Using Pressure Treated Wood For Raised
Is It Safe For Growing Food??
A few months back, Popular Mechanics Magazine ran an article about their partnership with
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
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
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
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
- 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!!