Using Pressure Treated Wood For Raised Gardens...
Is It Safe For Growing Food??

A few months back, Popular Mechanics Magazine ran an article about their partnership with Rebuilding Together Philadelphia.  In June of 2010, they worked with local volunteers (as part of a wider series of repair and rebuilding projects) to build a series of raised gardens so the local folk could grow some of their own produce (see photo below).

What struck me is the use of pressure treated wood for the gardens!  I have been pelted with questions for years concerning the safety of using pressure treated wood for vegetable gardens.  Though I had feelings on the subject (and have built dozens of them for friends and clients) it was only recently that I felt there was enough information to make a firm recommendation on this website.

Yes, the "new" pressure treated wood is safe for use for raised garden frames... with a few precautions!

Up until 2003, the most common preservative used for pressure treated wood was chromated copper arsenate (CCA), a compound using arsenic as its primary rot protectant.  Over years, the industry, in cooperation with government recommendations, phased out the use of CCA for all residential and most commercial wood pressure treatment.  Part of the reason was the fear that the arsenic would poison the soil and anyone who touched it.  Though actual cases of poisoning via pressure treated wood use by the public were hard to find, there was enough circumstantial evidence of soil contamination to warrant a change.

New preservatives with either copper or chromium as the primary preservative replaced CCA, and that changed the safety dynamic dramatically.  Unlike arsenic, which is well absorbed into and retained by the body (explaining its toxicity even in long-term, small exposures), these new products (though toxic in large amounts) are not absorbed efficiently by the body so the miniscule exposures from touching or working with these products are safe provided simple exposure precautions are taken, such as hand washing and collection of the sawdust. 

Do plants absorb the preservative in pressure treated wood raised garden frames?

Yes, plants can absorb these preservatives, but tests have shown that the amount of preservative leached from the newer PT wood products is so low that it is virtually undetectable.  According to my reading, the primary toxicity concern raised so far in the effect of the new preservatives on lower plant life such as algae, which wouldn't affect most homeowners unless you have a lake nearby.

Applying a sealer can protect against CCA exposure...

According to the EPA, studies show that the application of a penetrating oil finish can reduce or eliminate exposure to CCA in older decks and to the preservatives used in newer decks.  So it is recommended that all pressure-treated surfaces that have human contact be coated with an oil finish as needed.  It has been noted in some studies that paints and opaque exterior stains do not offer the protection of stains that are absorbed more deeply into the wood.  They should be recoated at least every few years or when water no longer beads on the wood surface.

Some manufacturers are adding a water repellent to the preservative, which would make the need for a sealer less important or unnecessary for garden frames and rough structures, though for decks subject to abrasion and sun exposure regular sealing is still a good idea to preserve the surface.

How applicable or even necessary this is for a raised garden frame I'll leave to your judgment.

Tips for working with pressure-treated wood...

• Predrill any nail or screw holes within an inch of the end of the board. This lessens the chance of splitting the board while fastening it. Even if it doesn't split when you initially fasten it down without predrilling, it will very likely do so later as the wood dries out.  BE SURE TO CLEAN UP ALL SAWDUST!
• Use the right fasteners. Use only screws or nails that are galvanized and designed for use with the newer copper-based pressure-treated wood.
  
• Be sure you are purchasing the correct grade of PT wood for your project. The newer PT wood products are more expensive so there are more grades available to keep the cost down.  You must use ground-contact grade for raised gardens if you want them to last the "test of time".

So here we are!!

Obviously, there will be more information on this topic and I will keep on top of any changes or new studies that might help you make the best decision.  Best of luck with your project!!

Last edit October 2010