Pressure treated wood – proximity to raised veggie bed

(Question)

I have a 20 year old pressure treated wood fence that I know was treated with the now banned process that allowed arsenic to leach into the soil. I want to create a raised veggie garden against the fence. Should I expect that arsenic and other dangerous contaminants remain in the wood after 20 years? If so, how do I prevent contamination of a raised garden – how high / deep would I need to create the raised bed and should I include a non treated wood or other type of bottom for the raised bed?

 

(Answer)

The treated wood can leach toxic chemicals for many years, so it would be wise to assume that the wood remains toxic. That said, there are a number of strategies you can use to minimize exposure of your raised veggie garden to the treated wood, as noted below. There is no recommended height/depth of the raised bed – the main issue is to ensure that the treated wood does not come into contact with the bed.   I’ve included links to earlier posts to our website, which discuss materials to use for the bed, as well.

Health Canada’s Staying Safe around Treated Wood  notes that chemicals leach “slowly” from the wood, at a quantity and rate that depends on several factors, including age, type of wood used, rain, rain acidity and the soil that contacts the wood. Splinters and wood dust can also release chemicals.   As well, the treated wood should not be used as edging for a veggie garden.

Consider isolating the treated fence from the vegetable garden by lining the raised garden bed with heavy plastic. This would prevent any leaching of toxic chemicals into the soil of the bed. See Fine Gardening’s Are pressure treated woods safe in garden beds?  You could also apply a sealant to the CCA-treated wood, to stop the leaching.

PennState Extension’s Garden Use of Treated Lumber  provides a good overview of what was in the lumber, along with toxicity to plants and humans.  The chemicals you are most concerned about are inorganic chemicals like chromated copper arsenate (CCA) [the most common], ammoniacal copper arsenate (ACA), and acid copper chromate (ACC), which were among the chemicals used to treat the wood to protect it against deterioration and rot. It is important to know that low concentrations of arsenic, copper and chromium are found naturally in water, soil, plants and even in our bodies. Humans can tolerate some copper and chromium, while arsenic is more likely to be toxic on a  a chronic, long-term basis.

The amounts of these metals that leach into the soils from treated wood is generally believed not to be significant, to the extent that it would affect human health. Copper and chromium are usually bound to soil particles so don’t migrate much through soil, while arsenic is more mobile — although this would vary according to soil characteristics (pH, amount of organic material incorporated, etc).   As well, the plants variety may be important – plants differ in their ability to tolerate these chemicals, and in the rate at which they take up these chemicals. However, as you have raised concerns about this issue, we want you to happily harvest your veggies, confident in the assumption that they are not laden with toxins!

The PennState article also includes additional recommendations on how to minimize risks of treated wood, including:

  • Use wood that naturally resists decay, e.g., certain cedars, or even plastics
  • Do not permit CCA-impregnated wood scraps or sawdust to fall on the beds
  • Keep soil pH around neutral range to decrease solubility of copper and chromium
  • Keep a lot of organic matter in the soil – this binds arsenic, copper and chromium, so they are unavailable to plants
  • Plant veggies at least 30 cm (12 inches) from the treated wood – the highest concentration of toxic metals will be in the soil right next to the wood
  • Clean/wash veggies grown close to treated wood thoroughly
  • Root crops (e.g., carrots, radishes, turnips) grown close to the wood should be peeled before eating.

Here are links to a few posts on the Ask a Master Gardener website, which respond to your question about the type of wood to use:

Of note, organic preservatives like creosote, pentachlorophenol and coal tars, have been used as preservatives for outdoor lumber. Don’t use wood that has been treated with these agents, which can be irritating and toxic!

Finally, you should not need a “bottom” for the raised bed — unless you want to use cardboard or some other weed barrier, or netting to discourage burrowing critters.  Mixing the soil that you will be adding to the bed with the existing soil under it should help improve the rooting of your veggies.  The arsenic from the treated wood should not reach “under” the bed.  As indicated above, the soil next to the treated lumber is at most risk.