Wood Chip Mulch Myths

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Robert Pavlis

The use of wood chips for mulch is very popular but there are also many myths about it. Will it rob nitrogen from the soil? Is the dye on black mulch toxic? Do wood chips attract termites and can it ignite spontaneously?

These are just some of the myths I’ll investigate in this post.

Wood Chip Mulch Myths
Wood Chip Mulch Myths, source: Cindy Shebley

What Are Wood Chips

For some, the word mulch is synonymous with wood chips, but that is not correct. Straw, hay and even stones can also be used as mulch. Wood chips are a special kind of mulch that is made from chopped wood.

Arborist wood chips are those that are produced when trees or shrubs are chopped up into small pieces. They tend to be a bit courser than wood chips sold by nurseries and they might include some leaves from the branches.

Colored mulch is available from nurseries and landscape supply companies. These are finely chopped pieces of wood that have been dyed in colors like red, brown and black. The wood used for making these could be arborist wood chips or reclaimed wood products from demolition sites. Chopped skids are frequently used.

Microbe Science for Gardeners Book, by Robert Pavlis

Cedar mulch is  chopped up cedar wood. It is usually more expensive, but it does decompose more slowly, reducing the cost of replenishing the mulch. Some claim that it is better at repelling termites.

Ramial wood chips are made from small to medium sized branches, and contains leaves. Because it has a higher ratio of cambium it is a bit higher in nutrients.

The difference between these mulches is fairly minor and the following myths apply to all of them, unless noted otherwise.

DO NOT PLACE WEED BARRIER (LANDSCAPE FABRIC) UNDER WOOD CHIPS!

Will Wood Chips Steal Nitrogen From Soil

Wood has a very high carbon to nitrogen ratio. In order for it to decompose, nitrogen needs to be added. The early stages in wood decomposition is done mostly by fungi with bacteria playing a role later in the process.

Fungi have hyphae that penetrate into the soil and they pull up nitrogen to the wood, to aid decomposition. This nitrogen is taken from the top couple of inches of soil, which is above most plant roots. This removal of nitrogen might affect seedlings which is a good thing because it keeps weed seeds from growing. This is one reason wood chips are not recommended for vegetable gardens.

Bacteria are very small and can only pull nitrogen from the surface of the soil.

In short, wood chips do remove nitrogen from the soil, but this does not impact the nitrogen level at plant roots. Also, as the wood reaches the end of the decomposition cycle, some of the nitrogen is returned back to the soil. The net effect is an increase in nitrogen levels.

Wood Chips Need to be Aged

People who believe wood chips rob nitrogen from soil, suggest that you shouldn’t use fresh wood chips. As they age, they are less likely to rob nitrogen.

As far as nutrients go, there is no reason to age chips. They age perfectly well sitting in garden beds.

See the disease section below for a possible reason to use aged wood chips.

Is Colored Mulch Toxic?

The dye used to color wood chips is made from iron oxide, carbon or vegetable-based colors.  They are natural, non-toxic and safe to handle.

The sun does bleach the color and over time the wood becomes lighter. It is one reason why many people stop using black mulch.

The dye may not be toxic, but it still has to be manufactured and applied. As with every manufacturing process, there is a negative environmental impact. For this reason, don’t use colored mulch.

Evergreen Wood Chips Acidify Soil

This myth is based on the incorrect notion that evergreens are acidic and I have even had arborists tell me “You don’t want my next load of chips because it was too acidic”. Neither wood nor needles from evergreens are acidic. Wood chips will not make the soil acidic.

Wood Chips Attract Termites

Termites live underground in large social communities. They like dark, moist places and the conditions under mulch are perfect for them, but that does not mean mulch will attract them.

“In the field, termites were detected with equal frequency beneath mulches of eucalyptus, hardwood, pine bark, pea gravel and bare, uncovered soil. ”

Keep mulch several inches away from the house foundation and never allow it to cover windowsills or contact house siding.

What about using wood chips from areas that have termites? The chipping process is very efficient at killing termites and other insects. Any that do survive and live on the remaining wood have a very low chance of survival.

There are some claims that cedar (Thuja) mulch repels termites. Studies show that Western red cedar, WRC (Thuja plicata) has variable resistance to termites, from low to high, and the European standard EN 350-2 (CEN 1994) lists WRC as susceptible to termites, while the Australian standard AS5604 (Standards Australia 2005) lists WRC as resistant.

Keep in mind that resistance is not the same thing as “repelling”. I found no study that showed cedar mulch repels termites.

Wood Chips Introduce Tree Pests

The concern here has to do with the larvae of pests, such as the emerald ash borer (BER). Will the larvae in contaminated chipped wood infect healthy trees?

Most such pests are killed during the mulching process. The other thing to consider is that the larvae itself will not infect trees. For infection to happen the larvae would need to mature, become an adult, and then the adult would need to lay eggs in the tree being mulched. That is unlikely.

Bulk wood chips are normally a local product. Any ash tree that is chipped up near me, won’t affect my trees, because my ash trees are already infected. It is probably a good idea not to move mulch from infected areas to ones that have zero infection, but moving it in an area that is already infected won’t make the problem worse.

For new types of infections, it is probably better to burn the wood rather than use the chips. For example, the Asian longhorned beetle was found in a very limited area in Ontario. All of the trees were removed in the infected area to stop its spread. As of right now, we don’t have the pest in the province, so it would be unwise to use trees that might be infected with the Asian longhorned beetle.

The risk that wood chips spread pests is very low due to the chipping process, but there is a risk. It is important to understand each pest and their potential for spreading.

Spread of Diseases From Using Wood Chips

Most diseases are not going to be spread by using wood chips but there are a few concerns.

honey fungus, source:
honey fungus, source: Lord Mayonnaise

Armillaria root rot, also called the honey fungus, is a fungal disease that is particularly deadly. “The fungus spreads between plants by root-to-root contact and can also survive on wood chip mulches. Spores disseminated from the mushrooms are unlikely to be a source of spread. A pot culture experiment found that ” Wood chips made from infected wood can infect other plants, but the rate of infection was low (2.5%) and approaches zero if the chips are cut small enough (1 cm). Another field experiment found no transfer of the honey fungus to 3 types of trees.

The last study also looked at the rate of infection of Sphaeropsis tip blight, Botryosphaeria canker, and found only the former infected trees from infected mulch. Heat treating the mulch killed the fungus and prevented infection.

“Data describing extended survival of tree pathogens in mulch are accumulating. Verticillium dahliae, pinewood nematode (Bursaphelenchus xylophilus), and the canker-causing fungus Thyronectira austroamericana were found to survive periods of at least 17, 80, and 98 weeks, respectively, in wood chips in the field”

All of the studies I looked at either showed no transmission or a low level of transmission, but the potential is there. In most cases, you have no idea if the wood was gathered from diseased trees. There is some indication that letting the wood sit for a while, reduces the chance of infection because the pathogen dies over time. This is especially true if the pile reaches higher temperatures.

Using wood chip mulch also increases the fungal population in soil, and most of these fungi are good guys which help control potential diseases. This benefit may overcome any negative impact from using diseased wood.

Wood Chips Are a Fire Hazard

A flame can ignite wood chips. So having wood chips near a home, in a fire zone, is a concern.

A study looked at different mulches to see which type was safer:

  • Shredded rubber, pine needles and shredded western red cedar were the most hazardous
  • Wood chips were less hazardous, and thinner layers burned less.
  • Wood chips treated with a flame retardant were marginally better than untreated wood chips.
  • Composted wood chips (proprietary product) were the least hazardous.

Spontaneous Combustion of Wood Chips

I have seen numerous newspaper reports that reference statements from fire professionals who claim that wood chip mulch can catch fire on their own – spontaneous combustion. These reports are wrong.

Piles of wood chips start to decompose and this process produces heat. If the heat can’t escape fast enough, it can ignite the pile of wood. Guidelines for the storage of wood chips and
mulch (from NFPA 230) recommend that “Piles should not exceed 60 feet in height, 300 feet in width or 500 feet in length”.

What this says is that piles smaller than this are safe from spontaneous combustion. A wood chip mulch layer will never combust on it’s own.

Mulch from Black Walnut Trees

Nothing grows under a walnut because of the juglone – or at least that is what many think. What about wood chips made from black walnuts?

Juglone does prevent some seedlings from growing, but it has almost no effect on mature plants. Wood chips are not a problem provided you are not growing seedlings.

If Some Mulch is Good, More is Better

Use 2-3 inches of wood chips. This is enough to stop weeds and still allow water to reach the soil. There is no evidence that more is better.

How often do you have to add it? If you start seeing bare spots or more weeds it is time to add more.

I regularly see people complain the wood chips don’t work. “I applied wood chips and still have weeds.” If you see a lot of weeds after adding mulch – you did not add enough. If done properly you should see almost no new weeds.

It does compact. You really need to add 4″ to end up with 3″.

Adding Fresh Mulch Each Year

You do not need to add it every year.

What do you do with the old mulch? Some people say that you should remove it, but that makes no sense. Just add the new mulch right on top of the old one.

The longevity of mulch depends on many factors; type of wood, size of chips, age at time of application, climate and the level of microbial activity in the soil. In my zone 5 garden, I find that a good 3″ layer lasts about 3 years.

Landscape Fabric Under Mulch Makes it Last Longer

I don’t know if this is true, but using landscape fabric is a bad idea.

Besides, you want the wood to decompose. The faster this happens, the quicker you improve your soil. Besides, landscape fabric does not prevent weeds.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

29 thoughts on “Wood Chip Mulch Myths”

  1. Thanks for the informative article.

    If you possible, would you clarify if arborist mulch is recommended in an herbaceous perennial garden?

    Thanks

    Reply
  2. ” This nitrogen is taken from the top couple of inches of soil, which is above most plant roots. This removal of nitrogen might affect seedlings which is a good thing because it keeps weed seeds from growing. This is one reason wood chips are not recommended for vegetable gardens.”

    I’m confused by this. If wood chips keep weed seeds from growing, then why are they NOT recommended for vegetable gardens? We don’t want weeds growing, right?

    The article now made me worried about my straw mulch that covers much of my vegetable garden. It looks like it’s decomposing pretty quick, so it’s robbing the top soil of nitrogen?

    Reply
    • wood chips can work in veg garden, provided it stays as a mulch. However, the extra planting and harvesting in a veg garden usually results in some being buried. Straw and hay tend to stay above the soil

      Reply
  3. I’m in the process of buying my first house and gearing up for being able to grow in the ground for the first time rather than in containers. I’ve been watching lots of your videos recently to help me make plans (including for a wildlife pond). Now I can’t help but hear your voice in my head when I read your articles! This was an interesting one, thank you.

    Reply
  4. Regarding free mulch from tree lopping firms – I find weeds come up quite soon in this mulch (spread about 3 inches deep). I guess that it’s because the mulch has a lot of fines in it, mainly from leaves. The green leaves decay into a mix of compost-like material mixed up with the bigger woody chips that last much longer. That compost-like material supports a rather variable amount of weed growth. Sometimes I resort to using Roundup on them. I’m in a warm, humid climate with mostly evergreens (Sydney, Australia).
    But the wood chips supplied by local soil dealers is much cleaner being usually just wood chips and no fines. It works much better at suppressing weed growth.

    Reply
  5. Wonderful post, Robert, my only suggestion would be to advise never to place landscape fabric (or black plastic) on the ground before mulching with wood chips at the start of the article, then a few more times scattered throughout, before ending with it one last time. The number of homeowner mulch installations I’ve seen that feature wood mulch atop a plastic barrier of some kind is disheartening, to say the least.

    Reply
  6. Great article. 2-3 years ago you wrote an article on fertilizers (eg. different 20-20-20) tree fertilizer stakes, manure, compost etc. comparing these fertilizers to mulch and i thought your results indicated that wood chip mulch resulted in superior growth of plants, shrubs and trees. i am happy to report that after many years of us using only mulch and no other fertilizers that our plants, shrubs and trees have done much better than before in regards to growth and health.

    Reply
  7. Thank you for investigating this timely topic. You answered three important questions arising yesterday in a conversation with a horticulturalist. It is so very well written.

    Reply
  8. Clarifies a lot of questions I had, thank you. I also heard wood chip mulch attracts Carpenter Ants (from a guy in the Pest Control Business)??

    Reply

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