Is colored mulch safe for plants? There is certainly a lot of controversy surrounding these colored mulches. We may be asking the wrong question when we put the emphasis on the colorant. The problem with colored mulches is that often they are made of scrap wood. This is actually the most dangerous aspect when dealing with them. While recycling wood from old pallets, fence posts and demolition and construction products might seem like a sound ecological principle, much of it is treated or has become contaminated with chemicals. Pallets especially are subject to all sorts of exposure from the materials stacked on them or from the environment in which they are stored. Pallets can be exposed to everything from gasoline to a myriad of unknown chemicals. Treated lumber can contain creosote and pressure treated lumber is preserved with chromated copper arsenate.
While arsenic compounds have been banned, there is still a lot of wood out there that has been treated with it. Wood reclaimed from demolition projects can contain lead based paints. These are not things that you want leaching into the soil and indeed they can be harmful to plants as well as the gardener.
This type of recycled wood is used for coloring because, beside the fact that it is cheap and easily obtainable, it is dry and will absorb the dyes. Wood chips and bark are more difficult to color. These more natural wood products are more attractive than recycled wood so they are more likely to be left in their natural state. The recycled wood chips are dyed to make them not only more attractive but more marketable
Not all dyed mulches are from recycled wood and, if possible, only colored mulch certified by the MSC (Mulch and Soil Council) should be purchased. This council was formed to help consumers and retailers identify exactly what is in the products that they are using. All certified products must: conform to proper labeling requirements, pass laboratory testing, pass greenhouse growth testing (soils) and chemical testing for CCA-treated wood contaminants (mulches) and pass random audit testing in retail markets. Since adherence to standards is voluntary, this certification is the only guarantee the mulch does not contain chemicals that are harmful to plants, to the environment and to you.
The dyes themselves are not considered toxic to plants, animals or people. Red dyed wood is colored with iron oxide. This is the chemical composition of rust. Iron oxide has been used for centuries in cosmetics, paints and other industrial products. It is also used in the horticultural industry to dye flowers. Iron in the soil has no detrimental effect on plants nor is it harmful to the environment. Black mulch is colored with carbon black. Carbon black is the result of incomplete combustion. Again, it is pigment that has been used extensively in inks and paints and is deemed safe for products that come in contact with food. It is essentially the equivalent of putting charcoal in the soil. Basically, studies have shown that the dyes have no detrimental effect on plants or soil.
While the large producers of colored mulch generally use organic and non-toxic dyes there is no guarantee that all colored mulches are dyed in this way. Smaller companies may be using other things to dye the mulches and your only recourse for a safe product is to buy certified mulch or depend on the retailer to provide a safe product. Colored mulches should be used with caution and due diligence.
All other arguments against using colored mulches are also true of any wood or bark mulch. Some research will come up with sources that insist that these mulches are harmful to plants because they leach nitrogen from the soil. This is true of any wood or bark mulch to a certain extent. Bacteria need a source of nitrogen to decompose wood. Approximately 1-2 % nitrogen is required for the bacterial decomposition of wood to take place. Since wood or bark mulch only contains a fraction of a percent of nitrogen, the nitrogen involved in this process is indeed taken from the soil. According to the University Of Idaho College Of Agriculture this only becomes a problem when the wood products are actually incorporated into the soil. The superficial contact with the soil when mulch is used to suppress weed rarely causes a problem. Also, the surface mulch dries and doesn’t provide and environment conducive to bacterial growth. Actually, since colored mulches deteriorate more slowly than any other kind of wood mulch they remove nitrogen at a slower rate. Curiously, hardwoods deteriorate at a faster rate than softwood. In any case the addition of a high nitrogen fertilizer can be added should a problem seem to develop.
The second complaint that is associated with wood mulches is that they lower the pH of the soil. Wood mulches are often acidic with a pH ranging from 3.0-7.0 (with 7.0 being neutral) depending on the species of wood involved. Unless a plant is extremely sensitive to pH wood mulch isn’t going to cause a problem. Plants such as blueberries, conifers and rhododendrons generally benefit from the lowering of the pH because they grow best in an acidic soil. A sprinkling of lime will alleviate any problems that occur but again, according to the Idaho College of Agriculture, there isn’t likely to be a sufficient lowering of pH to be detrimental to ordinary plants. .
There are two situations where it is best to avoid using wood mulches whether dyed or otherwise. Fermentation can occur in wood mulches. This can happen in unturned mulch piles or in the bags of mulch that are purchased at the store. These mulches have a strong odor of vinegar and the pH can be as low as 2.7. It is best to confine these mulches to acid loving plants or toss them. Occasionally, you will open a bag of mulch to find a white fungus growth. This is artillery fungus, (Sphaerobolus). It is a wood rotting fungus and it will also attack wood siding and other untreated wood. This is not something that you want to introduce into your yard. Especially it shouldn’t be used near or against wood siding.
There have been few studies as to whether natural products in wood mulches are toxic to plants. It is well known that black walnuts produce juglonss, a substance toxic to plants, but there isn’t much chance that black walnut is going to turn up in mulches. Other natural occurring substances in wood mulch that might be harmful to plants are monoterpenes, tannins, resins and turpentines. Monoterpenes are a kind of essential oil found in plants and there is some suggestion that their presence in concentration can inhibit seed germination. Mulch that has been composted or well watered is less likely to contain these substances.
Bottom line in the colored mulch controversy is that the dyes are probably harmless. The source of the wood is more important. If the mulch is actually bark or wood chips there is less likelihood that there are toxic substances in the mulch than if it is made of recycled wood. As with anything you use, read the bag and buy from reputable dealers.