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Gardening Answers Knowledgebase

Plant Answer Line Question

Keywords: soil contamination, Wood preservatives, Pesticides and the environment

We are thinking about putting in a retaining wall and a fence on our property, which is near a lake. Should we avoid using pressure treated wood? If so, what are some alternatives?


There are many reasons not to use treated wood for your fences and/or retaining walls. The chemicals most used to preserve wood---creosote (on railroad ties, among other things) and penta---were banned by the EPA in 1986 for indoor use and for many outdoor uses. The chemical used to pre-treat wood (CCA, a mixture of copper, chromium, and arsenic called chromated copper arsenate) has been shown to leach into the soil and to transfer to human skin through contact.

There are safe paints and preservatives for coating wood; there are safe types of pre-treated wood; some people use stone, cement blocks, or other materials instead of wood.

Below is lots of info about treated wood and alternatives.

Start with the page on the EPA site, which is full of information on treated wood. It includes a section on alternatives and some questions and answers about studies.

If you find this too technical, try the next two links below.
The Natural Handyman website has good information.

Toxic-Free Future (formerly known as Washington Toxics Coalition) has a fact sheet about safe and unsafe paints and wood preservatives. Lots of background information on the toxicity of treated wood is included as well.

Date 2017-08-04
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Plant Answer Line Question

Keywords: soil contamination, Soil testing, Plant-soil relationships, Wood preservatives

This is about composting from raised beds constructed of railroad ties. I went to a workshop on growing edible plants, and was informed that one cannot eat anything grown in a railroad tie bed because of arsenic and other nasties, and if one has such beds, they should ONLY BE USED for ornamentals.

I try to compost everything in my garden, so I need to know if it is safe to use compost made from plants growing in railroad beds on the beds where I am growing edibles. If it is not safe, would time, weather, or decomposition EVER make it safe? I'm willing from now on to put all the soil-contaminated clippings in the city yard waste bin that goes to Cedar Grove, but I'd rather be able to make use of them in my own garden.


Your question about the safety of compost made from plant matter grown in a railroad-tie bed is complex. Railroad ties are treated with wood preservative that contains arsenic. Arsenic never goes away entirely, but the amount may be at lower levels than Washington State's law on clean-up, based on parts-per-million. I would definitely recommend a soil test. Here is information from the Northwest Coalition for Alternatives to Pesticides, which has a page on chromated copper arsenate.
The Center for Disease Control has published a Public Health Statement on arsenic, excerpted below:
"About 90% of all arsenic produced is used as a preservative for wood to make it resistant to rotting and decay. The preservative is copper chromated arsenate (CCA) and the treated wood is referred to as 'pressure-treated.'
Arsenic cannot be destroyed in the environment. It can only change its form, or become attached to or separated from particles. It may change its form by reacting with oxygen or other molecules present in air, water, or soil, or by the action of bacteria that live in soil or sediment. Arsenic released from power plants and other combustion processes is usually attached to very small particles. Arsenic contained in wind-borne soil is generally found in larger particles. These particles settle to the ground or are washed out of the air by rain. Arsenic that is attached to very small particles may stay in the air for many days and travel long distances. Many common arsenic compounds can dissolve in water. Thus, arsenic can get into lakes, rivers, or underground water by dissolving in rain or snow or through the discharge of industrial wastes. Some of the arsenic will stick to particles in the water or sediment on the bottom of lakes or rivers, and some will be carried along by the water. Ultimately, most arsenic ends up in the soil or sediment. Although some fish and shellfish take in arsenic, which may build up in tissues, most of this arsenic is in an organic form called arsenobetaine (commonly called 'fish arsenic') that is much less harmful."

Washington State University has information on gardening on arsenic- or lead-affected soil which may be of interest to you.

To be cautious, you should keep the compost from these beds separate from your other compost, and only use it on your ornamental plants already being grown in those beds. I don't recommend putting even slightly arsenic-contaminated yard waste into the city compost, since that means the problem is being spread farther afield. It would be worthwhile testing every so often for soil contaminants. Here is King County Public Health's guide on arsenic, and testing information.

Date 2017-05-24
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Plant Answer Line Question

Keywords: Food crops, Wood preservatives

My son wants to use wooden pallets for a vegetable garden. Is the wood in these pallets safe for contact with food crops?


Pallets (especially those used in international shipping) are very likely to be treated, since wood packaging must now comply with the International Plant Protection Convention's International Standards for Phytosanitary Measures Guidelines for Regulating Wood Packaging Material in International Trade (ISPM 15). Here is more information from an article by Wendy Priesnitz, editor of Natural Life Magazine:

"Pallets made of raw, untreated wood are not compliant with ISPM 15. To be compliant, they must be debarked and either heat-treated to certain specifications or fumigated with methyl bromide, which affects the central nervous and respiratory systems. Heat-treated pallets bear the initials HT (or sometimes KD for kiln dried) near the IPPC logo. Pallets treated with methyl bromide bear the initials MB. In 2010, a phase-out of the use of methyl bromide began because it is an ozone depleting substance under the Montreal Protocol. However, many pre-2010 pallets are still around and, in fact, are likely to be the ones nearing the end of their useful lives as pallets.

"Older pallets could also have been pressure-treated with chromated copper arsenate (CCA), which has been phased out for many residential uses. The arsenic in CCA-treated wood can be dislodged so that direct contact with wood can lead to exposure, thought to be a problem especially for children, and it can leach into ground water. A 2008 Australian study found that one percent of pallets tested contained CCA. Copper-treated wood varies in color from a very light green to an intense green color, depending upon the amount of chemical impregnated into the wood. However, it ages to a silver color, as does untreated wood, so color is not a reliable indicator, especially with older wood."

A related question about reusing wooden pallets appeared in the online journal Grist, and their columnist raised the issue of chemical treatment as well as the possibility of the wood having absorbed dangerous bacteria, so I think it's wise not to use wooden pallets for growing food.

If you want to recycle the pallets, there are places in King County which will accept wooden pallets for reuse, listed on the King County Solid Waste website What Do I Do With.... ?. If you know where the pallets came from, you might ask that business if they will let you return them.

Date 2018-03-01
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May 31 2018 13:14:08