Does Cedar Mulch Repel Pollinators and Other Insects

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Robert Pavlis

We all know cedar chests repel moths and cedar shavings are routinely used in homes to control insect pests. Based on this, gardeners have concluded that cedar mulch will repel insects in the garden and will negatively impact pollinators trying to get to flowers. For these reasons they recommend you should not use cedar mulch.

Is there any science to support these claims? Does it harm bees? Does it affect ants and termites?

Should you stop using cedar mulch?

Does Cedar Mulch Repel Pollinators and Other Insects
Does Cedar Mulch Repel Pollinators and Other Insects

What is Cedar Mulch?

It is always good to start with fundaments. Lots of people use the term “cedar mulch” but almost nobody defines what it is. Clearly it is bits of wood from a cedar tree, but what is a cedar tree?

In Eastern North America the term cedar is used mostly to refer to the eastern red cedar Juniperus virginiana, or the eastern white cedar (or northern white cedar, or arborvitae), Thuja occidentalis, neither of which is a cedar. The western red cedar (Thuja plicata) is also not a cedar. The term can also be used for trees in the genera Cupressus (cypress) and Chamaecyparis (false cypress).

True cedars belong to the genus Cedrus which includes things like the Atlas Cedar and the Cedar of Lebanon.

So which mulch is a problem? Are they all a problem? Proper tree names are rarely used by people who make claims about cedar mulch. In this post I will use the term cedar in the more general way and clump all of the above together, except in cases where the tree species is known.

Food Science for Gardeners, by Robert Pavlis

The Claims for Cedar Mulch

The most common claim is that “cedar mulch is an insect repellant because insects hate the smell”. Some see this as a positive and recommend cedar mulch over other types of mulch. Others see it as a problem because it keep beneficials and pollinators out of the garden.

It is odd that “all insects” hate the smell? But just a minute – do cedars not have pests? Sure they do. So that means not all insects hate the smell of cedars.

Other claims say that cedar mulch is toxic to bees, it stops seed germination and it can kill plants. But none of these claims comes with references that support the claim.

Cedar Mulch and Bees

There are a number of claims about bees and cedar mulch. Some say it kills bees. Others say the mulch keeps bees away because they don’t like the smell.

Did you know that cedar mulch is used by beekeepers around their hives to keep insect pests away from the hives. Some of the best beehives are also made from cedar wood.  I think we can be quite sure cedar is not going to kill bees or even repel them.

Cedar bee hives,
Cedar beehives, source: Sweet Mountain Farm

However, mulch does pose a problem for native ground nesting bees. They like open soil for making the nest and won’t make a nest if the layer of mulch is too thick. But that is a problem with any mulch, not just cedar mulch.

Plant Science for Gardeners by Robert Pavlis

Thujone Repels Insects

Thujone is an essential oil found in Thuja species as well as many perennials including, tansy, yarrow, wormwood, sage and hyssop. It’s been studied extensively and has shown to repel insects. It is important to note that these studies used concentrated extracts, not wood mulch. One study found that neither Thuja occidentalis essential oil (containing 69.80% of α-thujone and 9.47% of β-thujone) nor pure α-thujone repelled the aphid, Myzus persicae, while pure β-thujone was a strong deterrent. The science on this is much more complicated than gardeners think.

Results with concentrated extracts can’t be extrapolated to imply a result using cedar mulch. This is a common mistake in the gardening world.

Do Cedar Chests Repel Insects?

"Cedar is great for moth and other insect deterrent both inside and outside the home." - Only $10 for a 13g sachet!
“Cedar is great for moth and other insects both inside and outside the home.”, so the ad says – Only $10 for a 13 g sachet! Are you kidding me?

I think a lot of the claims about cedar mulch stem from the fact that everybody knows a chest of drawers made from cedar will repel moths and other insects. Or is this a myth too?

Actual studies looking at this are quite old and I could only find parts of the reports. None showed good evidence that cedar chests repelled cloths eating moths.

Jason Dombroskie, manager at the Cornell University Insect Collection and the university’s Insect Diagnostic Lab, suggested that cedar chests are effective primarily because of their airtight seals and it is unlikely that the cedar linings on their own would be effective against moths or their larvae. In any case, cedar loses its oils as the wood ages. The University of Kentucky says, “contrary to popular belief, cedar closets or chests are seldom effective in deterring clothes moths because the seal is insufficient to maintain lethal or repellent concentrations of the volatile oil of cedar.”

Cedar has been used to repel insects as far back as Ancient Greece, but it is mostly a myth. At best, there is some effect when the wood is new.

So a cedar wooden box that is almost air tight is not very effective at repelling insects. Consider the garden with some cedar wood chips laying on the ground and wind blowing away most of the chemicals emanating from the wood. It’s going to be even less effective. Cedar mulch has almost no effect on flying insects.

Does Cedar Mulch Repel Ants

I just said it is not very effective at repelling insects – why would ants be different? There are thousands of insects and they don’t all behave the same. Almost every general comment about “insects” is wrong in some cases.

In a lab study, the Argentine ant, (Linepithema humile), and the odorous house ant (Tapinoma sessile), avoided aromatic cedar mulch as a nesting substrate. Both ant species were killed when confined with fresh cedar mulch in sealed containers, but only one species was killed with aged cedar mulch.

Cedar Mulch May Attract Mosquitoes

Areas mulched with Ashe juniper, had more mosquitoes than un-mulched areas.  This was true for both fresh and aged mulch. The number of mosquitoes found in this study was small.

Cedar Oil Saves Mummies

Egyptian mummies are deteriorated by insects. Cedar wood oil diluted in ethanol had a toxic effect on 4th instars larvae of Dermestes maculatus, a serious pest of mummies. If you ever decide to keep a mummy in your garden, you now know how to protect it. 🙂

Cedar Mulch And Invertebrates

Invertebrates are small animals that include things like, earthworms, woodlice, spiders, springtails and mites. Most are beneficial to the garden. A study looking at invertebrates found that mulched areas had a higher level than bare soil and this included cypress mulch. In some cases the numbers were higher in the mulch itself than in the soil under the mulch.

Cedar Mulch And Termites

When termites were given a choice between different kinds of mulch as food, they used cedar less than other types. Cedar is clearly not toxic to termites and given no alternative they will use it. Another study looking at a different subterranean termite, Coptotermes formosanus, found similar results.

New packing material made from cellulose and fungus mycelium is prone to termite attack. These materials were tested with several natural oils including cedar oil and borax. Heavy mortality  was found with vetiver oil and guayule resin, but cedar oil and borax produced only moderate results.

Cedar mulch does not seem to attract termites, nor is it toxic to them.

Cedar Mulch And Pollinators

I found no studies that looked at the effect of cedar mulch on pollinators (except for the ground nesting bees discussed above). Since most pollinators are more interested in flowers, which are located a fair distance from the mulch, I doubt that the mulch has much of an effect.

Should You Use Cedar Mulch?

The idea that cedar mulch repels pollinators is probably false. It may repel some insects like ants, but it also attracts others.

Cedar mulch ages, and as it ages the impact of any cedar oil decreases. If the mulch is a byproduct of the oil extraction process, it would have even lower oil levels than material from the forest industry.

Overall, cedar mulch is a good option, provided it is available locally. Don’t buy it if it was trucked a long distance to get to you. Try to use local material.

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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!

2 thoughts on “Does Cedar Mulch Repel Pollinators and Other Insects”

  1. Thank you for another eye opener.
    I use any mulch I can find for reasonable price or free when possible never really thinking about its effect. It works fine in my perennial and vegetable gardens. Lots of pollinators and other types of insects seem to be “happy” 🙂

    Reply
  2. Oh thank god an answer to my mummy troubles.

    In all seriousness though, I personally avoid cedar (Thuja plicata) because someone in my household is allergic to the wood. To avoid rashes & respiratory trouble, we use 100% douglas fir rather than ambiguously sourced mulch.
    (These are both, of course, trees of the Pacific Northwest.)

    Reply

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