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What is the best wood for a raised garden bed?

Decay-resistant heartwood such as cedar or redwood for untreated beds; ground-contact treated lumber if you want the longest life for the money. How long each lasts and what extension services say about safety.

The short answer

For an untreated bed, use the heartwood of a decay-resistant species such as western red cedar, redwood or white oak. The University of New Hampshire Extension says cedar or redwood beds can last 10 years or more, against three to five years for ordinary untreated wood. Modern treated lumber labelled for ground contact is another option; extension services describe it as low risk for vegetable beds with a few precautions.

How the woods compare

The USDA Forest Products Laboratory rates heartwood by its natural resistance to decay. It says resistant heartwood generally serves well above ground, while only very resistant heartwood generally performs in contact with the ground.

Heartwood decay resistance (USDA Forest Products Laboratory Wood Handbook)
WoodDecay resistanceNotes
Black locust, Osage-orangeVery resistant
Western red cedar, eastern red cedar, white oakResistant
Redwood, old growthResistantSecond-growth redwood generally has low to moderate resistance
Douglas-fir, western larchModerately resistant
Pine (most species), spruce, true firs, hemlockSlightly or not resistantUntreated southern pine may last only a few years, Virginia Cooperative Extension says

Sapwood, the lighter outer wood, has no natural decay resistance even in cedar and redwood, so lower grades that include it last less long.

How long an untreated bed lasts

Estimates vary. UNH Extension gives 10 years or more for cedar or redwood and three to five for other untreated wood. Washington State University Extension notes that untreated wood often begins to decay within a few years, even rot-resistant cedar. In an Oregon State University Extension trial, untreated Douglas-fir beds were due for replacement after the sixth growing season.

Treated lumber

The older CCA treatment, which contained arsenic, was withdrawn from almost all residential uses at the end of 2003, according to the EPA. Today’s residential treated lumber uses copper compounds: the University of Maryland Extension says micronized copper azole is the most widely available and is considered low in toxicity.

In the Oregon State trial, beds of treated lumber raised soil copper only within 1 inch of the boards, and the vegetables and herbs grown in them showed no increase in copper. Clemson Extension recommends lumber rated for ground contact (AWPA UC4A or higher) and planting vegetables 12 inches in from the inside edge; the University of Minnesota Extension suggests lining the inside of the walls if you want to limit copper reaching the soil.

Board thickness and fasteners

A 2-inch board is 1½ inches thick once dried and planed, the size the American Wood Council lists. UGA Extension says boards only 1 inch thick need a vertical support about every 4 feet. For cedar and redwood, the Western Red Cedar Lumber Association recommends stainless steel screws, and hot-dipped galvanized will do. For treated lumber, the American Wood Council says hot-dip galvanized fasteners follow ASTM A153, and aluminum should not touch copper-treated wood.

The 4 × 8 cedar bed plan uses these sizes and screws. For metal beds, see whether galvanized steel is safe.

References