Mulch, Mulch…Too Much Mulch!

Is your association plagued by the “too much mulch” syndrome? You know, it’s the one where the landscaper comes out each spring and puts on another three inches of fresh mulch in all the beds. And then someone comes to a monthly meeting and asks why there are eight inches of mulch in all the beds. This is a fairly common problem and one that we are often asked about. People generally know that bark mulch is good for the shrub beds but when does it shift from being good to being bad?

First let’s cover the basics about mulch and discuss the types of mulch. In looking at the product list for just one mill in Maine, I counted nine different mulch products to choose from. Many of these products tend not to be “pure” but rather some blend with the named bark being the predominant component. We at The GroundsKeeper tend to stick with the basic three; hemlock, spruce and pine. Each mill tends to have its own blending ratio and it just doesn’t work to say “same as last year” if you change landscapers. Chances are a match won’t be possible unless the new landscaper happens to use the same mill as the source for his mulch.

Several mills have begun a color enhancement program with some of their products in an effort to have the mulch retain its color a little longer in the season. The only problem with color enhancement is when the color begins looking like a Halloween decoration

Remember the bright orange mulch that began appearing several years ago? It wasn’t really bark mulch but rather recycled pallets that were colored in an effort to convince the consumer that it wasn’t recycled wood. Recycled wood products are no substitute for bark mulch. Wood products are from the heartwood of the tree while the bark is the outer covering. When ground up, the two have distinctively different characteristics, should not be confused with each other and are not interchangeable. A primary characteristic of good quality bark mulch is its ability to retain moisture. Recycled wood products don’t have that characteristic. While it may be spray painted to look like bark mulch, it doesn’t have any significant moisture retention capability.

Weyerhaeuser Company research several years ago determined that two inches of bark mulch reduced the moisture loss in summer by 21 percent while at the same time reducing soil temperature by 10 degrees. The third most important characteristic of bark mulch is to reduce weed growth, however we find that weeds seem to thrive in mulch beds too.

Generally speaking, two to two and one half inches of mulch is the optimum total depth to achieve the objectives of moisture conservation and temperature modification. As mulch depth decreases below two inches, the effectiveness experienced for these two objectives also tends to decrease.

When the depth of the mulch covering exceeds the three inch mark, the previously mentioned benefits start becoming detrimental. Deep mulch retains moisture so long that water-logged soil becomes a problem. Research has shown that azalea, rhododendron, and yews in particular suffer most from extended periods of continuously wet soils. Also, if the mulch is piled up against the stem of these plants, it will lead to constantly wet bark which is a favorable condition for disease development

We have seen properties that had mulch accumulations in excess of six inches. It wasn’t that the landscape contractor put down six inches of mulch, it just accumulated over the years. The association’s spring clean up procedures did not include a requirement to remove excess mulch from the previous year and the new application of mulch simply added to the accumulated depth.

Mulch this deep creates another problem. The ground accumulates layers of fine decomposing mulch which becomes wet and virtually seals the soil surface. Roots under a sealed surface layer are often starved for the oxygen necessary for growth and development. When the root system is weakened, the plants develop poor foliage color (rhododendrons in particular become chlorotic with pale green leaves), flowering trees and shrubs will often flower late or sporadically, the plants will develop weak stems and new growth tends to have smaller leaves. None of these symptoms is sudden or (unfortunately) readily apparent. It just tends to happen over a period of years. The plants just won’t thrive any more and then one begins to see whole stems die out or they become diseased. Much of this goes back to the excessive mulch depth.

There isn’t a magic depth where one can say “Ahah! It’s the mulch depth.” Four inches of mulch does not necessarily constitute a hazard. It’s almost like our human lifestyle thing. If a human tends to eat a lot of calories and fat they will tend toward heart disease. Likewise, if the mulch depth tends to be deep, the plants in the mulch beds will tend to have more problems than those that are mulched properly. The deeper the mulch, the more pronounced the problems may become. Also, time is a factor. One or two seasons where the mulch may accumulate to four or five inches does not guarantee plant failure. Successive years of significant over mulching will cause plant stress and the probability of plant failure increases.

Unfortunately correcting the mulch depth problem does not produce instantaneous changes in the plants. The effect of removing the excessive mulch may not be apparent for two to three years and some plants which were overstressed may die even after the excess mulch has been removed. Depending on the length of time the plants suffered, some may be irreparably harmed and never return to be thriving plants.

My last subject is tree “volcanoes.” Certainly you have seen the tree rings with mulch piled up the tree trunk to look like the base of a volcano. This is not a good horticultural practice and should be stopped as soon as possible. The tree bark begins to decompose under the layer of mulch just like the mulch itself decomposes. This will likely result in an open wound (canker) in the bark that does not heal. What this does is break the essential connection between the food producing foliage and the nutrient absorbing root system. Research has shown that flowering cherry, dogwood and ash trees seem to be most susceptible to this type of bark injury.

Another response to over mulching is for the tree to send small fibrous roots up into the mulch bed itself. When one finally pulls back the deep mulch from around the tree, a mass of fine roots will be on the surface of the ground. The best technique in this situation is to cut these roots down to the soil level. Normally, if not too many years have passed, the fibrous roots will shrivel and die within one season and the tree will get back to using the established root system.

So, be vigilant but don’t panic about the mulch depth. If the mulch beds all tend to be over three or four inches deep, get your landscaper to remove the excess mulch and reapply to an accumulated depth of not more than three inches. This one time removal procedure may be an extra cost item because removing excess mulch is a labor intensive job. If the beds are stripped of excess mulch each year, it should be included in the cost of spring clean up. Get the specification on spring clean up to include a requirement that the total depth of the mulch should not exceed three inches.

Properly mulched tree. See tree volcano in background.

 

Mulch volcano on three year old planting.

 

Mulch pulled back to root flair. Note fibrous roots.