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Lenahan Landclearing & Grinding, Inc.
4 Toddy Hill
Road/Route 34
Sandy Hook, CT
Phone: 1-203-426-2909
Fax: 1-203-426-1637

Beware of Sour Mulch

Mulching garden or landscape plants with bark, straw, leaf, or sawdust mulch can be beneficial to plants in a number of ways. Mulch not only helps conserve moisture around plants, but also serves as a barrier to weed growth and can be an effective method of weed control. It is generally beneficial to plant growth as long as it has been composted properly and is applied properly to the plants. Proper application of mulch includes not applying it in too thick of a layer and avoiding direct contact with the base of plant stems, especially those of tender herbaceous plants. According to the Plant Protection Newsletter (Vol. 10, No. 5), when organic material used for mulch has been composted improperly, the result can be "sour mulch," which is toxic to lawns, bedding plants, and newly planted shrubs.

Toxicity of sour mulch is due to the by-products of the decomposition process, such as methane, alcohol, ammonia gas, or hydrogen sulfide gas, that build up to levels toxic to plant growth. Symptoms of mulch toxicity occur within 24 hours after application and include marginal leaf chlorosis, leaf scorch, defoliation, and/or death of plants.

Methane, alcohol, and other toxic by-products of composting may build to toxic levels in piles of organic materials subject to anaerobic conditions (i.e., low in oxygen and containing over 40 percent water). Large piles, such as those accumulated at lumber mills or those used to compost mulch by companies that manufacture it, can develop pockets low in oxygen and high in water content. Reputable companies turn the windrows when necessary, thus avoiding low oxygen/high moisture conditions.
The only way to protect plants from mulch toxicity is to avoid using toxic mulch. Although most companies that manufacture mulch and compost are concerned with producing a quality product and would not knowingly sell an inferior or dangerous product, it is possible to purchase toxic mulch. Therefore, it is important to be able to recognize toxic mulch before you apply it around plants. Good mulch generally smells like fresh-cut wood or garden soil. Sour mulch may smell like vinegar, ammonia, sulfur, or silage. If you are unsure of the smell, you can have the pH of the mulch tested. Sour mulch is very acidic, and usually has a pH of 1.8 to 3.6, whereas properly composted organic material has a pH of 6.0 to 7.2. The pH of sour mulch is too low to be neutralized by liming; however, the pH of the mulch usually does not change soil pH around plant roots significantly.

Diagnosis of mulch toxicity after it occurs can be difficult because the toxins that cause the damage dissipate quickly after they cause the damage and cannot be detected. However, pH of the mulch can still be measured and is a good indication of sour mulch. Circumstantial evidence is often the easiest means of diagnosis -- if plants develop any of the symptoms listed above within 24 hours of mulch application and no pesticides or fertilizers have been applied recently, damage is due most likely to mulch toxicity.

Removing the mulch is usually not much help at this point since the damage has already been done, and the toxins have dissipated. Plants often recover from the damage by putting out new growth.

At J.F. Walsh Mulch Company, we strive to produce only the highest quality wood mulch. We protect against sour mulch by turning our product over every 60 days, on average, and more often if needed.


Mulches have become a standard element of landscape plantings. For many years well-aged bark was the standard mulch used in landscapes. It contained little wood and was so thoroughly decomposed that few of the fungi living in it produced fruiting bodies. However, the current demand for landscape mulches is so great that wood is collected from a wide variety of sources to be processed into mulch. A segment of the industry is devoted to coloring fresh wood chips prior to use as mulch.

According to Grounds Maintenence Magazine, March 2002, the increase of the wood content in mulch, as well as the decrease in the length of time mulches are aged prior to use, raises several concerns that landscaper professionals must address on a regular basis. Will harmful insects or disease-causing organisms be brought in with the mulch, or be attracted to it? Will unsightly things grow in the mulch? Will something grow in the mulch that could damage structures near it? All of these are reasonable concerns of customers, but are they real problems?

INSECTS IN MULCH. Termites are the insects of most concern to people. Customers see all of this wood-based material being spread around their homes and worry that termites will be brought in with the wood, or will be attracted to it. This is not a real problem. Termites do not live or breed in mulches in large storage piles, or in the thin layers of mulch spread in landscape plantings. The same can be said for carpenter ants, which live and nest in unexposed wood.
People are also concerned that insects that could damage landscape plants will be brought in with mulch. There is a very slight possibility of this happening with fresh wood chips. Bark beetles, wood borers and pine wood nematodes may survive for a short time in fresh wood chips. However, composting or aging the wood for four weeks or more will eliminate this problem.

PLANT DISEASE-CAUSING ORGANISMS IN MULCH. Homeowners may also be concerned that plant disease-causing organisms could be brought in with mulch. A small chance exists that certain vascular diseases, such as verticillium or oak wilt, could survive in fresh wood chips. However, there are no disease-causing organisms that will survive composting or even four weeks of aging in piles that are at least eight feet tall.

FUNGI THAT GROW IN MULCH. Like other organic matter, wood and bark decompose over time. The primary organisms involved with their decomposition are bacteria and fungi, which derive their energy for growth from the carbon-based compounds found in wood and bark. Bacteria are microscopic organisms that are not visible in the mulch. Fungi also may be microscopic, but many develop reproductive structures (for example, mushrooms) that are plainly visible.
The bacteria and fungi involved in the decomposition of landscape mulches are natural components of the mulch environment. They are not harmful to landscape plants, and no known health hazards are associated with them unless they are eaten. They can be found growing in mulches anytime the daytime temperatures are consistently above 40 degrees, usually following rainy weather. Four common types of fungi growing in landscape mulches are mushrooms, slime molds, bird's nest fungus, and the artillery fungus.

MUSHROOMS. The fungi that produce mushrooms are not harmful to the plants in the landscape. The only risk associated with mushrooms is that some may be poisonous if eaten. Removal is recommended if small children are present. Otherwise, mushrooms can be disregarded (or admired!).

SLIME MOLDS. Slime molds are fungi that feed on bacteria growing in mulch. They initially appear as brightly colored slimy masses that range in size from an inch to over a foot across. Many tiny, dark spores are produced in these structures before they dry out and turn brown, eventually as a white powdery mass. Slime molds cause no problems for plants, animals or humans. The fungi may be left in place to decompose, or physically removed if found offensive looking.

BIRD'S NEST FUNGUS. The fruiting bodies of this fungus are produced in structures that resemble small gray to brown bird's nests filled with eggs. They are natural decomposers and not harmful to plants or animals. Some may find them fascinating to look at.

ARTILLERY FUNGUS. The artillery fungus produces tiny, short-lived fruiting bodies resembling tiny cream or orange-brown cups with one black "egg" or spore mass. The spore mass can attach to nearby objects and resembles small specks of tar. Once in place, the spore mass is very difficult to remove without damaging the surface to which it is attached. When removed, it can leave a stain.
Current research at Penn State University suggests that the artillery fungus may not grow as well on wood or bark from decay-resistent trees, such as cedar. Pure pine bark chips or nuggets do not appear to support the growth of the fungus as well as some other materials.

The chances of importing or attracting damaging pests or diseases with wood mulch are usually small. We strive to produce only the highest quality wood mulch. We avoid the few problems that can occur with wood products by properly aging and composting our mulch.

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