Wood Chip Mulch – Free Friend or Fungusy Foe?

Wood chip mulch. Some people love it, others fear it! Wood chip mulch can be purchased for less than two bucks a bag at many home centers. Fancier, colored chips can be had for a little extra moolah. In many towns, wood chips from tree trimming and brush clearing are free for the taking at the local landfill or compost facility. Wood chips are really effective as mulch for suppressing weeds and retaining moisture, as well as adding carbon to compost piles… so what’s not to like about wood chip mulch? I mean, we are talking FREE here!!

Actually, lot of people have concerns about wood chips. Some of the concerns are well founded, but other fears about wood chips are about as legit as worrying that Bigfoot will attack your garden. Hey, I’m not saying it can’t happen, but…What is fact, and what is fiction? Can you run the risk of importing pests, pathogens and pesticides along with the free landfill mulch? And what about the store bought varieties? Are they safer?

There is some excellent information out there on the benefits of using wood chip mulch, but you have to look a little harder to find good, reliable information on the downsides. For those of you who may be unfamiliar with the benefits of woodchips as mulch, I would refer you to a 2010 article from Mother Earth News entitled Building Garden Soil With Wood Mulch. It explains a lot of the benefits you can see from using it in your garden. From personal experience, I can tell you that adding wood mulch on top of our clay soil does a lot to improve the structure of the soil. Although decomposing wood can reduce nitrogen in the soil, adding mulch on top and not working it into the soil reduces the impact on nitrogen, and helps the growth of healthy bacteria and fungi in the soil.

Now, let’s talk about the scary bits. What got me wondering about the safety of bringing wood mulch onto my property was the dreaded Emerald Ash Borer. In much of the country, this nasty little critter is wreaking havoc, infesting and destroying Ash trees. In many areas, it’s out-and-out Ashageddon. Transporting ash logs is strictly verboten just about anywhere you go. But do these little devils survive the shredding process? According to study by the Department of Entomology and Department of Forestry at Michigan State University (pdf), larvae do not survive in chipped wood. Chips of under 1” are considered to be safe, and in fact chipping is the recommended method to destroy infested Ash trees. My gut feelings tells me that nothing is 100% safe, but it sounds to me like the risk of bringing in Emerald Ash Borers is very low.

What about other pests and pathogens? There is a chance of attracting carpenter ants or termites, but it’s pretty low, according to most sources. Cedar mulch actually repels pests, so if you want to mulch around the foundation of the house, cedar is a good choice. As for tree diseases, the fungal activity in the mulch generally will kill off pathogens before they can reach the roots of the tree. To be safe, age your wood chips, or get them from the oldest pile at the landfill. This also helps assure that any pesticides that have been used on trees have the chance to break down.

Another concern is that chips from trees with a lot of oil, like Cedars, will kill plants. In truth, only a few trees like Black Walnut have been found to contain allelopathic chemicals (“allelopathic” is a fancy way to say “kills other plants.”) Most city wood piles are pretty well mixed, so a small amount of black walnut probably won’t make too much of a difference, and the bagged Cedar chips are no problem.

I have talked mostly about the wonderful bounty of free, local wood chips and the bagged commercial cedar chips. What about the other kinds of bagged mulch? Are the fancy dyed mulches safe? The answer is, “it depends.” Most of the colored chips are dyed with vegetable based dyes, and for the most part are safe. Some wood chip mulches have been treated with other chemicals, though, and you will want to do a little research on the particular product before you buy it. A lot of products out there at the big box stores may say things like “organic” or “Eco-Friendly,” but in most cases, they are not actually “Certified Organic,” unless they have the USDA seal. In any case, don’t use the dyed mulches near your vegetables.

After doing a lot of research, I feel a lot better about bringing in piles of wood chips to mulch around my gardens. A thick layer in the paths means no more weeding! And, they add a really nice rustic look to things. I gotta say, I’m all in. I’m going to spread wood chips everywhere!! Just like Johnny Appleseed! Except they’ll call me Chip Chipperson!