Mulching Tips

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One of the most common problems we encounter while working on homes all over New Jersey is improper mulching. Not only is over-mulching a waste of mulch, (and a potentially costly one at that), it is rapidly becoming the number one cause of death to shrubs and trees.

One of the most common causes of stress by over-mulching is suffocation of plant roots. Repeated applications of mulch can contribute to a water clogged soil and root zone by slowing soil water loss through evaporation. Roots must respire (breathe) and take in oxygen unlike leaves which give off oxygen. When oxygen levels in the soil drop below 10 percent, root growth declines. When too many roots decline and die, the plant will eventually succumb.

It is most important to remember that the problems caused from yearly over-mulching are not immediate, but progress slowly with time. The symptoms may take 3 – 5+ years to express themselves and sometimes longer, depending on the species of plants and trees.  Unfortunately, by the time the symptoms are recognized (off-color foliage, abnormally small leaves, poor twig growth, and die-back of older branches) it is generally too late to implement the corrective measures. At this point, the plant has usually gone into an irreversible decline and  most likely die or continue to look so poor  that you’ll just remove it and replace it ($) .

A second major cause of plant decline and death from overmulching comes from the piles of mulch being placed against the trunk base of trees and shrubs. The above ground stem and trunk tissue is very different from root tissues. Roots have evolved many mechanisms to survive in continually moist environments, the trunks of most woody species have not. Above ground trunks and stems must be able to freely exchange adequate amounts of oxygen and carbon dioxide through lenticels. When mulch is piled onto the trunks, gas exchange decreases with phloem tissue eventually becoming stressed and later dying. When the phloem dies, roots are malnourished and weakened to the point where they suffer reduced water and nutrient uptake, which subsequently affects the health of the whole plant.

A third mortality factor which is associated with the application of mulching next to trunk bases and stem tissue involves fungal and bacterial diseases. Most plant diseases require moisture to grow and reproduce. Trunk diseases are no exception and will usually gain entry into the stressed, decaying bark tissue caused by the homeowners unknowingly piling the mulch next to the tree trunk. Once established, even secondary fungal invaders such as Phytophthora and Armillaria species will eventually kill the inner bark, thereby starving the roots, and ultimately killing the plant. Many times, bark beetles and borers. which are also attracted to the stressed trees, will assist in the decline of the tree and allow other fungal pathogens entrance into the tree. This has been observed with clear-winged borers which normally attack higher on the stem.

Excess heat can also be generated when excessive mulch is placed up against the trunks and stems become wet and begin to decompose. The decomposing of mulch, similar to a compost pile, where inner mulch layers may reach 120° to 140 fahrenheit plus, this will enough heat to kill young trees and shrubs. Older trees and shrubs will be put into additional stress and be more susceptible to drought, insect damage, frost damage or many other possible stress problems. The combination of these  stress problems, weather primary problems or secondary problems,  will become the cause of death of your very expensive plant material. Over mulching will also prevent the natural hardening off of the bark, a natural process that trees and shrubs must go through, to prepare themselves for winter.

The continuous use poor mulch will also contribute to plant stress by ultimately changing the soil’s acidity level, commonly referred to as soil pH. Acid mulches like pine bark may have a pH of 3.5 to 4.5 and when applied year in and year out, may cause the soil to become too acidic to grow many alkaline requiring plants. Here at Best Horticultural Services we send out soil samples and test soil areas where plant material, year after year, required replacement.  We found due to the increased solubility of many micronutrients in acid soil and trace of toxic elements all led to additional plant stress which in turn allows secondary pathogens and insects to invade the plants.  Studies also showed that hardwood bark mulch, which is initially acidic, may cause the soil to eventually become too basic or alkaline causing acid loving plants to quickly decline because of micronutrient deficiencies. Soil pH’s above 6.5 – 7.0 usually create micronutrient deficiencies of iron, manganese, and zinc for many common, acid-loving, landscape plants. Changes in soil acidity can be avoided by periodically testing your soil for pH levels and nutrient availability. We use independent laboratories which test your soil  so we can condition the soils for the proper growing environment. Best Horticultural Services will then structure and implement the correct solution for any of you horticultural problems.

Placing piles of mulch adjacent to tree trunks can also kill plants by providing cover and habitat for chewing rodents such as small mice, meadow voles, etc.. With lots of cover from predators, the rodents will usually live under the warm mulch in the winter and chew on the tender and nutritious inner bark to get at the sugars. This chewing off of the bark many times goes unnoticed until the following spring or summer when the tree doesn’t look good. If the chewing is extensive or goes around the whole tree, there is little that can be done to save the tree. Bridge grafting with strips of bark over the girdled or damaged area can be done but this service is time consuming and  unpredictable, so we don’t recommend this procedure , as most arborists would not recommend that you go to these extremes.

CHEAP MULCH or I GOT A GREAT PRICE MULCH may be a great deal of trouble

Finally, many fresh or non-aged mulches may cause nitrogen deficiencies in young trees, shrubs, and flowers. Decomposing bacteria and fungi which ultimately break down mulch must have an ample supply of nitrogen to do their job. Most landscaping mulches are comprised of bark or wood which have high carbon to nitrogen ratios and have very little nitrogen available for the decomposing bacteria. Hence, the bacteria in the soil utilize the existing nitrogen to break down the mulch. This process may cause nitrogen deficiencies on new growth. Although nitrogen deficiencies may occur, they are usually considered temporary as the mulch will eventually release its nutrients into the soil and the decomposition will taper off.

 

The best way to determine if you have a problem with excessive mulch piling in your landscape is to go out and simply dig through the mulch layer to see how thick it really is. A light raking of the existing mulch is all that is needed to break up any crusted or compacted mulch layers that can repel water and to give it that finished landscape appearance. As a rule-of-thumb, install only a thin layer of mulch a minimum of 3-6 inches away from the trunks of young trees and shrubs and 8-12 inches away from mature tree trunks.

Conducting a visual inspection of the root flare is the best way for an arborist to check a tree or shrub for a possible root collar disorder. If no root flare or buttress roots are found, the chances are good that at least some of the root crown has been buried. When burial is suspected, the arborist must first carefully probe downward to determine the extent and depth of burial. If the root collar is buried, you must remove the soil or mulch below the surface of the junction of the roots and the trunk collar (without damaging the roots or collar) to expose the root collar. This is necessary to allow the collar to dry out and begin respiration of essential oxygen and carbon dioxide.

In very sever cases Best Horticultural Services take has taken a small strip of bark and sapwood from the root collar following excavation to determine the presence of fungal pathogens such as Phytophthora or Armillaria . The resulting exposed well must be left open unless the root collar disorder is so severe that the resultant tree decline or hazard potential warrants removal of the tree.

Best Horticultural Services has been part of an amazing number of plants which have improved rapidly in color and vigor within months of a root collar excavation. Observations also indicate that far less winter injury has been recorded  in such plants because the healthy roots, once an excavation has been conducted, produce the growth regulators responsible for above ground winter hardiness.

Of course, pruning of any dead and or dying branches should be conducted to reduce the introduction and spread of disease in treated trees. A light fertilization with a low salt index, slowrelease, nitrogen fertilizer (at 1 – 2 pounds of actual nitrogen per 1000 square feet) may also be required of trees treated for root collar disorders to renew vitality and growth.

In summary, over-mulching and root collar burial is needlessly killing many landscape trees and shrubs by oxygen starvation of the roots, lack of gas exchange and death of inner bark, promoting stem and root diseases, prevention of hardening off via increased mulch temperatures and declining root vigor, rodent girdling, development of water repelling mulch layers, allelopathic mulches, potential short-term nitrogen deficiencies, and nutrient and acidity problems from sour mulch.