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The Hidden Dangers of Wood Mulch

5 Dangers of Wood Mulch



Danger to:


  1. Mulch from treated wood can contain chromated copper arsenate, or CCA, which includes a form of arsenic--a poison that can leach into your soil -- a problem around your vegetables. It can also get on your hands as you handle the mulch.

FYI: Arsenic is a known human carcinogen and is acutely toxic.

2. Toxic liquids and gases can form in improperly prepared or stored wood mulch, such as methanol, acetic acid, ammonia, and hydrogen sulfide which is more dangerous than any pesticide.


to plants: Toxic to leaf tissue as shown by greatly decreased metabolic rates and waterlogged appearance. Root tissue is even more sensitive, showing toxicity when briefly exposed to 10 % methanol solution.

to humans: Exposure can lead to symptoms of weakness, dizziness, headache, nausea, vomiting and blurred vision. In severe cases of accidental or reckless ingestion, methanol poisoning may lead to permanent blindness or death.

Acetic Acid

to plants: A concentration of 20% acetic acid will kill some plants.

to humans: Concentrated acetic acid is corrosive to skin and can cause skin burns, permanent eye damage, and irritation to the mucous membranes. These burns or blisters may not appear until hours after exposure.


to plants: Stunts the growth of plants and dries out the soil before killing the plants. Ammonia also affects plants by discoloring them and burning their roots. Concentrations as low as 3.5 parts per million are toxic to seedlings.

to humans:

Inhalation: Causes immediate burning of the nose, throat and respiratory tract. This can cause bronchiolar and alveolar edema, and airway destruction resulting in respiratory distress or failure. Lower concentrations can cause coughing, and nose and throat irritation.

Skin or eye contact: Produces rapid skin or eye irritation. Higher concentrations may cause severe injury and burns, permanent eye damage or blindness. The full extent of eye injury may not be apparent for up to a week after the exposure. Contact with liquefied ammonia can also cause frostbite injury.

Ingestion: Results in corrosive damage to the mouth, throat and stomach.

Hydrogen Sulfide

to plants: Although a specific amount of hydrogen sulfide aids plant growth, even the most minor misuse easily leads to crop mortality.

to humans: Hydrogen sulfide is considered a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous system is most affected.

3. Wood mulch may become very acidic, with a pH in the range of 1.8 to 3.6. (Properly composted organic material has a near-neutral pH of 6.0 to 7.2.)

This leads to...

Aluminium toxicity

A small drop in pH results in a large increase in soluble aluminium. In this form, aluminium retards root growth, restricting access to water and nutrients, leading to poor crop and pasture growth, yield reduction and smaller grain size.

Nutrient un-availability

All the major plant nutrients (nitrogen, phosphorus, potassium, sulphur, calcium, manganese and also the trace element molybdenum) may be unavailable, or only available in insufficient quantities.

4. Wood mulch can breed 'shotgun' or 'artillery' fungi that shoot tar-like spores as far as 30 feet towards light colored objects, like the side of your house or car.

A lawn expert once shared…

“I’ve received some emails asking me about little black spots that appear on the siding of homes. They seem to multiple until they are covering large sections of the side of the house. At first, homeowners think they are being attacked by birds or bats. They even think that some type of shell-less snail is gathering. It’s none of those things. It’s actually a self-firing little spore called Artillery fungus.”

5. Wood mulch takes nitrogen right out of the soil, out-competing your nitrogen-needy plants. Nitrogen deficiency in soil causes:

  • Yellowing of leaf edges

  • Scorched-looking leaves

  • Defoliation

  • And/or death of plants.

1 comentário

01 de jun.

Cedar wood chips are excellent for mulching, but watch out for potential problems. They can attract insects and hold moisture, leading to mold. Apply them properly and maintain regularly to enjoy their benefits without the hidden dangers of wooden mulch.

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