Wooden planter boxes add texture and interest to the home landscape. All woods can be made weather- and insect-resistant by coating them with a weather sealant, but if you’re looking to bypass the chemicals for a wood that lasts a long time naturally, there are many choices. Keep in mind though, the type of wood is not the only aspect to consider when planning for your planter box's longevity. Do not use wood treated with creosote or chromated copper arsenate (CCA), as the preservatives are toxic. These chemicals leach into the soil and are absorbed by the plant's roots and if eaten, can affect humans in a negative way.
Attributes of Wood
When choosing wood for a planter box, look for wood that contain naturally occurring oils that protect it from rain and termite damage if you don’t want to use chemical treatments. Also, heartwood, the part of the tree near the center of the trunk, contains higher concentrations of insect- and weather-resistant compounds than outer regions, referred to as sapwood. A planter box made of heartwood lasts much longer than one made of sapwood of the same wood type. And slow-growing tree species have more naturally occurring rot-resistant compounds than fast-growing ones, and thicker boards last longer than thinner ones.
Redwood (Sequoia sempervirens) and bald cypress (Taxodium distichum) contain naturally occurring compounds that make them resistant to termite and weather damage. While old growth species provide the most resistance, many are protected and they can be pricey compared to other woods. Second-growth redwood and bald cypress are not as resistant as old-growth trees but a planter box made from their heartwood lasts many years. To revive the color of a weathered redwood box, coat it with a colored sealer. Redwood is hardy in U.S. Department of Agriculture zones 8 to 10, and bald cypress in USDA zones 5 through 10.
Tropical woods naturally resist decay because their native range is hot and moist. Spanish cedar (Cedrela odorata) is imported to the United States from Central and South America and is hardy in USDA zones 9B through 11. It is very resistant to rot from moisture. Teak (Tectona grandis) is moderately to very resistant to rot depending on whether the wood is old growth or second-growth. However, this native to southeast Asia is in high demand worldwide and is not as readily available as other woods. Teak is hardy in USDA zones 10 through 12.
Tried and True Woods
Black locust (Robinia pseudoacacia), native to the southeastern United States, is a rot-resistant, extremely long-lasting wood. The heartwood of this tree is reddish-brown and gets its durability from its high density. Because of its high ability to resist rot, black locust has commonly been used for fence posts and has been reported to last up to 50 years without chemical treatment, making it ideal for use as a planter box. Black locust is hardy in USDA zones 3 to 8. In its native range it is available, though you may have to look for it at local lumber yards as opposed to big-box stores.