A few months back, Popular Mechanics Magazine ran an article about their partnership withRebuilding TogetherPhiladelphia. In June of 2010, they worked with local volunteers (aspart of a wider series of repair and rebuilding projects) to build a series ofraised gardens so the local folk could grow some of their own produce (see photobelow).
What struck me is the use of pressure treated wood for the gardens! Ihave been pelted with questions for years concerning the safety of usingpressure treated wood for vegetable gardens. Though I had feelings on thesubject (and have built dozens of them for friends and clients) it was onlyrecently that I felt there was enough information to make a firm recommendationon this website.
Yes, the "new" pressure treated wood is safe for use forraised garden frames... with a few precautions!
Up until 2003, the most common preservative used for pressure treated woodwas chromated copper arsenate (CCA), a compound using arsenicas its primary rot protectant. Over years, the industry, in cooperationwith government recommendations, phased out the use of CCA for all residentialand most commercial wood pressure treatment. Part of the reason was thefear that the arsenic would poison the soil and anyone who touched it.Though actual cases of poisoning via pressuretreated wood use by the public were hard to find, there wasenough circ*mstantial evidence of soil contamination to warrant a change.
New preservatives with either copper or chromiumas the primary preservative replaced CCA, and that changed the safety dynamicdramatically. Unlike arsenic, which is well absorbed into and retained bythe body (explaining its toxicity even in long-term, small exposures), these newproducts (though toxic in large amounts) are not absorbed efficiently by thebody so the miniscule exposures from touching or working with these products aresafe provided simple exposure precautions are taken, such as handwashing 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 theamount of preservative leached from the newer PT wood products is so low that itis virtually undetectable. According to my reading, the primary toxicityconcern raised so far in the effect of the new preservatives on lower plant lifesuch as algae, which wouldn't affect most homeowners unless you have a lakenearby.
Applying a sealer can protect against CCA exposure...
According to the EPA, studies show that the application of apenetrating oil finish can reduce or eliminate exposure to CCA in older decksand to the preservatives used in newer decks. So it is recommended that allpressure-treated surfaces that have human contact be coated with an oil finishas needed. It has been noted in some studies that paints and opaqueexterior stains do not offer the protection of stains that are absorbed moredeeply into the wood. They should be recoated at least every few years orwhen water no longer beads on the wood surface.
Some manufacturers are adding a water repellent to the preservative, whichwould make the need for a sealer less important or unnecessary for garden framesand rough structures, though for decks subject to abrasion and sun exposureregular sealing is still a good idea to preserve the surface.
How applicable or even necessary this is for a raised garden frame I'll leaveto 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 gradesavailable to keep the cost down. You must use ground-contact grade forraised 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 ontop of any changes or new studies that might help you make the best decision.Best of luck with your project!!
