By Jeff Rugg- Q: Our flowerbeds need mulch. We have seen mulch that is colored red or black. The black mulch looks nice, but we are concerned that the coloring may not be safe for our dogs, and for any vegetables we plant.
A: The dyes used to color wood mulch are primarily carbon-based and iron oxide-based. Iron oxide (rust) is the most commonly used dye. It is simply a compound of iron and oxygen. The rust decomposes, and iron is released into the soil, but it is not considered to be toxic.
The carbon-based dyes used to color mulch 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.
The concern some people have with colored landscape mulch is the source of the wood chips and the possibility of contamination with toxic substances. Most of the wood used for making colored mulch comes from recycled wood. Wooden pallets, and wood reclaimed from construction and demolition waste, are ground up in huge chippers to make the mulch.
The wood is not very attractive unless it is dyed. This keeps it out of landfills. The wood is also very dry, and it absorbs the dyes better than wood from freshly cut down trees.
The wood from pallets may be contaminated by chemicals spilled onto them. The wood from a demolition project may be contaminated with wood preservatives, especially if it is old decking or fencing. Both of these potential contaminates are known, and efforts are made to prevent them from being used in colored mulch.
It is unlikely that the colored mulch would be harmful to people or pets that occasionally walk in it. Vegetables growing in it have the potential to be contaminated, but it is very small.
Q: I was told that before I spread new mulch onto my flowerbeds, I need to add lime to the bark mulch pile that was delivered to my driveway. I have never heard of this, and the person telling me this didn’t know why it should be done. What do you think?
A: If freshly ground-up organic matter (fresh bark mulch) sits in a very big pile for too long, the decay organisms in the pile will consume all the oxygen. Other decay organisms that can grow without much oxygen will thrive. The fermentation and heating up of the organic matter will release a variety of organic acids. These acids can lower the pH of the mulch pile drastically. The mulch will smell fermented, and spreading this acidic mulch on flowerbeds can harm plants. Agricultural lime will help neutralize the pH of the mulch.
The only time lime needs to be applied to a mulch pile is if it has not been turned over on a regular basis and anaerobic conditions have existed in the pile. This used to happen when mulch piles were piled very high, but now they are usually turned over frequently and not piled too high.