Promising new alternatives to tropical hardwoods come with caveats.

By Meg Calkins, FASLA

The past decade has brought an explosion in the use of tropical hardwood decking and furnishings in public, institutional, and commercial landscapes. Whereas wood decking was once largely the purview of residential landscapes, now it can be found in urban settings from the High Line to West 8’s sculptural Wavedecks. Tropical hardwoods are so durable, hard, and decay-resistant that they appear to be the ideal material, yet the impacts of using even hardwoods certified by the Forest Stewardship Council can be substantial and threaten the most critical ecosystems of our planet.

Wood is a renewable construction material if you compare the amount of time a wood member is in use to the amount of time it takes to grow a tree to yield a comparable piece. But that is not always the case. For example, it takes 90 years to grow a Handroanthus heptaphyllus tree to yield ipe lumber for deck boards, but the deck boards will likely not be in use for 90 years even if they are reclaimed, refinished, and reused.

As concern for the health of tropical forests is increasingly recognized in sustainable design thinking, alternatives to tropical hardwoods for decking and site furnishings are making their way into the market. The most promising alternatives are thermally modified lumber, polymerized lumber, acetylated lumber, and black locust lumber.

The environmental impacts of logging for tropical hardwoods are well documented (see, among other reports, “A Trail of Stumps” by Jane Hutton, LAM, May 2013, which explained how the legal market for such woods can lend cover to gray- or black-market products). A 2014 Greenpeace study of logging in the Amazon estimated that in Pará State, one of Brazil’s main logging regions, 78 percent of hardwood lumber is harvested illegally. Documents are forged or obtained illegally, trees are cut and dragged from pristine old-growth forests in the dead of night, and logs find their way onto the wrong trucks. The result is that the Amazon rain forest is disappearing at an alarming rate of one acre per second, resulting in an 18 percent loss in the past 30 years.

But FSC-certified tropical hardwoods are OK, right?

Although the Forest Stewardship Council (FSC) sets a rigorous, well-respected standard, some experts believe it is possible to cheat the system, particularly in the Amazon and other tropical forests where fraud and illegal harvesting are rampant. The 2014 Greenpeace study states: “Several situations could occur that might result in illegal timber being traded under the FSC label, such as a company further up the supply chain violating laws in the country of origin or the timber’s true origin being hidden from the buyer by means of misappropriated documents.” Additionally, critics cite FSC’s willingness to certify old-growth tropical hardwood forests around the world, including those in the Amazon.

Even selective harvesting of tropical forests (promoted by FSC as a sustainable logging method) for species such as ipe or cumaru can result in the forest’s ecological decline. Research has shown that within years of selective harvesting of high-value species, the forest is likely to be clear-cut and turned to agriculture, contributing to forest fragmentation.

Plantation-grown tropical hardwoods are an alternative to old-growth forest tropical hardwoods, particularly if the plantations are FSC-certified—but they are not a comparable alternative. Tropical hardwood lumber from plantations is not as dense, decay-resistant, or durable as the forest trees that have been growing for well over 100 years. And the most valued tropical hardwoods are very slow-growing, which means their growth on plantations is a truly multigenerational investment.

The European Union and some U.S. municipalities are actively encouraging alternatives to tropical hardwoods for site and architectural uses. European Union legislation requires all importers of tropical hardwood to certify that the wood comes from sustainable sources. And going a step further, the government of Norway has banned the use of tropical hardwoods in all public projects, stating: “There is no international certification that can guarantee in a reliable manner that imported wood is legally or sustainably logged.” New York City, San Francisco, and many smaller municipalities in California have banned tropical hardwoods on projects that receive public funding. This has spurred the development of new wood treatments.

Three new wood modification technologies have come to the U.S. market from Europe in recent years. These technologies—thermal modification, acetylation, and polymerization—use physical, biological, or chemical processes to produce property improvements in softwoods and U.S. hardwoods, resulting in lumber that performs competitively with tropical hardwoods. Manufacturers of modified woods seek to improve durability through resistance to decay and termites, and to improve properties of lumber such as dimensional stability and hardness.

Thermally modified wood

Thermal modification, or TM, heats wood to temperatures above 400 degrees Fahrenheit, permanently modifying the carbohydrates (hemicelluloses) and rendering the wood inedible to decay organisms and insects. By limiting oxygen and using methods such as steam or hot water, the wood doesn’t burn. During the process, the wood is dried to lower its moisture content and to reduce expansion and contraction owing to moisture changes over the life of the lumber. The result is a lumber resistant to decay and insects that is dimensionally stable and darker in color. There is some loss of strength as well, but testing shows that it is not significant.

No chemicals are introduced in the nontoxic thermal modification process, so the lumber is easily recyclable. Energy to heat the wood and transport it to and from Europe is the main environmental impact; some companies take steps to capture waste heat and use renewable fuel sources.

Both hardwoods and softwoods can be thermally modified, with varying results. Thermory, one of the main suppliers of TM decking, cladding, and lumber in the United States, uses ash, spruce, and southern yellow pine. Some of the ash and pine are sourced from the United States, and although FSC-certified woods are not standard, they are available by special order.

Thermory’s TM ash has a European durability class rating of 1, “very resistant” to decay; its TM southern yellow pine is rated 2, or “resistant.” Mark Challinor, a principal at Thermory, explains that a durability rating of 1 means the wood can be used in ground contact, but it is not guaranteed for that use. In a deck application, the understructure is typically built with conventional pressure-treated lumber, and the decking is Thermory lumber.

Challinor explains that owing to its decay resistance, durability, and stability, Thermory is priced at a premium, like ipe, but cost savings come in the ease of constructability. Challinor offers that the product is easier to work than ipe and requires no special tools or predrilling, which cuts installation time and waste. Ipe is very hard and sometimes irregular, with board-end splits and cracks, resulting in time-consuming connections and layout of wood members.

Emily Mueller de Celis, an associate principal of Michael Van Valkenburgh Associates (MVVA), used TM lumber for a deck and custom benches at Wellesley College. She concurs with Challinor’s statement about TM lumber’s ease of constructability. “It allows complicated radial cuts that are difficult to achieve with ipe and black locust lumber.” She adds: “Because it is not as dense as ipe, it does not hold heat, making sitting on the bench or walking barefoot on the deck more appealing in the heat of summer.”

Acetylated wood

The acetylation process uses acetic anhydride and heat to modify wood’s composition, greatly reducing the ability of the wood to absorb water and rendering the wood more dimensionally stable and durable. Wood’s swelling and shrinking are reduced by 75 percent, and its durability is substantially increased. One manufacturer, Accoya, estimates a 50-year use life for its acetylated radiata pine in aboveground applications and a 25-year life given ground contact. As with TM wood, microorganisms and fungi are no longer able to recognize the wood as a food source. Stake tests show that the wood performs better in-ground than lumber treated with chromated copper arsenate (CCA), a standard treatment since the 1930s that was voluntarily phased out beginning in 2003.

The acetylation process, similar to that of TM, alters the full cross section of the wood, so cuts and machine holes do not need to be treated with a preservative. Because there is minimal shrinking and swelling of the wood, paints and finishes are expected to last longer.

Accoya, the main acetylated wood supplier to the United States, is given the second-highest rating, gold, by the Cradle to Cradle Products Innovation Institute, a sustainability certification organization. The institute recognizes it as a nontoxic, biodegradable product that involves recycling of processing waste; an energy-efficient manufacturing process; and responsible corporate policies. However, transport energy is one of the major environmental impacts of Accoya’s most durable product, radiata pine, as it is grown in New Zealand, shipped to the Netherlands for treatment, and then shipped to the country where it will be used.

Polymerized wood/kebonization

Kebonized lumber is softwood impregnated with furfural alcohol (a monomer and agricultural by-product) then heated to penetrate and surround the cell walls of the wood. Kebony, a Norwegian company, is the main manufacturer employing this polymerization process. It ships southern yellow pine and maple from the United States, radiata pine from New Zealand, and Scots pine from Scandinavia to a treatment plant in Norway. The company’s deck boards, cladding, and dimensional lumber are distributed around the world.

Kebonized lumber shares its durability (European durability class 1—“very resistant” to decay), dimensional stability, and color traits with thermally modified and acetylated lumber. But because of the polymerization, it is heavier and harder, comparable to black locust in hardness. Given the hardness and increased brittleness, the manufacturer recommends predrilling screw holes and using self-sealing stainless steel screws. The kebonization process adds an additional 50 percent to the dry weight of southern yellow pine.

Thomas Balsley Associates (now a part of SWA Group) specified Kebony for decking and custom seating at Hunter’s Point South Waterfront Park in Long Island City. Owing to New York City’s ban on the use of tropical hardwoods in city-funded projects, Balsley’s firm searched for alternatives among wood lumbers, as the designers did not want to use plastic lumber. They proposed Kebony to the NYC Department of Parks and Recreation, and the department decided to approve its use as a test case for other city parks. Kebony was installed on the project four years ago, and it has been so successful that designers are using it on the final phase of the project.

Black locust

Black locust (Robinia pseudoacacia), native to the Appalachians and the Ozarks, is a fast-growing pioneer species that is considered invasive in some parts of the United States. Black locust trees grow large enough to yield high-quality, dense, decay-resistant heartwood lumber in 25 to 40 years—just 20 to 40 percent of the time needed to grow comparably decay-resistant tropical hardwoods. Its Janka hardness rating, a measure of how well the wood can withstand dents and dings, is 1,700 pounds-force (lbf), tougher than white oak at 1,360 lbf or red maple at 950. The Janka hardness number also can predict challenges in nailing, screwing, sanding, or sawing, and although black locust is hard compared to U.S. softwoods, it is only half that of ipe and cumaru—notoriously challenging woods to build with.

Visually, black locust is known for its knots, mineral streaks, surface checks, and bark insertions. Many proponents of black locust, like Thomas Amoroso, ASLA, of Andropogon Associates, appreciate the natural character of the wood. Clients must be prepared for the appearance—and changing appearance—of the wood. The lumber grain can open up and look like it’s splitting, then close up again within one season. It is quite durable, but clients can be alarmed at what looks to them like failure. Premium grade has few imperfections; No. 1 grade is less expensive but has more imperfections.

In the United States, black locust is not grown on plantations. It is harvested from open areas where it sprouts up at the edge of farm fields and vacant lots, which creates the largest challenge of using black locust lumber—procurement issues. The lead time to source and air-dry good-quality wood to the ideal 11 percent moisture content can be substantial. For benches at Shoemaker Green on the University of Pennsylvania campus, Amoroso sourced the lumber from a local woodworker in Lancaster County, Pennsylvania. Yet at the U.S. Coast Guard headquarters in Washington, D.C., a much larger Andropogon project, the contractor resisted using black locust because large quantities were required, and the project was on a tight construction schedule. The team settled on FSC-certified ipe for the decking. Emily Mueller de Celis cites black locust procurement issues as a reason MVVA shifted its specifications at Wellesley College to thermally modified lumber. She found it far easier to procure, with no lead time at all for the contractor.

Until modified lumber products and black locust lumber came on the market, there were really no comparable alternatives to tropical hardwood lumber. Modified lumber and black locust lumber offer durability, decay resistance, and dimensional stability that sometimes outperform the hardest tropical hardwoods. As most installations of the modified lumber products have been in use for less than a decade, time will tell if they can perform as expected for 25 to 50 years, but their future looks bright.

Meg Calkins, FASLA, is a the Head of the Landscape Architecture and Environmental Planning Department at North Carolina State University.  She is the author of Materials for Sustainable Sites and the editor of The Sustainable Sites Handbook.