf litter, or wood chips.  They can also be inorganic such as gravel or landscaping fabric.  Inorganic mulches need less attention while organic mulches must be replenished.  This can be annually or several times a year depending on the composition of the much.  The finer the mulch the more quickly it will break down. 

The benefits of mulching trees, shrubs, and flowerbeds are numerous.  Mulching reduces water loss to evaporation by protecting the soil surface from the wind and sun.  It also holds water droplets giving them time to soak into the soil and reduces run off.  Mulch decreases soil erosion, soil compaction, and increases soil fertility.  It can also reduce weed germination.  It moderates soil temperature, warming soils in the winter and cooling them in the summer.  One of the greatest contributing factors for survival of a newly planted tree is proper mulching.   

Because mulch holds water, an excessively deep layer of mulch can act like a sponge preventing moisture from reaching roots.  It also prevents the exchange of fresh air needed for healthy soil conditions.   Part or all the original roots the tree had at the time of transplant can die under excessive mulch.   A condition called mulch girdling can occur where the tree stem below the mulch grows less in diameter than above, and eventually leads to the tree’s death.  Root loss can make the tree unstable, causing it to lean or fall.  The extra moisture against the trunk increases the chance of fungal decay.   Symptoms to look for on over mulched trees are browning on leaf edges, wilt, or yellowing, early fall color and/or leaf drop, tips of branches dying, stunting of tree growth or leaf growth.

So how much mulch should you use?  Even a thin layer of mulch, ½ inch, can reduce soil erosion, moderate soil temperatures and reduce evaporation.  However to control weed germination the mulch should be 2-4 inches thick.  To mulch a newly planted tree, place the layer of mulch over and beyond the original tree planting hole.  Keep mulch  1 to 2 inches away from the trunk. If a berm or mound of earth around the planting hole called a watering ring is present, mulch it as well.  Organic mulches are better for most tree plantings.  Though it needs to be replenished more often, composted mulch is better than raw mulch. The microorganisms that decompose raw mulch can rob the soil of nitrogen.  If you must use raw mulch, opt for a courser type such as wood chips or pine bark.  They will deteriorate slowly and consume less nitrogen.  Finer mulches such as sawdust of fine leaf litter may necessitate the addition of a slow release nitrogen fertilizer.  Mulches that are too fine can compact easily and reduce air exchange.  When replenishing mulch, do not add too much.  Keep your mulch layer 4 inches thick or less.  If you desire that fresh mulch appearance and already have a 4 inch layer, try raking the mulch to expose the darker layers beneath.  Remember, it is possible to have too mulch of a good thing.

If you would like more information on mulching, ISA offers several tree care informational brochures, including “Proper Mulching Techniques”.  They can be downloaded from their site at http://www.isa-arbor.com/consumer/consumer.html.