While we know that arsenic can hurt humans, you may well wonder what effect a soil containing a greater than normal amount of arsenic (via colored mulch derived in part from CCA-treated wood) might have on plant health.
I am not a scientist, so I will merely mention here that in my reading on the subject, I did not encounter much that would indicate that an "uncertified" colored mulch would be harmful to surrounding plants. Speir et al. (1992) conducted two related studies on the "effects on soil biochemical and biological properties of a soil amended with Cu-, Cr- and As-(CCA)-treated, or boric-treated, sawdust":
- "soil samples were analysed before and after a pot trial conducted to assess the effects of these amendments on plant growth and element uptake"
- "an incubation study conducted without plants and with soil samples taken for analysis"
The results, according to the abstract, "warrant a cautious acceptability of CCA-treated sawdust as a mulch or garden amendment."
Consult my separate article on CCA-Treated Wood to learn more about it.