The benefits of mulch
There are a ton of wonderful benefits that mulch provides for a landscape such as water retention, improved water infiltration, temperature moderation, compaction reduction, and erosion control. Two other key benefits, ones that are often hard to see unless you are digging for them, are the food source and aeration that it provides to the soil environment. Mulch alone can actually bring a mostly dead soil back to life. The one caveat though, is that it needs to be a mulch that actually decomposes into the soil.
Mulch in action, a real application
I have an upward slope at the back of my property that is about 12 feet in depth and it runs the entire length of my backyard. The previous owners had a yard maintenance crew that walked along the hill and blew around dyed mulch, leaves, and topsoil every week (not much help there, but that’s another story). I was left with scattered patches of faded mulch, bald clay spots and hybrid-t roses that weren’t doing well. Before planting my design, I pulled the roses, and removed all of the existing mulch. There was no topsoil beneath the mulch; there was no topsoil anywhere. I was working with heavily compacted, anaerobic clay loaded with large rocks. I used a pick-axe to make 25 holes for shrubs and trees; I never saw a single earthworm, grub, or viable insect. Organic and natural gardeners know this is not a good sign. After planting, I topped the soil with 5″ of wood chips from a tree service and let things go. I lost a few plants (not surprising) and I learned a lot along the way about how to make a graded residential lot suitable for growing natives (keep an eye out for an upcoming post about Mediterranean plants on standard residential sites). But what was most surprising to me was what happened to the soil just one year later. When I was digging around the following season to check on my plants, I dug under the mulch and found all kinds of life, earthworms, beetles, and lots of little crawly things. Underneath the mulch it was cool and moist where it touched the existing soil and it was beginning to decompose, feeding the soil below. A new topsoil was being created and it smelled healthy and earthy. Roots and rhizomes from my plants were occupying part of the I/O layer (the inorganic / organic layer of soil, where soil meets organic matter) and worms and beetles were burrowing between the two layers creating much needed air spaces. There was life in a once lifeless environment.
Toxicity and the issues with colored mulches
There are a couple other potential issues with these kinds of mulches. Sometimes they are made from chipped pallets or recycled wood from construction waste. Some of these woods could have been treated with various chemicals like chromated copper arsenate (CCA). In addition the dyes and chemicals that are used to color the wood should be checked as well. I know some of the black chips are treated with carbon black (which is basically charcoal) and the reds sometimes are made from iron oxide (rust). As far as safety goes, these are fine to add to the soil but the iron dioxide may stain concrete. If you are going to use these mulches I’d definitely check the source and verify that they are free from chemicals that will adversely affect your soil. Here’s a quick and informative read on the topic from the University of Massachusetts about the potential toxicity in these products: https://ag.umass.edu/fact-sheets/colored-bark-mulch-0.
The differences in mulches
What can we take away from this? Wood chips decompose and provide soil and plants with many healthy benefits that dyed chips and inorganic mulches do not. Think of natural mulch as a very slow acting compost. Rubber mulch and the dyed wood chip mulch that’s usually seen in black, red, and brown colors doesn’t decompose (at least not in our lifetime). These products are not natural and lack the most important benefit that mulch has to offer: a long term food source for your plants and soil. These aren’t my opinions, they are observable facts. My opinions on dyed mulches are also not very favorable…
If you don’t like the rustic look of natural wood chips…
I understand the attraction to dyed mulch, my husband likes it because it has a uniform color and provides a good contrast to plants…. But to me it looks much worse whenever anything foreign (leaves, twigs, flower petals, weeds) drop on or grow through it. It also fades out in about 2 years and looses its initial appeal. If you want to have a more uniform look in your planting beds, I highly suggest buying natural bark mulches, or screening your own tree chips (if you don’t mind the extra work). You can always put down an initial layer of tree chips first (ask the tree company for chips from healthy trees, a good company will know when it’s pulling and chipping trees that are diseased and will not want to pass that on to you, if you are only doing a small area then I’d look at getting bags of a partially decomposed forest bark, redwood soil conditioner, leaves from your yard or leaves mixed with grass clippings — something that will decompose in a year or two, but not as fast as compost so you are feeding the soil at different intervals ) Then top with a 3″ layer of fir or redwood bark. Bark has a lot of natural resins and waxes that prohibit rapid decomposition so it will last longer than natural wood chips and keep its color for several years, but it will eventually feed the soil faster than the dyed mulches. With this approach you will get looks and function.