The Sentinel-Record

How much mulch is too much for your beds?

- Luke Duffle

This year, before adding mulch to your beds, first measure how much mulch is already there. A 2-4 inch layer is ideal and provides many benefits to plants. These include weed suppressio­n, moisture retention, and the slow addition of organic matter to the soil as the mulch decomposes. Another important benefit of mulching around trees and shrubs is that it keeps mowers and weed trimmers away from their trunks, which can be easily damaged by such equipment.

Since mulch does so many good things, it is easy to think more would be better. Thicker layers of mulch can harm plants. When mulch builds up over 4 inches, water simply sheds off the top of the mulch rather than soaking into the soil below, leaving plants thirsty. Thick layers of mulch can also suffocate plant roots. We don’t often think about the role of air in the soil, but roots need air to survive. Burying root systems under excessive layers of mulch reduces the amount of air in the soil, causing plants to decline. This decline is usually slow, often taking years for plants to die from over mulching

If you already have a 4-inch layer of mulch on your beds but wish to add more to brighten up the bed’s appearance you have two options. One is to remove part of the mulch that is already there before adding new mulch. You can add the old mulch to your compost pile or use it as an underlayer for new beds. The other option is to just stir up the mulch you have. Often only the top layer loses its bright appearance and if you mix the mulch up a bit you will bring some of the more richly colored lower layers to the surface.

Another common mistake is to treat mulch like a soil amendment, mixing it into the soil when you plant. Soil amendments like compost have already broken down into small particles. When mixed into the soil, they slowly release nutrients, help retain moisture and also improve drainage. Most mulch is too coarse to make good soil amendments.

Mounding mulch against trunks, aka “volcano mulching,” can slowly kill trees. Especially harmful is the practice known as volcano mulching, where deep layers of mulch are piled up against the trunks of trees. In addition to suffocatin­g roots, this type of mulching encourages pests. If your trees look like they are erupting out of a mountain of mulch, you have volcano mulch. To protect the health of your trees, pull excess mulch away from tree trunks. Taper the mulch level down near the trunk so no mulch actually touches a tree’s trunk.

When using hardwood bark mulch, be aware of a rare but destructiv­e problem called “sour mulch.” Most of the time, hardwood bark is stored in piles that have adequate oxygen present so that aerobic microbes can help with normal decomposit­ion. When storage conditions favor anaerobic microbes, significan­t problems can develop, leading to plant injury or death of the plant. Sour mulch is easily identified by the presence of a pungent odor similar to vinegar, rotten eggs or ammonia. Vapors often dissipate quickly after the affected mulch is spread out in a shallow layer. When spreading sour mulch, you may even experience a burning sensation in your eyes. While the vapors can cause damage to plants, they are usually not harmful to people or pets. The vapors are simply an irritant to people.

To prevent mulch from becoming sour, simply monitor circumstan­ces (i.e., large pile size, waterlogge­d bark) that favor anaerobic conditions. Suppliers should avoid storing mulch in piles over 4 feet high. For taller piles, periodic turning will allow for some level of aeration. Mulch should also be stored in a well-drained area to prevent water accumulati­on. Gardeners and landscaper­s should not buy, spread or allow the applicatio­n of mulch with a foul odor or mulch that is hot. Good mulch should have fertile compost or fresh-cut woody smell.

4-H informatio­n

There are several 4-H clubs for Garland County young people who are 5 to 19 years old. For more informatio­n on all the fun 4-H activities that are available, call Carol Ann McAfee at the Extension Office at 501-623-6841 or email her at

Master Gardener informatio­n

Master Gardener meetings are held on the third Thursday of each month at the Elks Lodge. They’re open to the public and guests are welcome. For more informatio­n call Luke Duffle at 623-6841 or email him at lduffle@uada. edu.

EHC informatio­n

Are you interested in joining an existing Extension Homemakers Club? EHC is the largest volunteer organizati­on in the state. For informatio­n about EHC, call Alison Crane at 501-623-6841 or email

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