Robinia

Overview and range in Europe

Robinia, black locust or false acacia (Robinia pseudoacacia), is a North American tree that became one of the most widely planted broadleaves in Europe and a major source of naturally durable European timber. The Wood Database gives its native range as the central-eastern United States. A 2020 review in the Journal of Forestry Research (Springer; the page I opened did not show the article title) says it reached Europe around 1601, now covers more than 2,306,000 ha, and is the most common broadleaved tree on the continent by that count, with Hungary and Romania among the countries holding at least 100,000 ha each.

Hungary and Romania are the centres of robinia growing. A 2018 review of the species there states in its abstract that they "have the most highly-developed black locust growing techniques and experiences". Historic figures from an FAO Unasylva article by Keresztesi (adapted from a 1979 paper) put Hungary at about 271,000 ha in 1976 (19% of forest area) and Romania at about 100,000 ha, with the Calafat sands as the largest contiguous plantation area in Europe. These are 1970s numbers, not current ones.

Germany has far less: a waldwissen.net recommendations page, citing the third national forest inventory, gives about 34,000 ha, with 8,100 ha inventoried in Brandenburg (the page's own statement on Brandenburg's share is inconsistent). The Bavarian LWF paper says sawlogs used in Germany come mostly from imports from Hungary and Romania. For Poland I found no sourced area; a Polish study (Bijak and Lachowicz, Forests 2021) tested wood by age class but its page gives no hectare figures.

Invasive status, as sourced only: the Springer review summary says robinia ranks among the 100 most invasive alien species in Europe and is formally considered invasive in 13 of the 35 European countries that reported; German foresters call it "partly invasive" because it mainly spreads into open sites. The waldwissen.net page says opinions are divided and names risks to dry grasslands and orchard meadows. Rules differ by country, so check local regulations before building supply on new plantings.

Appearance and identification

The Wood Database describes heartwood from pale greenish-yellow to darker brown, darkening to russet brown with age. A Teknos data sheet (a coatings manufacturer's document developed with the Thünen Institute, undated) adds a cathedral figure on tangential surfaces. The LWF paper gives the sapwood as yellowish-white to light yellow, about 1 to 2 cm wide (usually 3 to 5 rings), and says heartwood forms early, from age 2 to 6 years.

The wood is ring-porous: large earlywood vessels in rows two to three wide, medium latewood vessels in clusters and tangential bands, abundant tyloses and narrow to medium rays (Wood Database). Grain is usually straight and texture medium. Heartwood fluoresces yellow-green under a blacklight (Wood Database), which is a useful check on mixed stacks. Separating robinia from oak by colour and weight should be easy, but that is my inference, not a published key.

Properties that matter

Robinia is among the densest and hardest timbers grown in Central Europe. The LWF paper gives a mean density of 0.74 g/cm3 (range 0.58 to 0.90) against 0.71 for oak and beech. Sources differ and often omit moisture condition or sample origin.

Property	Value	Source	Region
Density, mean	0.74 g/cm3 (0.58-0.90)	LWF Wissen 84	Germany/Europe; moisture and origin not stated
Air-dry density, 12-15%	0.74-0.80 g/cm3	Teknos sheet	Europe
Specific gravity, 15%	about 0.73-0.74	Springer 2020 review	Europe
Average dried weight	770 kg/m3	Wood Database	North America; specimen origin not stated
Bending strength	150 N/mm2 (LWF); 133.8 MPa (Wood Database)	as listed	Europe; North America
Modulus of elasticity	13,600 N/mm2 (LWF); 14.14 GPa (Wood Database)	as listed	Europe; North America
Compression parallel	73 N/mm2 (LWF); 70.3 MPa (Wood Database)	as listed	Europe; North America
Hardness	Brinell 64-78 N/mm2 along grain (LWF); Janka 7,560 N (Wood Database)	as listed	Europe; North America
Shrinkage green to dry R/T/vol.	3.9-4.4 / 5.8-6.9 / 11.4% (LWF); 4.6 / 7.2 / 10.2% (Wood Database)	as listed	Europe; North America
Heartwood durability class	1-2 (EN 350:2016-12)	LWF; Teknos	Europe

The Wood Database states 12% moisture only for specific gravity. Teknos gives 4.0% radial and 7.7% tangential shrinkage from fresh to 12-15%, a different basis from green to oven-dry; do not mix them when sizing allowances. The T/R ratio is about 1.6 (LWF, Wood Database).

Age and juvenile wood matter. Bijak and Lachowicz tested three age and size classes (about 38, 60 and 71 years) and found modulus of elasticity, bending and compression strength at 12% moisture highest in the youngest class, with bending strength of 178.1, 153.2 and 148.8; units are not shown on the page and are probably MPa, and the Polish origin is my inference. The LWF paper puts juvenile wood at roughly the first 7 to 15 rings, with reduced durability and strength; Latorraca, Dünisch and Koch (2011, eastern Germany and Hungary) put it at the first 10 to 20 years.

Sawing and log handling

The sources say far more about log form than about sawing, and I found no sawing parameters or yield figures. The LWF paper reports that today mostly small diameters, short lengths and low quality are available, with crooked stems, sweep, spiral grain and eccentric growth, and that growth culminates early so large diameters are rare. The FAO article (1970s Hungary) names small logs, crookedness, branch stubs and rotten heartwood, with an average diameter over bark of 17.6 cm in final fellings and 11.9 cm in thinnings in 1975. The waldwissen.net page asks for straight, largely knot-free stems with a target diameter above 40 cm for quality sawlogs and warns of significant stem rot risk from about 50 to 60 years.

It is reasonable to expect blanks, posts and short lengths rather than long clear boards (my inference). The LWF paper says spiral grain and growth stresses can make machining much harder, so expect movement when opening logs; I found no data on its size. Sapwood is narrow but not durable, and juvenile heartwood near the pith is weaker, so log sorting by heartwood share and pith position matters for exposed products.

Drying

Drying is the main technical difficulty. The LWF paper says fresh wood dries slowly and strongly tends to warp, worsened by irregular grain. It recommends covered air drying with careful stacking and, if needed, sealed ends; vacuum drying is described as gentle with good results, followed by thorough conditioning. Teknos says poor permeability makes drying slow, with a high tendency to lag and often warp and crack, and that open-air drying before kiln drying plus conditioning gives good results. Neither source gives times or temperatures, and I offer no schedule.

A Slovenian laboratory study of park-tree boards (Plavčak and co-workers, Les 2019; Omovšek thesis, Ljubljana 2020) dried black locust at 22, 28 and 46 mm and at 20, 40 and 60 degrees C. Thicker boards took longer, moisture gradients stayed low, and diffusion was generally lower in black locust than in the less dense box elder and horse chestnut. The thesis abstract reports good overall quality and flags collapse risk for box elder, not robinia. These were small urban-tree samples, not production kiln runs.

The 1979 FAO article states that the wood seasons well, which disagrees with the modern German sources; I could not resolve this.

Machining, gluing and finishing

Straight-grained, stress-free robinia is easy to work despite its hardness (Teknos, LWF), though suitable tooling is needed. The Wood Database calls it harder to machine than many domestic hardwoods, with a moderate blunting effect, and says it turns and steam-bends very well. Planing gives a clean surface; screws and nails hold well but need pre-drilling (LWF, Teknos). Dust may cause skin allergies (LWF).

Gluing is where sources disagree. The LWF paper says bonding high-density wood is generally difficult, heartwood extractives make it harder, finger jointing has worked, and suitable face-gluing systems have been identified, while a glulam project from short thin lamellae aims at building-authority approval. The Wood Database says it glues well. Test adhesives before committing.

For finishing, Teknos says outdoor use needs heavily pigmented, UV-absorbing coatings applied soon after planing, and warns of iron-tannin discolouration, so use stainless fittings. The LWF paper notes that colour can be homogenised by steaming or thermal treatment; the FAO article says heat treatment improves workability and reduces shrinkage but lowers strength, with no numbers.

Durability and treatment

The LWF paper rates the heartwood class 1 to 2 under DIN EN 350:2016-12 and calls it the most durable European timber, ahead of oak and sweet chestnut. In 16-week EN 350-1 laboratory tests (Latorraca and co-workers), mature heartwood from 50 samples lost on average 0.7% mass to the brown-rot fungus Coniophora puteana and 1.7% to the white-rot Coriolus versicolor (class 1), whereas juvenile heartwood from 64 samples lost 10.1% and 17.0% (class 2 for brown rot, class 4 for white rot). These are German and Hungarian samples in the laboratory, not field data.

The waldwissen.net page says untreated poles can last up to 50 years; I did not find the underlying service-life data. Landgraf and Böhm (AFZ-DerWald 14/2024) say the wood is accepted without chemical preservation for outdoor weathering and for water and soil contact, without naming a use class. The LWF paper notes low resistance to shipworm in seawater.

The heartwood is hardly impregnable even with pressure treatment, while sapwood takes preservative more easily (LWF). Exclude or treat sapwood and juvenile wood in ground contact. I did not read an EN 335 use-class assignment and state none.

Typical uses and market notes

The LWF paper lists unpreserved outdoor and ground-contact products (posts, palisades, bollards, decking, garden furniture, play equipment, avalanche protection together with sweet chestnut), vine stakes, hop poles and fences, hydraulic works, parquet, stair treads, furniture, tool handles and sports equipment, turned goods, facades, windows and doors, citing about 508 m3 for the Letzigrund stadium. The Springer review adds firewood, highly valued in Hungary and Romania, and biomass.

Hungarian 1975 data (FAO) show a small sawlog share: of 1,386.3 thousand m3 felled, sawlog and veneer logs were 10.5%, other sawmill timber 12.2% and firewood 52.7%. Landgraf and Böhm say demand has risen in recent years, positioning robinia against tropical imports. The Wood Database says prices are moderate in the native range, comparable to white oak in the eastern US, and much higher for imported lumber. I found no sourced European price, so none is given. The waldwissen.net page gives optimal rotations of 20 to 30 years for coppice and about 60 years for high forest, which explains scarce large diameters.

What is uncertain or varies

• The Cirad Tropix datasheet could not be opened (robots restriction); none of its figures are used.
• No sawing yields, drying times or kiln schedules were found; drying evidence is qualitative or from small laboratory samples.
• Moisture condition and sample origin are missing for several values, and the Teknos figures come from a coatings manufacturer's sheet whose underlying data I could not check.
• No sourced Polish area, Romanian property data or European price was found; the Polish strength values lack units.
• Sources disagree on gluing, on how well the wood seasons, and on juvenile wood extent (7-15 versus 10-20 years).
• No EN 338 strength class or EN 335 use class is claimed.

Key points

• Density averages about 0.74 g/cm3 in the main European source, with a wide range and a fall in strength with tree age and juvenile wood share.
• Mature heartwood is durability class 1 to 2; juvenile wood and sapwood are not.
• Logs are typically small, crooked and spiral-grained, so expect short lengths.
• Dry slowly with air drying first, end sealing and conditioning; expect warp and lag.
• Test glues and coatings, keep wet wood away from iron, and expect moderate tool wear.
• Check national invasive-species rules and the origin of supply.