Are Hem-Fir Landscape Timbers Safe for Vegetables?

Hem-Fir

• Hem-fir accounts for about one-fifth of all timber production in the United States. This timber is harvested from mixed forests in the Pacific Northwest, comprising both western hemlock and a wide array of fir tree species. Hem-fir wood is heavily used for construction purposes, as well as for landscape timbers. Because of landscape timbers' outdoor use, where contact with the soil and weather elements exists, the timbers are treated to resist decay or insects.

Chemical Treatment

• Concern over the safety of hem-fir landscape timbers in a vegetable gardens centers on the chemical treatment conducted on the wood. Timber may be treated with organic or oil-borne solutions or inorganic water-borne compounds with or without pressure treatment to push the compounds deeper into the wood's pores. The majority of wood available to the public in retail sale is treated with inorganic compounds chromated copper arsenate (CCA), ammoniacal copper arsenate (ACA) and acid copper chromate (ACC). However, the much less toxic preservative alkaline copper quat (ACQ) is more appropriate to use in an edible garden setting. ACQ-treated landscape timbers contain no arsenic, chromium or any other chemical the Environmental Protection Agency considers toxic.

Weighing Concern

• The concern centers on treated hem-fir landscape timbers, which may leach molecules of copper, arsenic and chromium into the vegetable garden soil. These elements, along with other compounds in the preservatives, may harm certain vegetable crops, or be absorbed into crops that are later consumed. Copper and chromium harm plants faster than the same amounts in humans. These two heavy metals also are strongly bound by soil particles and organic matter. However, arsenic is movable through soil and is toxic in lower levels in comparison.

Advisory

• When purchasing hem-fir landscape timbers, ask to see specific information regarding which preservatives are in the lumber. ACQ preservatives are preferred to CAA, ACA and ACC types. Leaching of heavy metals from treated hem-fir landscape timbers is exacerbated and more troublesome if certain vegetable garden conditions exist. The more wood surface in contact with the soil and moisture, the greater the leaching. Acidic soils cause more leaching from the wood, and the presence of organic matter increases leaching. At least two of these conditions readily exist in the soils created and maintained in vegetable gardens.

Options

• Use only ACQ or nontreated, raw hem-fir timber in a vegetable garden. It will rot at an increased rate. Alternatively, use more naturally rot-resistant timber such as cedar, redwood, locust, white oak or Osage orange. If treated hem-fir landscape timbers are used, cover them with a thick, impermeable membrane such as plastic or a thick PVC liner and place soil inside the liner walls. If vegetables are planted near exposed treated timber, do not plant root crops -- carrots, turnips, potatoes, radishes -- any closer than 12 inches to the timber. Highest concentrations of leaching heavy metals and other chemicals is highest immediately next to the wood as well as on vegetable roots and tuber skins.