Q. There has been a lot of controversy about the use of colored wood chip mulches in landscapes.
1. What is the problem with these mulches?
2. Are the dyes used in coloring the wood chips safe?
A. The primary concern with colored landscape mulches is not the dyes used for coloring. Rather, it is about the sources of wood chips and the possibility of contamination with toxic substances.
Most of the wood used for making colored mulch comes from recycled wood, i.e., wood scraps, wood pallets, and wood reclaimed from construction and demolition (C&D) waste. Besides the benefits of recycling waste wood materials, the reason why these wood materials are used for colored mulches is that they are very dry and readily absorb or adsorb coloring agents. With their high moisture content, fresh wood chips do not easily absorb or adsorb the dyes commonly used for coloring. Also, mulch produced from recycled wood is not viewed as attractive as other mulches such as pine bark. This is the most compelling reason for coloring the recycled-wood mulch.
Nevertheless, it has been found that some of the recycled waste wood used for making landscape mulch products is contaminated with various chemicals, such as creosote and CCA (chromated copper arsenate). CCA, of course, is the chemical that was used in the manufacture of pressure-treated wood. With bans on arsenic-based wood preservatives in recent years, this may become less of a problem, though there is still plenty of CCA preserved wood to be found in older decks and fences. Sometimes wood pallets that have been used in the transport of chemical agents can become contaminated by spills of these chemicals. The bottom line is that CCA and other toxic chemicals have been found to be contaminating soil where colored wood chip mulch has been applied. The most egregious source of the contamination appears to CCA treated wood recycled from C&D waste.
The dyes used in coloring wood mulch are primarily of two types: carbon-based dyes and iron oxide-based dyes. Iron oxide, the most commonly used dye, is simply a compound of iron and oxygen. As the compound oxidizes, iron is released to the soil but is not considered to be toxic. Iron oxide dyes are often used in the floriculture industry to dye flowers. Dyes that are not absorbed by or adsorbed to the wood would come off with contact, especially if the mulch is wet. There are some carbon-based dyes used on mulch. These carbon-based colorants are similar to those used in ink and cosmetics. At this time, there is no evidence that the dyes used to color wood chip mulch are toxic.
It should not be assumed that all colored mulches are contaminated. However, anyone planning to use colored mulch should become familiar with the supplier and the source of the wood used in making it. If C&D waste wood is used, it should be a red flag that there is a possibility of CCA contaminated mulch.
UMass does not have a lab that tests for possible contaminants in bark mulch. If you still have concerns after contacting the supplier about the source of the wood used, contact a private environmental testing lab in your area. These can be found online or in the Yellow Pages under Laboratories.
Written by: Ron Kujawski
UMass Extension Landscape, Nursery and Urban Forestry Program