MULCH//Are Wood Chips Harmful to Gardens?
I’ve spent an entire morning perusing studies to answer some questions and concerns I read/hear when it comes to using wood chips. And when I say “using wood chips,” I don’t mean on the bed surface like Back-to-Eden gardening. Instead, what I’m talking about is: are there any dangers to adding wood chips to our pathways?
Let’s take a look at some more common concerns…
Will they steal nitrogen from the bed?
So, it seems this concern comes specifically from the nitrogen that bacteria use to decompose wood. The bacteria consume a heap of nitrogen, immobilizing it temporarily (immobilizing nutrients is basically stabilizing them in the soil inside of one microorganism or another). However, based on what I can find, it appears as if this nitrogen thievery is happening strictly around where the wood chip is in contact with the soil, which is not a large space, nor necessarily a growing space, if only in the pathways. Obviously, if the chips are tilled into the soil too soon, and especially if they are fine (like sawdust) where there is a lot more surface area from which nitrogen can be stolen, there may be some N-theft. But if the chips are on the surface, the danger is pretty low. Generally speaking, if the woodchips are in the pathways, nitrogen loss for plants in the bed should not be a great concern.
Can they import harmful chemicals?
If you don’t know the provenance of your wood chips this could actually be a potential problem. There was at least one interesting study done on a particular broad-leaf turf herbicide (used on golf courses but similar versions are used in hay fields as well). Trees in the area were dying from herbicide spray and they were subsequently chipped. Those chips were then used to mulch greenhouse tomatoes for the study. The tissues were later measured and that same herbicide was detected in the soil and plant tissue of the tomatoes. Again, for pathway use this may not be a concern, especially if not certified organic. However, if mulching beds at all, it may be prudent to either test your chips or get them from a trusted source (whomever that might be). I should say, though, this is concern with compost, too—some broadleaf herbicides are incredibly persistent and can survive the composting process. Yeah, it’s just annoying all around.
Will chips add allelopathic chemicals that will harm crops?
Trees are geniuses and have figured out how to generate their own herbicides to stifle competing seeds and seedlings. Although these “herbicides”, like juglone from walnut trees or thujone from cedars, can potentially harm plants, they are not likely to do so from the pathways so far as I can tell. They may inhibit some bacterial and microbial activity right below them from the leaching of these water soluble “chemicals” and tannins, but I can’t find anything to suggest it’s a big issue or that it would harm plants adjacent to—but not below—them. The weeds in the pathways, though—those may not fair as fair as well. Which is what we’re after, right?
Will they attract pests?
Wood chips can be, at least in theory, the ideal habitat for slugs (though bark can actually be an irritant!). So, if slugs are an issue in your area wood chips may not be your best choice... or maybe you’ll also want to get a load of ducks delivered with your chips. Other predators include frogs, snakes, birds, predatory snails (!!!), and even fireflies—so anything you can do to encourage those will discourage the bugs. Otherwise, wood chips create habitat for all sorts of micro and macro organisms—many of them good, some likely bad. As long as you have good bird habitat around, healthy soil, and plenty of flowers for beneficials, this shouldn’t be an issue. Every situation will be different, so I can’t say definitively and it may take a season or so to start seeing the many benefits of woodchips, such as increased organic matter and fungal activity and overall prettiness. Yeah. I said it. Wood chips are pretty.
In an ideal world, you would create all of your own wood chips, from specific trees, with specific purposes. They would all be ramial wood chips, made from the young growth of hardwoods, from your farm, and that would be that. All of that. Unfortunately, that’s unrealistic for most of us, but if wood-chipped pathways is of interest to you, I think importing them is a good and relatively safe option. I would not, however, put them on top of my beds unless I was absolutely certain of their provenance. Even then, I would probably hesitate. Of all the no-till methods, I am the most incredulous about using chips on top of beds as a mulch in a commercial setting, but please feel free to prove me wrong!
For more on where to find wood chips, watch this amazing video. Someone give this guy an Emmy (or the YouTube equivalent).
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