Is Hem-Fir Landscape Timber Safe for Vegetables?

Hem-Fir Landscape Timber

• Hem-fir is not a single tree species. Lumber stamped as hem-fir is made of any of five softwood conifer tree species harvested in the Pacific Northwest. Both Western hemlock (Tsuga heterophylla) and various firs (Abies spp.) in the mixed forest share roughly the same wood characteristics. Hem-fir timber is comparatively inexpensive, light colored and moderately strong for use general contracting and construction. Landscape timbers are often pressure treated prolonging their lifespan with resistance to rot and insects.

Treated Wood

• When using wood to create a vegetable patch, avoid treated hem-fir timber containing heavy metals in the preservation treatment. Chromium, arsenic and lead are harmful in elevated levels, often causing cancers. Do not use lumber with these heavy metals near edible plants in the garden. Chromate-copper-arsenic treated lumber is no longer legal or available in the United States. Acid-copper-chromate wood preservatives are available but contain chromium and are not a good in a vegetable patch. Hem-fir timber treated with alkaline-copper-quaternary-ammonium does not have carcinogens and is the better pressure-treated wood to use in edible gardens. Plant roots may absorb the elevated levels of copper in ACQ-wood.

Alternatives

• While ACQ-treated hem-fir landscape timbers are the best treated wood for creating a vegetable patch, a nontreated hem-fir timber is best. Such a product may not be available or inexpensive compared to treated wood supplies. Alternatives to hem-fir that naturally resist decay, fungi and insects include red cedar, northern white cedar, Osage orange, white oak, locust or redwood. None endures as long as pressure-treated hem-fir wood. Cinder block, brick or stone also make viable, nontoxic wood alternatives.

Vegetable Insight

• Residues from pressure-treated wood used in a vegetable gardens travel vertically in the soil. In time, rainwater moves the wood residues into the soil, but not horizontally. Planting food crops no closer than 12 inches from the pressure-treated lumber diminishes the chance of plants absorbing the chemical compounds. Root crops, such as turnips, carrots, radishes and potatoes, are best kept the farthest from treated lumber in the veggie patch, as these underground tissues may absorb and store the chemicals.