Mulching basics: Are you covered?

During the last decade or two, mulching around landscape plants has gained wide acceptance. While it's true that proper mulching can dramatically improve the health and vigor of landscape plants, it's just as true that improper mulching can stress and even kill plants. Let's look at the right, and wrong, ways to use mulch.

Mulch materials There are two basic choices of mulch: inorganic and organic. Inorganic mulches such as lava rock, mineral rock and gravel, pulverized rubber, geotextile fabrics and so on do not readily decompose. This makes them long-lived and, therefore, low-maintenance, which is why many contractors prefer them.

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Organic mulches are usually derived from plants or plant parts and include materials such as cocoa hulls, conifer needles, leaves, grass, newspaper, straw, hardwood and softwood (conifer) bark, wood chips and other wood products.

When considering mulch longevity, remember that conifers (softwoods) decompose more quickly than hardwoods, finely textured mulch more quickly than coarse mulch, succulent tissues more quickly than woody materials and fresh tissue more quickly than dry.

Wood content (cellulose, for practical purposes) is important in determining longevity. Wood decomposes more quickly than bark. For example, conifer bark nuggets from mature pine, cypress or other softwood trees contain high amounts of lignin, wax and protected cellulose that resist decay. Wood from these same species (especially from young trees), by contrast, rots quickly. Likewise, hardwood tree bark, even from large trees, contains large amounts of cellulose (like most wood) that is not protected from rotting.

Because of decomposition, organic mulches need periodic replenishment, and this is where many problems arise. The "monkey-see, monkey-do" plague of over-mulching landscape plants unfortunately continues because of a lack of education. Not only is over-mulching a waste of mulch (and clients' money), it is becoming a major cause of death for azaleas, rhododendrons, dogwoods and many other landscape ornamentals.

What happens with too much mulch? Over-mulching may cause stress and plant death in several ways.

* Low oxygen. One of the most common causes of stress by over-mulching is oxygen starvation of plant roots. Roots must "breathe" and take in oxygen to survive (unlike leaves, which are net producers of oxygen). When oxygen levels in soil drop too low, roots decline and die, and the plant will succumb. Excessive applications of finely textured mulch can prevent water loss via soil evaporation and increase the likelihood of root stress and dysfunction. The increased soil moisture displaces air and reduces oxygen levels.

Oxygen deprivation from finely textured mulch is especially prevalent in soils that do not drain well, and during rainy periods. Plants are more susceptible to low oxygen levels in spring and fall, when root growth is at a maximum. Saturated soils also enhance attack by root-rot pathogens.

If you use leaves for mulch, chop or shred them before application to prevent matting. Grass clippings should be dry and applied no more than 1 inch thick.

* Inner bark (phloem) stress. A second major cause of plant decline and death results from piling mulch directly against the stems of trees and shrubs. Unlike roots, trunks and stems of most plant species do not possess mechanisms for surviving in continually moist environments and must be able to freely exchange oxygen and carbon dioxide with the atmosphere.

Studies have documented that soil or fine-textured mulch piled onto stem tissue decreases gas exchange with inner bark (phloem) tissue. Researchers from the Bartlett Tree Expert Company have verified that continual moisture on trunk tissue inhibits the movement of oxygen and carbon dioxide in and out of the phloem (inner bark). This condition can be exacerbated in landscapes if sprinklers adjacent to the plant keep the mulch in a saturated state.

Without gas exchange, the inner bark ceases to function normally and eventually may die. If this happens, roots become malnourished (a major function of the phloem is to transport photosynthates from the leaves to the roots) and weakened to the point where they become much less efficient with water and nutrient uptake. If wet conditions persist long enough, the plant may experience top dieback and death.

* Fungal diseases and opportunistic borers. Fungal and bacterial diseases can be serious problems with excessive mulching. Because most plant diseases require a moist environment to grow and reproduce, piling mulch against a trunk can provide the moist conditions necessary for fungi-notably Phytophthora and Armillaria-to gain entry into the plant.

Some concern has been raised about a somewhat different disease problem. Can mulch that was derived from a diseased plant transmit the disease to landscape plants? Research has shown that Verticillium dahlia, a common fungus that causes wilting and death in many ornamentals, can be transmitted in fresh mulch. Similarly, Rhizoctonia solani, a fungal pathogen that causes damping-off of seedling plants, can kill herbaceous ornamentals and is actually stimulated by fresh mulches, especially following nitrogen applications.

Despite these studies, disease transmission in this manner apparently is rare with established woody landscape ornamentals. If it were a frequent problem, tree-care companies such as Davey and Bartlett (which have applied fresh wood chips for decades) would have documented it. It appears that the mulch must come into direct contact with an open injury on a susceptible plant during ideal environmental conditions for the fungus to be transmitted. Though possible, the probability of all these conditions being met at once is small.

Regardless, potential disease transmission is easy to eliminate by composting fresh mulches for 6 weeks under high-temperature conditions (130 to 160 degrees F). It is critical to compost the mulch correctly-turn the pile frequently and add the necessary material to achieve the ideal (20:1) C:N ratio. Apply 1 to 3 pounds of nitrogen per cubic yard of fresh mulch, add grass clippings (be sure they haven't been treated with herbicides) at 10 to 20 percent by volume or add any other nitrogen source to reduce the carbon to nitrogen ratio to 20:1. That is the quickest way to compost fresh mulch and make it usable.

Insect borers and bark beetles have also been associated with stressed, over-mulched trees and shrubs. Not only are stressed plants less able to repel pests when they attack, some pests, including borers, actually can sense stressed plants and attack them preferentially.

* Hardening-off. When wet mulch layers placed against the stem decompose, they generate heat, just as they do when composting. Temperatures may reach 120 to 140 degrees F, which is enough heat to kill young tree and shrub phloem.

The extra warmth can cause another kind of problem. In autumn, plants naturally "harden off," which prepares them for the winter cold. The extra warmth of decomposing mulch can trick a plant into delaying the hardening-off process. If trunk-flare tissue is not adequately hardened off before freezing weather arrives, the tissue may die, the roots may starve and the plant will go into decline.

* Soil pH. Long-term use of the same type of mulch may change the surface soil's pH. Acidic mulches such as pine bark, pine needles and peat moss may have a pH of 3.5 to 4.5. When you apply them continually, they may, over several years, cause the surface soil to become too acidic. Acidic mulches such as pine needles are beneficial for maintaining the acidic conditions required by acid lovers such as rhododendrons and azaleas.

Conversely, hardwood bark mulch (even though it is initially acidic) may cause the surface soil to eventually become too alkaline (soil pH above 7.0), causing shallow-rooted, acid-loving plants to decline because of micronutrient deficiencies (the chief problem stemming from high pH levels).

Inorganic mulches also can affect soil pH. For example, limestone gravel used as mulch will quickly kill acid-loving plants. Be sure to test inorganic mulches for pH and toxic elements before using them.

Studies have shown that mulches do not significantly alter subsurface soil pH because of the buffering capacity of the soil. Therefore, concerns about pH are mostly relevant to surface-rooted plants.

* Rodent chewing and girdling. Mulch piles provide cover and habitat for chewing rodents such as mice and meadow voles. With protection from predators, the rodents can live under the warm mulch in the winter and chew on the tender, nutritious inner bark (see photo, page 44). This activity may go unnoticed until the following spring or summer. If the chewing is extensive (more than 50 percent of the circumference is girdled) or total (it completely girdles the tree), there is little you can do to save the tree. Prevention is the best solution-keep mulch away from the trunks of plants.

* Anaerobic or "sour" mulch. "Sour" mulch can occur when finely ground mulch is piled so high (usually greater than 10 feet) that inadequate air exchange occurs in the center of the pile. Without adequate oxygen, anaerobic microorganisms become active and produce several organic acids and alcohols, causing the mulch to give off pungent odors and produce extremely acidic pHs ranging from 1.9 to 4.8. Such mulch is highly toxic to plants-especially tender, herbaceous annuals and perennials, and recently transplanted woody ornamentals. If mulch smells bad or is extremely acidic, don't use it until it is properly composted.

* Nutrient deficiencies and toxicities. Many uncomposted, "fresh" mulches can cause nitrogen deficiencies in recently planted trees, shrubs and flowers. Most landscaping mulches have little nitrogen available (in other words, they have a high C:N ratio) for the decomposing bacteria and fungi. Thus, microbes will use the existing nitrogen in the surrounding environment (which may be scarce already) to break down the mulch. This may cause temporary soil-nitrogen deficiencies, especially if the mulch is mixed into the soil and is finely textured. Fortunately, such nitrogen deficiencies are temporary. The mulch will eventually release its nutrients into the soil and the decomposition will taper off. Supplemental nitrogen in the meantime can alleviate the deficiency.

The opposite problem-nutrient toxicity-has been demonstrated in a few studies using fresh mulch. For example, one study found that green hardwood mulch caused toxic levels of manganese in newly planted trees. USDA research has also shown that rubber tires contain zinc. When ground up and used as mulch, this rubber can cause severe zinc phytotoxicities, especially in acidic soils or when incorporated into growing media. Fortunately, toxicities such as these are rare in landscapes.

* Allelopathic mulch. Allelopathic toxins are compounds produced by plants that inhibit the growth of other plants. Such substances might be present in mulch, depending on the source. The classic example of allelopathy is the black walnut. It produces juglone and juglonic acid, which inhibit the growth of many plants. Juglone is found in all parts of the plant. Therefore, fresh wood chips and sawdust from black walnut should not be used as a mulch unless well composted. Even then, small amounts of juglone can be detected.

For a list of plants that black walnut negatively affects, see the Ohio State University Extension Fact Sheet HYG-1148-93 (which is available online at

Besides black walnut, other allelopathic mulches include uncomposted sawdust of redwood (Sequoia) and cedar (Cedrus); the bark of spruce (Picea), larch (Larix) and Douglas fir (Pseudotsuga); and both the leaves and sawdust of Eucalyptus. The bark of needled evergreens sometimes releases toxic volatile gases that can be harmful, especially to herbaceous ornamentals and vegetable crops. Toxin-containing mulches are more likely to cause injury if: * the mulch is "fresh" * the mulched plants are young * the mulch particles are fine * the mulch is piled deeply * heavy rains or watering follows application * a large number of roots are near the soil surface.

To neutralize toxic substances in mulch, I recommend composting.

* Fungi. Slime molds (also colorfully referred to as "dog vomit fungus") initially appear as brightly colored (orange, yellow or red) slimy masses that may be several inches to more than a foot across. The mold eventually dries and turns brown, sometimes leaving a white, dry powdery mass. Slime molds are a temporary nuisance and are harmless.

Bird's nest fungi look like tiny brown or gray bird's nests about 0.25 inch in diameter growing on top of the mulch. The "eggs" are actually masses of spores that are dispersed by rain. These fungi are natural decomposers and do not warrant removal or other action.

These, and other fungi that may produce mushrooms, occasionally grow on organic mulches but are harmless to most ornamentals. They usually arise from wet organic mulch following rainy weather. If the appearance of these fungi is offensive, you can remove them (a step you definitely should take if children have access to mushrooms growing on the mulch) or rake them into the existing mulch where they will not be so evident.

- Artillery fungus. "Artillery" or "shotgun" fungus, caused by Sphaerobolus spp., is not so harmless. It grows on wood chips, "double-shredded" bark, leaves and dung, and occurs throughout the United States. Artillery fungus produces tiny fruiting structures that eject spore masses. The fruiting structures look like tiny cream- or brownish-colored cups with small (0.10 inch), black spore masses in them. Problems arise because the fungus can "shoot" the spore masses up to 20 feet (but they can be blown much farther or higher with wind). When they land, the spore masses resemble specks of tar and are extremely difficult to remove, leaving stains on house siding, cars and any other surfaces.

Artillery fungus has caused more than $1 million in homeowner damage claims in Pennsylvania alone. Unfortunately, many insurance companies will not cover damage claims due to "molds." Fungicides are ineffective; so control solutions involve prevention and avoidance.

One solution is removing infected wood mulch near foundations and replacing it with an inorganic mulch. Another is topdressing the infested mulch with additional, very coarse, mulch. This won't eliminate the fungus, but it can create an obstacle that blocks spores from being "fired" into the air.

Preliminary research at The Pennsylvania State University indicates that the Sphaerobolus fungus has difficulty growing on rot-resistant woods like redwood, cedar and cypress, but this is preliminary and other studies are needed to confirm this. Research also is underway to find effective solvents for removing the spore stains.

- Hydrophobic mulch. Hydrophobic mulches are common, and frequently occur when mulches are applied too deeply. This commonly occurs during the summer with fresh woody mulches that become hot from decomposition, eventually drying the mulch pile. Fungi then colonize these dry, dusty mulches causing them to become water repellent (hydrophobic). This can cause irrigation water to run off to the sides, endangering newly planted ornamentals by depriving the root balls of water. However, you easily can remedy this by raking the mulch and breaking up the crusted, hydrophobic layers.

* Termites. Subterranean termites live in nests or colonies in the soil and feed exclusively on wood and wood products containing cellulose. According to research by Ohio State University entomologists, termites can infest and consume wood mulches and thereby be lured closer to residential structures. However, the chances of introducing termites to a site with infested mulch are slim. This is because the reproductive queen termites needed to establish a new colony only live in the soil and are not found in the mulch.

I should emphasize, however, that you should never apply mulch so that it touches the foundation or lowest course of siding on your home, nor contact any wood surface on the home. Though it may look nice, termites can use the cover of mulch to invade homes undetected. An unmulched 6-inch buffer makes it much more difficult for termites to conceal their mud tunnels.

Mulching recommendations Mulching can be one of the best or one of the worst things you can do to landscape plantings. I've just explained what happens with improper mulching; what is the proper way to mulch? Knowing the moisture needs of the plants you care for, as well as your soil's drainage characteristics, is imperative.

For shallow-rooted species in poorly drained soils (clays), avoid mulching more deeply than about 2 to 3 inches. With perpetually wet soils, which need as much oxygen as possible, it may be wiser to control weeds with herbicides and avoid mulches altogether. This will aid soil aeration and encourage drying. The worst thing you can do is apply a thick layer of mulch against the trunk and over the roots.

On the other hand, more deeply rooted species growing in well-drained soils can tolerate 2 to 4 inches of mulch. Coarser textured mulches (such as large "nuggets") can be applied relatively thickly. You must be more cautious with the finer, more compactable, "double-shredded" mulches. A 2-inch layer usually is adequate to provide the benefits of mulching-weed suppression, moisture conservation, reduced frost heaving-without the negative consequences of overdoing it.

As a rule-of-thumb, keep mulch 3 to 6 inches away from the trunks of young trees and shrubs, and 8 to 12 inches away from the trunks of mature trees. If decomposition creates the need for more mulch, measure first so you know how much to apply. Do not pile on more simply to "freshen" the appearance-raking is usually all that is needed.

Mulch out to the tree's dripline (if possible) and add only enough mulch every 2 to 3 years to maintain the desired depth. When planting young trees and shrubs, mulch ideally should extend 12 inches beyond the periphery of the root ball to promote faster root growth into the surrounding soil. Extend the radius of the mulch 12 to 24 inches each year until the tree or shrub's root system becomes re-established.

Investigating problems If you suspect over-mulching may be stressing a tree or shrub, simply dig into the mulch to assess its depth. If it appears that the root flare has been buried, visual inspection is the next step. Carefully probe downward and remove the soil or mulch to below 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 exchange essential oxygen and carbon dioxide.

Arborists often remove a small strip of bark and sapwood from the root collar following excavation to determine the presence of fungal pathogens. The resulting "exposed well" must be left open.

The problems caused by over-mulching often are not immediate and progress slowly with time. The symptoms on long-lived woody plants may take 3 to 5 years, sometimes longer, to manifest themselves. Unfortunately, by the time the symptoms (off-color, chlorotic foliage, abnormally small leaves, poor annual twig growth and dieback of older branches) are recognized, it may be too late. The plant is likely to die.

Nevertheless, scientists with Bartlett have noted that an amazing number of plants improve rapidly in color and vigor within months of a root-collar excavation. Observations also show such plants are less susceptible to winter injury because healthy roots produce the growth regulators responsible for aboveground winter hardiness.

All of us in the turf and ornamental industry can discourage over-mulching by educating the public and doing what is best for the plant, even when that will not benefit our pocket books. Many uneducated or unscrupulous contractors sell mulch even when the plants don't need it.

Submit articles and photos to local newspapers to get the word out about improper mulching. This helps people become intelligent consumers. Mulch can be worth its weight in gold, but you have to apply it properly for it to pay off.

Dr. Chris Carlson is asssociate professor and Director, Horticulture Technology, at Kent State University (Salem, Ohio).

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