We love our mulch, don’t we? It can be the eye-pleasing beauty bark or a plain plastic tarp — it doesn’t matter much, because what we’re really after are its practical results: fewer weeds and warm, moist soil. And, show of hands, how many of us have given the odd piece of cardboard a try? It’s cheap, it’s biodegradable, and that brown color matches almost everything — it’s all good, right? Well, has Linda Chalker-Scott got news for you…
I’ve discussed my dislike of cardboard mulch before: like other sheet mulches it restricts water and gas transfer between the soil and atmosphere. In published comparison studies, other mulch choices generally outperform cardboard in terms of plant growth, weed control, etc. But there’s one area where cardboard is tops compared to every other mulch material tested.
Termites.
Eeeew.
Termites LOVE cardboard. Did you know that termite researchers use cardboard feeding stations to lure termites? And cardboard is often used as the “control” in feeding studies, because termites will always eat it?
People seem to think that wood chips are termite magnets. Though termites can eat some types of wood, they prefer cardboard in taste testing. If they are given no choice and have only wood to eat, they will consume it but their survival rate decreases. Dead termites don’t reproduce.
To give termites a bit of a break, they are very useful in bringing life back to crusted, arid soils: studies have shown that just adding mulch and termites to these degraded soils is enough get biological processes going again.
But personally, I’m not providing a cardboard welcome mat for termites to the gardens surrounding my wooden house. Hopefully you won’t either.
Dr. Chalker-Scott is WSU Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture and Landscape Architecture. She is a regular contributor to The Garden Professors blog.
The Perennial Post 
We went through the same problem last year after I spent nearly $250 on red-dyed mulched. The mulch was infested with termites within about 3 months of putting it out. Very upsetting seeing how I put it around my house, in all my flowerbeds and around my greenhouse. Needless to say, I spent the beginning of Spring this year scooping it all back up and going with pinestraw!
Hi, Stephanie –
Thanks so much for your eye-opening comment! Was the red-dyed mulch made of cardboard or shredded wood?
Dolly.
It was made of shredded wood. We bought it from a local retailer who shredded and dyed it locally. The problem was, that when you buy it in bags from a retailer it is usually some form of redwood, cedar or cypress, therefor it usually doesn’t attract termites. This was made from whatever wood they chopped down from their land clearing business. THAT WAS THE PROBLEM… If you are going to use mulch it needs to be redwood, cypress or cedar. While none of them are 100% termite proof, you will RARELY find termites in those three types of wood. All the years before I used a red cypress and never saw a termite. It really does pay to pay a few extra dollars in the long run!
You can buy borax from the washing powder isle and sprinkle it on thick after pulling up your cardboard mulch and that will help get rid/prevent termites. :)
What did you mean about “red-dyed mulch”? Was the cardboard colored red?
My grandsons helped me finish spreading medium bark this weekend over a graveled area at my house. This is the second area where the bark over the gravel has been great for keeping the weeds down, soaking up excess water from down spouts, making it easier to hoe up the few weeds that appear, and making a softer more pleasant walking surface. Also use the medium bark on all my beds. Have eliminated shot weeds in late winter almost completely.
Judy, that sounds like a great solution: good-looking and effective! Thanks for the tip.
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Termites aside, cardboard is still my go-to weed barrier when establishing new garden beds over top of fallow ground or lawn. Something I do often with my business. It’s effective and lasts just long enough to do its job before quickly dissolving into the fluffy new soil. Fluffy because Earthworms also love cardboard, and they are tilling, composting, pH correcting, tunneling machines. Their numbers always increase whenever I combine my cardboard with a deep organic mulch.
There are some rules to using cardboard as a weed barrier though (I won’t call it mulch because it is a poor stand-alone option. When the mulch is away the wind will play, if you get my drift. And its ugly.). First, many cardboards contain toxins and/or are treated with fungicides. Luckily these are easy to identify. Any cardboard that has been used to contain refrigerated or frozen foods has been treated, and any with glossy, magazine-like print will contain heavy metals and should be recycled instead. Beer and frozen pizza boxes are definitely off the list! The glues and inks used in non-glossy cardboard are water/soy based and not harmful. I am now going to recommend not using cardboard adjacent to structures built of non-termite resistant woods thanks to this blog.
For my orchard care clients I will often double up my layers of cardboard before mulching with deciduous branch-wood wood chips to really snuff out sod, which negatively effects soil gas ratios to avoid being shaded out by trees, but breaks down into great soil if left in place.
A quick word about using woodchips as mulch. Small diameter, deciduous wood chips do wonderful things for the fungal communities in our soil, especially for perennials and trees. But, most of the wood chips you can purchase in our Evergreen State are conifer wood. Almost all of our soils were growing evergreen forests not long ago, which tend to create acidic soils not beneficial to our veggies, and most fruits. Conifer mulch will add to this condition and is best avoided if it makes up more than 20% of your total mulch volume. There is always the blueberry caveat though. They love acid, as do many other berries, azaleas, rhodies, etc.
When I ran an invasive weeds crew I developed a strategy of two or more layers of cardboard atop weed patches, with as much wood mulch as I could get. This not only kills weeds, but creates rot conditions for the seed bank still in the soil. In some cases this could otherwise re-sprout up to 90 years later. In time this heavy mulch helps restore native soil conditions and allows for the return of the local flora.