Answer: There is conflicting evidence about whether pressure-treated lumber is acceptable for use around food crops. Some research results say its fine, that it doesn't leach toxic chemicals; other results say that it does and should not be used around food crops. As far as sealing your existing wood, I've heard of several different methods. I don't know if anyone has studied the effectiveness of them, however. Perhaps a combination would be most effective.
You can paint the wood with several coats of an appropriate sealer.
You can line the structure with heavy plastic sheeting.
You can construct a "foundation" using masonry, that would lift the cold frame so it isn't in contact with the soil.
You can cover horizontal surfaces to eliminate water absorption.
Here are a few web sites with information about pressure treated wood.
The following is a message board about using pressure treated wood in gardens. I cannot vouch for the accuracy of any of the statements, but it does provide some interesting options and opinions:
Organic Gardening magazine has been involved in the discussion of pressure-treated wood:
"How can I preserve lumber without toxic chemicals?
I do not know if this offer still stands, but Organic Gardening magazine used to offer copies of an article they published in 4/97 on the subject of pressure-treated wood, including their updated bibliography of scientific studies. Their address is: Organic Gardening, 33 E. Minor St., Emmaus, PA 18098. I believe they requested $2.00 for postage.
The American Wood Preservers Institute
The U.S. Environmental Protection Agency
Hope this helps.
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