Question: There has been a lot of controversy about the use of colored wood chip mulches in landscapes. What is the problem with these mulches?
Answer: 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 absorb coloring agents. With their high moisture content, fresh wood chips do not easily absorb or absorb 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 complelling reason for coloring the recycled-wood mulch.
Neverthless, 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 the recent ban on arsenic-based wood preservatives, 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.
("Questions From You" as published in Hort News, UMASS Extension, by Ron Kujawski and Sonia Schloemann. Vol 18, Number 5)