The concept is deceptively simple: take a common, inexpensive wood species, heat it to extreme temperatures in a controlled environment, and permanently alter its cellular structure to resist rot, repel moisture, and remain dimensionally stable outdoors. No chemical preservatives. No toxic compounds. No copper, no arsenic, no chromium. Just heat and time, producing lumber that rivals tropical hardwoods in durability while using fast-growing, sustainably managed species as the starting material.
Thermally modified wood has been commercially available in Europe for two decades and is steadily gaining traction in North America. Three brands dominate the market for outdoor applications: Kebony, Thermory, and Accoya. Each uses a different modification process, starts with different wood species, and achieves different performance characteristics. If you are considering any of them for raised beds, garden furniture, decking, pergolas, or other outdoor garden structures, the differences matter — and the marketing does not always make them clear.
This guide breaks down the science, compares the three head-to-head on the metrics that matter for garden use, and helps you decide whether thermally modified wood makes sense for your project — or whether a naturally durable species is the better investment.
How Thermal Modification Works: The Science Behind the Heat
All thermal modification processes share a core principle: heating wood to temperatures between 400 and 500 degrees Fahrenheit (200 to 260 degrees Celsius) in an oxygen-depleted environment. The absence of oxygen prevents combustion — the wood does not burn, but its chemistry changes fundamentally at the molecular level.
At these temperatures, the hemicellulose in the wood cell walls breaks down. Hemicellulose is the component most responsible for absorbing and releasing moisture — it is why untreated wood swells when wet and shrinks when dry. When hemicellulose degrades, the wood loses its ability to absorb water efficiently. Moisture equilibrium drops from the typical 12 to 15 percent of conventional lumber down to 4 to 7 percent. That reduced moisture content creates an environment hostile to the wood-decay fungi that cause rot. Decay fungi need wood moisture content above 20 percent to colonize and consume wood fiber. Thermally modified wood simply never reaches that threshold under normal outdoor exposure.
The process also caramelizes the wood sugars, which is what produces the characteristic dark brown color of thermally modified lumber. These caramelized compounds have mild fungicidal properties of their own, adding a chemical dimension to the physical moisture barrier. The result is a product that resists rot through two mechanisms simultaneously: it stays too dry for fungi to grow, and what moisture does penetrate encounters antifungal compounds.
There is a trade-off. The same hemicellulose breakdown that improves durability also reduces the wood's mechanical strength. Thermally modified lumber is typically 10 to 30 percent weaker in bending and compression than the same species in its unmodified state. For decking, cladding, raised beds, and garden furniture, this reduction is usually irrelevant — the loads are low and the dimensions are generous. For structural applications like load-bearing beams, it matters, and engineers must account for the reduced values.
Kebony: Furfurylation, Not Just Heat
Kebony is technically not a pure thermal modification. It uses a patented process called furfurylation — the wood is impregnated with furfuryl alcohol (derived from agricultural waste like corn cobs and sugarcane bagasse) and then heated to polymerize the furfuryl alcohol within the wood cells. The polymerized resin permanently fills the cell structure, making the wood harder, denser, and more dimensionally stable than heat alone would achieve.
Kebony offers two product lines. Kebony Clear starts with Monterey pine (Pinus radiata) from New Zealand plantations and receives the full furfurylation treatment. The result is a uniform dark brown wood with a Janka hardness comparable to teak (approximately 1,100 to 1,300 lbf) and Class 1 durability (25+ years in ground contact). Kebony Character uses Scots pine with a lighter treatment, achieving Class 2 durability at a lower price point.
For garden applications, Kebony Clear is the relevant product. Its performance characteristics are impressive: dimensional stability is 50 to 60 percent better than unmodified pine, meaning boards maintain their shape through wet and dry cycles with minimal cupping, twisting, or gapping. The surface hardness resists dent damage from garden tools, furniture legs, and foot traffic. And because the modification extends throughout the board — not just the surface — any exposed end grain, cuts, or damage reveals treated material, not vulnerable unmodified wood underneath.
Kebony carries a 30-year warranty for above-ground residential applications and a 25-year warranty for ground contact. It is FSC-certified and markets heavily on its sustainability credentials: fast-growing plantation softwood transformed into hardwood-equivalent performance without tropical timber harvesting.
Thermory: Pure Thermal Modification
Thermory, based in Estonia, uses a straightforward high-temperature treatment with steam as the oxygen-displacing medium. The process heats the wood to 410 to 430 degrees Fahrenheit in pressurized steam chambers, which both prevents combustion and accelerates the chemical transformation. Thermory treats primarily ash (Fraxinus excelsior) and pine (Pinus sylvestris), both abundant European species from sustainably managed forests.
Thermory ash is the standout product for garden use. Ash is already a reasonably durable hardwood in its unmodified state, with a Janka hardness of approximately 1,320 lbf. After thermal modification, ash achieves Class 1 durability while the hardness decreases slightly to roughly 1,050 to 1,150 lbf — still substantially harder than cedar (350 lbf) and comparable to many tropical hardwoods. The color deepens to a rich chocolate brown with warm undertones, which many designers and homeowners find more attractive than the slightly cooler tones of Kebony.
Thermory pine is the budget option, achieving Class 2 durability (15 to 25 years above ground, less in ground contact). It is softer and less dimensionally stable than Thermory ash, but at roughly half the cost, it fills a gap for projects where maximum longevity is not required. For raised bed sides that do not contact the ground, Thermory pine performs well at a competitive price point.
Thermory ash is available in a wider range of profiles than most competitors — decking boards, cladding, fencing, and landscape timbers in multiple dimensions. This profile variety matters for garden projects because you can source matching material for beds, benches, fences, and walkways from a single product line, achieving a cohesive look across the entire garden.
Accoya: Acetylation, a Different Chemistry Entirely
Accoya uses a process called acetylation, which is chemically distinct from both thermal modification and furfurylation. The wood — radiata pine from FSC-certified New Zealand plantations — is soaked in acetic anhydride (a vinegar-derived compound). The acetic anhydride reacts with the hydroxyl groups in the wood cell walls, replacing them with acetyl groups. These acetyl groups are hydrophobic — they repel water at the molecular level.
The result is extraordinary dimensional stability. Accoya swells and shrinks less than 1 percent across its width with changes in moisture content, compared to 3 to 6 percent for unmodified pine and 1.5 to 3 percent for most thermally modified products. This makes Accoya the clear winner for applications where dimensional precision matters — window frames, doors, and any garden project with tight joinery that needs to stay tight year-round.
Durability is Class 1, with a 50-year above-ground warranty and a 25-year below-ground warranty. These are the longest warranties in the modified wood industry. Accoya also retains more of the original wood's mechanical strength than pure thermal modification — bending strength is reduced by only about 10 to 15 percent, compared to 20 to 30 percent for Thermory and Kebony.
The trade-off is cost. Accoya is the most expensive option, typically $8 to $14 per board foot depending on profile and thickness. It is also the most limited in availability — Accoya is manufactured at a single facility in the Netherlands and distributed through a network of authorized dealers. Lead times can be long, and off-the-shelf availability is not guaranteed in all regions.
Head-to-Head Comparison for Garden Projects
Durability class: Kebony Clear, Thermory Ash, and Accoya all achieve Class 1 (25+ years). Thermory Pine and Kebony Character achieve Class 2 (15 to 25 years). For ground-contact garden applications like raised beds sitting on soil, the Class 1 products are strongly preferred.
Dimensional stability: Accoya leads by a significant margin — less than 1 percent swell across width. Kebony Clear is second at roughly 1.5 to 2 percent. Thermory Ash is competitive at 2 to 3 percent. All three are dramatically better than unmodified softwoods (4 to 7 percent).
Hardness: Kebony Clear leads at approximately 1,100 to 1,300 lbf. Thermory Ash follows at 1,050 to 1,150 lbf. Accoya is the softest at roughly 800 to 900 lbf (still harder than cedar and comparable to unmodified pine). For garden beds and furniture, all three are adequately hard. For high-traffic decking around garden areas, Kebony or Thermory ash are the better choices.
Color: All three start as a dark chocolate brown. Kebony and Thermory Ash have warmer tones. Accoya starts slightly lighter. All three will weather to silvery gray if left unfinished, just like teak and white oak. This weathering is purely cosmetic and does not affect durability. A penetrating oil finish will maintain the brown tones if preferred.
Cost per board foot: Thermory Pine: $3 to $5. Thermory Ash: $6 to $9. Kebony Clear: $7 to $11. Accoya: $8 to $14. For comparison, clear western red cedar runs $4 to $7 and domestic white oak $5 to $9. Ipe runs $8 to $15. All prices are for standard 5/4 decking-profile boards from authorized distributors.
Availability: Thermory has the broadest North American distribution network with multiple stocking dealers and reasonable lead times. Kebony has strong dealer networks on both coasts but thinner coverage in the interior. Accoya is the most limited — few stocking dealers, longer lead times, and potential for extended waits on specialty profiles.
Warranty: Accoya leads with a 50-year above-ground warranty. Kebony offers 30 years above ground. Thermory offers 25 years for ash, 15 years for pine. All warranties have conditions and require registration. Check the fine print — some require annual oiling to maintain the warranty, which may or may not align with your maintenance preferences.
Thermally Modified vs Naturally Durable Species
This is the question most garden builders need to answer: should I buy modified wood or naturally durable species like white oak, black locust, or cedar? The answer depends on your priorities.
Choose thermally modified wood if you want consistent quality with minimal natural variation, dark brown aesthetics without stain, chemical-free durability from common wood species, and verified warranty backing from an established manufacturer. The standardization of factory-modified lumber means every board meets a specification. There are no concerns about sapwood versus heartwood, no grading questions, and no variability between trees.
Choose naturally durable species if you want lower cost per board foot (especially cedar and domestic hardwoods), no supply chain dependency on a single manufacturer, species with established track records measured in centuries rather than decades, and the character and variability that comes with natural wood. White oak and black locust have been proven in ground contact over generations. The thermal modification industry is newer — the oldest installations are 20 to 25 years old, which is compelling but not yet a century of evidence.
For detailed comparisons of natural species for raised beds and garden structures, see our materials and buying guides.
Best Applications in the Garden
Thermally modified wood excels in several garden applications. Raised bed construction benefits from the dimensional stability — boards that do not cup or twist keep bed walls straight and joints tight over decades. Garden furniture benefits from the hardness and uniform color — a bench or table built from Thermory ash or Kebony has a consistent, refined appearance that competes with teak at a lower price. Pergola and arbor construction benefits from the rot resistance in above-ground applications, and the lighter weight of thermally modified softwoods (compared to species like ipe) makes overhead installation less strenuous.
One application where thermally modified wood is particularly strong: any project where you need a dark, consistent color without staining. The thermal process produces an inherent color that is permanent and uniform. Unlike surface stains that fade, peel, and need reapplication, the color of thermally modified wood extends through the full thickness of the board. Cut it, sand it, plane it — the color remains the same.
For ground-contact applications like fence posts and retaining walls, naturally durable hardwoods like black locust still have an edge. The reduced mechanical strength of thermally modified wood is a bigger concern in load-bearing ground-contact applications, and species like black locust offer equal or better durability without the strength compromise. To understand how long different materials last in garden settings, our guide on raised bed lifespan covers the full comparison.
Frequently Asked Questions
Is thermally modified wood safe for vegetable garden beds?
Yes, all three products are food-safe. Kebony's furfuryl alcohol is derived from plant waste and polymerizes into an inert resin. Thermory's process uses only heat and steam. Accoya's acetylation produces acetic acid (vinegar) as a byproduct, and the resulting acetyl groups are chemically inert. None of these processes introduce toxic metals, pesticides, or preservatives. All three are safer for food gardens than pressure-treated lumber.
How does thermally modified wood handle fasteners?
Pre-drilling is recommended for all three products, especially near board ends. The modified wood is more brittle than unmodified lumber, meaning it is more prone to splitting if screws are driven without pilot holes. Use stainless steel or coated structural screws — the corrosion resistance of the wood means nothing if the fasteners rust and streak the surface. Kebony and Thermory both publish specific fastening guidelines; follow them for best results.
Will thermally modified wood splinter on bare feet or bare hands?
Less than unmodified wood, generally. The thermal process smooths the wood fibers and reduces raised grain, which is the primary cause of splinters. Thermory ash and Kebony both market their products for barefoot-friendly decking applications. However, any wood surface that weathers without maintenance will eventually raise grain — annual cleaning with a stiff brush or light sanding followed by an oil coat will keep the surface smooth if barefoot comfort matters for garden pathways or seating areas.
Can I paint or stain thermally modified wood?
You can, but most owners do not. The inherent dark brown color is one of the product's main selling points. If you want to maintain that color, use a UV-resistant penetrating oil annually. If you prefer the natural silver-gray weathered look, do nothing. If you want a different color entirely, penetrating stains work well — the reduced moisture content of thermally modified wood actually improves stain absorption and adhesion compared to unmodified species. Film-forming paints and varnishes are not recommended for exterior use on any wood, modified or not.
What is the environmental benefit over tropical hardwoods like ipe?
Thermally modified wood uses fast-growing plantation species — radiata pine, Scots pine, European ash — instead of slow-growing tropical hardwoods harvested from mature forests. The modification process uses energy (natural gas or biomass fuel for the kilns), but lifecycle analysis studies show a significantly lower carbon footprint than importing tropical hardwood from South America or Southeast Asia. All three major brands (Kebony, Thermory, Accoya) source from FSC-certified or sustainably managed forests, and the modification process adds value to species that would otherwise be too perishable for outdoor use.