Mulch: Can there be too much?
This year, before adding mulch to your beds, first measure how much mulch is already there. A 2- to 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. In reality, thicker layers of mulch 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 oxygen 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 under-layer for new beds. The other option is to just stir up the mulch you have. Often, only the top layer loses its bright appearance. 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, also known as “volcano mulching,” can slowly kill trees. Volcano mulching is where deep layers of mulch are piled up against the trunks of trees and this practice is very harmful. 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. The mulch layer should be 3 to 4 inches deep.
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
If you have any questions, contact the Garland County Cooperative Extension office at 623-6841.
Master Gardener information
Master Gardener meetings are held on the third Thursday of each month through Zoom. They’re open to the public, and guests are welcome. For more information on how to attend, call the Extension office at 623-6841 or email Alex Dykes at adykes@ uaex.edu.
EHC information
Are you interested in joining an existing Extension Homemakers Club? EHC is the largest volunteer organization in the state. For information on EHC, call 623-6841 or email Alison Crane at acrane@uaex.edu. Follow Alison on Facebook @garlandEGF and @Garland FCS, and EHC on Facebook @GarlandCountyEHC.
4-H information
For information about Garland County 4-H Club membership or program benefits, contact Carol Ann McAfee at the Garland County Cooperative Extension office located at 236 Woodbine in Hot Springs, call 623-6841, or email cmcafee@uaex.edu. More information is available at http://www.uaex.edu/garland.