Extension News

Wood in the Garden

Note to media editors: Garden Column for the week starting June 10


By Paul Wray
Extension Forester
Iowa State University

Gardening in raised beds and other wood structures can be just the answer for would-be gardeners who would love to grow their own vegetables and flowers, but lack the space or physical ability for a traditional garden. However, recent changes in wood preservation treatments have left many gardeners wondering about the safety of treated lumber.

Many gardeners have made use of treated lumber in their raised beds, decks, fences, benches, gazebos and other landscape structures. Up until about 2003, many of us used a then commonly available treated lumber product called chromated copper arsenate (CCA). But in 2002, the Environmental Protection Agency (EPA) announced that CCA treated lumber would be phased out for use in consumer/residential products by 2004 due to concerns regarding the safety of arsenic. Since then, several more environmentally friendly alternatives to CCA treatments have become available, and CCA lumber is no longer available for the consumer.

Alkaline copper quaternary (ACQ) is a water-based fungicide/insecticide combination that is higher in copper than CCA but is free of arsenic. This type of treated lumber can be used for above-ground structures, as well as those underwater. ACQ treated wood can be painted or stained and is now the most common type of treated lumber.  ACQ treated lumber is more corrosive than the CCA product; as a result, fasteners must be either stainless steel or hot dipped galvanized. 

Another treatment that may be available is sodium borate (sold as Advanced Guard or DuraBora).  This product is not as good as ACQ and the lumber should not come in contact with moisture or be used for decks or home construction.

When selecting treated wood, look for the “preservative quality label or mark.”  It will have the AWPA (American Wood Preserve’s Association) standard used for treatment, the type and amount of preservative retention in pounds per cubic foot (pcf).  For above ground outside use the pcf should be 0.25 and for ground contact it should be 0.40.

The alternative to treated lumber in outdoor projects is to use naturally durable wood such as western red cedar, redwood, or bald cypress.  Only the heartwood of these species is durable.  Properly treated wood will outlast these naturally durable species.

When using treated wood remember the following precautions:
*Do not use where food, animal food, or drinking water may be in contact with the wood.
*Do not burn treated wood in open fires, stoves or fireplaces; dispose of wood through ordinary trash disposal outlets.
*Avoid inhalation of sawdust; always wear a dust mask when working with treated wood.
*Always wear gloves when working with wood.

In Iowa, we have a few species that show some decay resistance, but they are not as durable as western red cedar, redwood or bald cypress.  Some Iowa species with moderate durability are:  Osage orange, black locust, black walnut, and white oaks.  Products using non-decay resistant wood will last 4-7 years in Iowa.  Using treated lumber will extend the life time of the project.

The right wood for the right project is essential for long-lasting gardens and landscapes.


Contacts :

Paul Wray, Forestry, (515) 294-1168, phw@iastate.edu

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

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