What’s Really in that Bag of Garden Mulch?

I was in a pinch. I needed half a dozen bags of pine bark mulch to protect the roots of some of my shrubs and a weeping cherry tree before freezing fall weather kicked in. I have no truck to make a big haul and didn’t need enough to warrant a delivery. The big box store is just a short drive, so I decided to go there and see what I could find. But there was no mulch of the pine bark variety which I prefer.

There was, however, brown rubber mulch, black rubber mulch, red rubber mulch. Nope. Rubber mulch inhibits water movement between the soil and plants, defeating the purpose of mulch. It’s also been known to leach heavy metals into the soil, so… no.

Don’t fall for “shredded” mulch.

There was also “shredded” brown mulch, black mulch, and a slightly deeper brown mulch (it was a major brand which you’ll find in every big box store). The front of the bag read, “made from forest products.” Sounds like trees, right? Some would have stopped reading there and bought the mulch, content in believing that they were buying a wood mulch made of pine, hemlock, cedar, or a similar tree species. But not me. I’m suspicious.

I flipped the bag over to read the ingredients: “..this mulch is made from pallets and construction refuse which has been dyed.” Nice.

So basically, this “forest products mulch” was made of old industrial pallets which carried God knows what, and construction debris including moldy old timber, flooring, handrails, stair cases, subfloors, pressure-treated wood, and anything that at some point was made from a tree. As long as it bears even a passing resemblance to what was once wood, providers of garden mulch they can call it “forest products”. And all of that debris was nicely shredded and dyed to look like pine or cedar mulch. Hell no. Off I went across town to the indie garden center to get the real thing.

A review by the University of Washington found that wood mulch made from construction debris can sometimes contain heavy metals and chemical residues:

As with composts, woody mulch quality is influenced by the source of materials. Mulches created from branches and tree trimmings often contain a diversity of leaves, wood, and bark, which contributes to a highly functional mulch. In contrast, woody mulch made from wood recovered from construction and demolition debris can contain pressure-treated lumber. In one Florida study, 18 of 22 samples collected from debris processing facilities contained arsenic (from chromated copper arsenate-treated wood) at concentrations greater than the state’s allowable levels. Similarly, mill wastes that contained formaldehyde and other wood processing residues reduced survival of tree seedlings when used as a mulch.

Linda Chalker-Scott, Washington State University, Puyallup Research and Extension Center (download study here)

It’s unfortunate that buying garden mulch practically requires a warning sticker on the bag and a robust suspicion of spurious marketing claims. But the health of your garden is at stake. I’ve bought that inexpensive construction debris mulch in the past when I didn’t know better. In it I found a rusty nail and wood that had clearly come from a pallet (I found a small piece with that very identifiable blue dye on it and nail holes). After 2 years the mulch still hadn’t broken down in my garden, which was of major concern to me. A layer of living mulch (that is, mulch from recently felled trees or yard waste) should break down within a year with the help of biological activity from the soil and a consistently wet or damp bottom layer.

Your garden should not be an alternative to a landfill. If the bag (or when buying bulk, the nursery) specifies the source and type of mulch, it’s legit (like that pictured above). Pass on the construction debris mulch and rubber mulch and opt for living mulches.