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A member of my family recently replaced some old pressure treated lumber for a retaining wall in their backyard. They used this darker colored pressure treated lumber identified by it's telltale pierced pattern used for the chemicals to embed and permeate the wood during its treatment. What kind of lumber is this exactly? I show a picture below of the old green style one and the darker one on top.

I know pressure treated is discouraged for gardening purposes but I'm wondering if an eco friendly product has supplanted the old style one. I'm also thinking that may not be the case and we're really just seeing dye on traditional pressure treated wood much like dyed mulch.

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Almost any species of wood can be pressure-treated. I don't know what species this post is, though the softer and most abundant pines tend to be used. The little pierce marks you rightly label as telltale indicate that it has been pressure-treated, whether or not it also has been dyed.

The liquid that used to be used contained copper, and gave pressure-treated wood that green copper color. Since the oxidized copper can wear off the surface, color isn't definitive identification. The liquid also contained chromium and arsenic, and thus was called CCA. In the U.S. it's now illegal to use wood treated with CCA in residences. Arsenic and chromium are heavy metals and are toxic or poisonous at low concentrations. I don't know about the other two but arsenic can migrate to soil and from there into plants. That is, it poisons the food you grow. Don't let your vegetable garden even look at that kind of wood!

When it comes to pressure-treated, eco-friendly is a relative concept. The point of pressure treating is to keep out bacteria, fungus, insects, and anything else that might like to eat the wood. So the liquid injected isn't going to be a substance that's friendly to living things. Here's a nice description of some of the alternatives and their relative merits: https://fortress.wa.gov/ecy/publications/publications/0604023.pdf

Short answer: Vegetable gardens and pressure-treated wood are a bad combination. Don't do it.

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  • Interesting background history; so the wood is without question pressure treated, and I wasn't so much looking for the species of wood per se but what variation of pressure treated wood this lumber is known as and hoping that the details on the treating liquid were something new. The retaining wall they built has some small fruit trees and grape vines on the upper mesa surface of the wall. I wonder if those plants should raise identical concerns for them as a vegetable garden would.
    – jxramos
    Commented Sep 13, 2017 at 20:39
  • Thanks so much for the link, I zoomed into my image on the barcode and barely read "Copper Azole" on the bottom right. It shows up in your pdf link described as: "C-A treated wood can be used for most applications including decks, walkways, picnic tables and play structures. It can be applied in fresh-waterimmersion applications. C-A is not approved for saltwater use or structural utility poles in high-decay regions.". The picnic tables is curious to me since that would be near a food source and present an opportunity for cross contamination and what not.
    – jxramos
    Commented Sep 13, 2017 at 20:43

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