Robinia outdoors – the complete guide
Robinia series · Pillar article – the reference guide
Robinia – also known as black locust, and often sold as acacia – is the most durable outdoor timber that grows in Europe. It lasts for decades in ground contact without any chemical treatment. This guide brings together the essentials: what the wood is, why it resists decay, what the figures say, where its limits lie and what it can be used for. It is the reference page of our series – each specialist article links back here rather than explaining everything again.
1. What is robinia?
Robinia (Robinia pseudoacacia) belongs to the Fabaceae family (the legumes, like peas or clover) – hence its ability to fix nitrogen from the air. It is a hardwood, not a softwood.
The trade name “acacia” is a misnomer: the true genus Acacia covers tropical trees (mimosas, Australian wattles). What Europe sells as “acacia” is in fact robinia. It was brought over from North America (the Appalachians) and planted in Paris in 1601 by Jean Robin, botanist to the kings of France; Linnaeus named the genus after him in 1753. The oldest robinia in Europe, planted by Jean Robin, still grows in the Square René-Viviani, opposite Notre-Dame; another, planted in 1636 by his son Vespasien, stands in the Jardin des Plantes. It is sometimes called “European teak”.
The tree reaches 20 to 30 m, can live for more than 300 years and grows quickly. It forms heartwood very early (from 2 to 6 years): this is what makes it usable in small diameters and, as we shall see, what explains its durability.
2. Why it lasts without treatment – the mechanism
This is the point almost nobody explains correctly. The trunk has two distinct parts:
The heartwood (the core of the trunk): golden brown, this is the “very durable” part. Robinia’s entire reputation rests on it.
The sapwood (the pale ring under the bark): in robinia it is narrow, and its thickness follows the tree’s growth rate – half a centimetre on mature, slow-grown wood, up to 1.5 to 2 cm on young, fast-grown wood. It is not durable (class 5). That is precisely why our sanded posts are “sapwood removed”.
Where does the durability of the heartwood come from? From its chemistry. The heartwood is packed with flavonoids – mainly robinetin and dihydrorobinetin – compounds that are naturally fungicidal and insect-repellent. They explain two things at once: why the heartwood resists and the sapwood does not, and why slow-grown robinia (tight growth rings, more dense heartwood) is even more durable. Another consequence: the heartwood is virtually impermeable – it cannot be impregnated with a preservative, and there is no need to: its protection is internal, not on the surface.
3. The figures: durability, classes and mechanical properties
Two European standards set the framework. Natural durability (EN 350) rates resistance to fungi from 1 (very durable) to 5 (not durable). Use class (EN 335) describes the exposure the timber can withstand:
Use class | Exposure | Example |
Class 1 | Indoors, dry | Furniture, flooring |
Class 2 | Indoors, damp / sheltered | Roof timbers under cover |
Class 3 | Outdoors, above ground | Cladding, decking |
Class 4 | Permanent contact with soil / fresh water | Stakes, posts, jetties |
Class 5 | Immersion in seawater | Mussel poles, marine piles |
Robinia is rated durability class 1-2 and naturally covers use class 4 – permanent ground contact – without any treatment. It is the only temperate European hardwood to do so; Forêt.Nature describes it as the only European timber to reach the best durability class against fungi. In practice, robinia is credited with around 30 years in ground contact, compared with about 15 years for sweet chestnut and 10 years for oak.
Mechanically, it outperforms the common European hardwoods. CIRAD-Tropix values (wood at 12 % moisture content):
Property | Robinia | Oak | Sweet chestnut | Larch | Cedar |
Density | 0.74 | 0.74 | 0.64 | 0.60 | 0.51 |
Hardness (French Monnin test, NF B 51-013) | 9.5 | 4.2 | 2.9 | 3.8 | 2.4 |
Bending strength (at failure) | 126 MPa | 105 | 71 | 90 | 82 |
Modulus of elasticity | 16,900 MPa | 13,300 | 13,300 | 11,800 | 10,100 |
Compressive strength | 70 MPa | 58 | 46 | 52 | 42 |
Durability (EN 350) | 1-2 | 2 | 2 | 3-4 | 1-2 |
Use class | 4 | 3 | 3 | 3 | 3 |
Outdoor treatment | None | — | — | — | — |
On the Janka scale more familiar to English-speaking readers, robinia measures around 7,560 N, against about 4,980 N for oak and 3,010 N for sweet chestnut: robinia is about 50 % harder than oak and about two and a half times as hard as sweet chestnut. And it is the only species in the table to cover use class 4 without treatment.
Two figures not to confuse – density and hardness do not measure the same thing.
Density is weight. It compares the weight of the wood with that of water: 0.74 means the wood weighs 0.74 times the same volume of water, or about 740 kg per cubic metre. A cubic metre of robinia therefore weighs ~750 kg – almost as much as a small car – against ~450 kg for pine. Be careful what you compare, though: this figure is measured at 12 % moisture content, the convention that allows species to be compared with one another. The same wood as it leaves the sawmill, still green, weighs 780 to 820 kg/m³ – that is the value given on our technical data sheets, and the one that matters when you size handling equipment or read the weight of a pallet.
Hardness is the resistance of the surface. The table above uses the French Monnin test (NF B 51-013), the reference used by CIRAD: a standard steel cylinder is pressed into the wood under a fixed load and the depth of the indentation is measured; the shallower it is, the higher the figure. On that scale robinia reaches 9.5. The Janka test, more widely used in English-speaking countries, measures the force needed to press a steel ball into the wood: robinia comes out at around 7,560 N, oak at about 4,980 N. In practice, this hardness means a decking board does not scratch underfoot, a stake takes a kick from a hoof or a knock from a grape harvester without crushing, and the wood does not dent. The downside: it is so hard that you need to pre-drill and use sharp tools.
The most telling point: oak and robinia have almost the same density (~740-750 kg/m³), yet on the Janka scale robinia is about 50 % harder. Proof that density and hardness really are two different things.
4. Where our robinia comes from – and does origin change the quality of the wood?
The species gives the durability; the forest gives the hardness. The durability of robinia (Robinia pseudoacacia) comes from the extractives in its heartwood, whatever country the tree grew in. Its hardness, on the other hand, depends on how the forest is managed – and that is where origin matters: our wood comes exclusively from dense, slow-growing Romanian forests, harvested at maturity between 25 and 30 years, from stems 15 to 30 cm in diameter.
In Romania, robinia stands are dense: many stems per hectare, competing for light. The trees grow slowly there, forming tight growth rings – a hard, dense and uniform wood. Our trees reach maturity at around 25-30 years with a diameter of only 15 to 30 cm: a robinia that has taken its time.
Durability, however, does not depend on the terroir. Robinia forms its heartwood very early (from 2 to 6 years) and keeps only a narrow band of sapwood – half a centimetre on mature wood, up to 1.5 to 2 cm on young, fast-grown wood. That heartwood concentrates the flavonoids (robinetin) that give it natural durability class 1-2 under EN 350, and allow it to be used in use class 4 without any treatment. These properties belong to the species: the Belgian study by Pollet et al. measured consistent durability on robinia grown in Belgium, far from its native range. What really changes how long a stake lasts is not the nationality of the tree – it is the proportion of sapwood and juvenile wood in the cross-section, the subject of our article “Round, sawn or split stakes – which to choose (sapwood and durability)”.
France has about 220,000 hectares of robinia (Inventaire Forestier National, 2022), or 1 % of its forest area. Silviculture there is practised differently from Romania: frequent thinnings, trees that put on girth quickly, a softer wood. Romania has about 250,000 hectares, managed as dense stands for timber: slow growth, straighter stems, steady volumes all year round, uniform grading.
These are two ways of managing the same species, and they produce two different woods. What we look for in Romania is the hardness that slow growth brings, and a reliable supply of long lengths.
All our stakes and sawn timber come from Romania, from around our sawmill in Teiu, Hunedoara County (Romania). The trees are harvested at maturity, between 25 and 30 years, at diameters of 15 to 30 cm: old enough for the heartwood – the durable part – to dominate the cross-section, without excess juvenile wood. Large straight logs remain rare, because robinia grows crooked: that is why finger-jointing makes so much sense for larger dimensions. Our finger-jointed timber was tested at the UCLouvain laboratory at 88 N/mm² in bending to EN 408; details in our article “Finger-jointed robinia decking and cladding – stable and durable”.
5. Its limits – to manage them, not hide them
A “lively” wood. Its dimensional stability is low and its shrinkage high: it tends to split at the ends and twist as it dries. The remedy: careful drying, anti-split plates, and finger-jointed timber wherever long, stable lengths are needed.
Hard and prone to splitting. Very dense, it blunts tools and splits if nailed carelessly: always pre-drill.
Slightly acidic (pH ~4.5). It corrodes ordinary steel (black stains, fixings working loose). All screws and fixings must be stainless steel (A2/A4) or hot-dip galvanised.
Large straight logs are rare. The tree is often crooked; long straight sections are precious. That is why finger-jointing and glulam make so much sense for larger dimensions.
6. Robinia compared with other materials (in brief)
vs oak and sweet chestnut: use class 4 instead of 3, harder, and a service life in the ground twice that of sweet chestnut and three to six times that of oak.
vs pressure-treated pine / Douglas fir: no injected biocide. And not all softwoods take treatment equally well: Scots pine, with its wide sapwood, impregnates properly (up to use class 4), whereas Douglas fir and fir/spruce are refractory – the preservative penetrates only a few millimetres – and struggle to go beyond use class 3. Robinia is durable right through, with no treatment at all.
vs tropical hardwoods (ipe, teak, cumaru): comparable durability, but European-grown and FSC-certifiable – without the long-haul transport or tropical deforestation.
vs modified wood (Accoya, thermally modified timber): the same “durable without biocide” advantages, but robinia needs no prior chemical or thermal processing.
Each of these comparisons is covered in detail, with figures, in our dedicated articles.
7. In water and in the sun
Permanently submerged, it does not rot. Wood-decay fungi need oxygen: underwater, a robinia pile can last for centuries. This is use class 4 (contact with fresh water / soil), which it covers without treatment.
The weak point is standing water, not water itself. It is not immersion that wears the wood out, but the alternation of air and water (the water-level fluctuation zone) and water trapped at the foot of a post. Hence the importance of always letting water drain away at the base.
It also holds up in salt water. It is used, for example, for the poles of mussel farms (bouchots), submerged at sea.
It greys in the sun – so what? Like all timber, UV light gives it a silvery-grey patina, purely on the surface: strength is not affected. You can keep the patina, or restore the honey colour with a penetrating oil every 2 to 3 years.
8. An ecological timber – with nuance
As a legume, robinia fixes nitrogen and enriches poor soils; its flowers produce the much sought-after acacia honey. As for our commitment, for every m³ of timber sold we plant a tree – more than 75,000 to date – to increase the area of robinia forest in Romania.
Not to be confused with regeneration after felling: there, no replanting is needed. Like bamboo, robinia regrows by itself from its roots (root suckering) – as long as a root remains in the ground, it sends up new shoots. That is what makes it so hard to eradicate, and why it is classed as an invasive alien species. Harvesting it therefore makes good use of an abundant local resource – an alternative to tropical hardwoods and treated timber. We cover this in the article “Is robinia an ecological timber?”.
9. Traced and certified timber
A timber’s durability only counts if its origin is beyond reproach. Our forests, our harvesting operations and our sawmill are FSC® certified (C153588, chain of custody SGSCH-COC-011793), and our production is ISO 9001 certified. A useful point when ordering: standard supply is from controlled sources, and FSC® 100 % delivery is available on request – mention it in your enquiry if your contract requires it.
Romania operates one of the strictest timber traceability systems in Europe: felling permits (APV), marking with the forest hammer, digital geolocated declaration (SUMAL), field inspections and public verification. At every stage we apply the same vigilance and rigour to guarantee sustainable, fully traceable sourcing.
For our customers – especially the most demanding – this is a decisive argument, particularly in view of the European EUDR regulation (“deforestation-free”), which comes into application at the end of 2026. Two dedicated articles cover these subjects in detail: “FSC and ISO 9001 – Acacia Bois certifications” and “Traceability of Romanian timber – SUMAL, APV and the EUDR”.
10. All its outdoor uses
Each use has its own dedicated guide – this overview is the starting point:
Fencing, stakes, cleft palings – agricultural, equestrian, vineyard, decorative.
Pergolas and structures – long spans, pegged joints.
Decking and cladding – often in finger-jointed robinia, non-slip and stable.
Playgrounds and school grounds – sound, sanded, splinter-free timber (EN 1176).
Shingle (shake) roofs and walls.
Soil bioengineering – riverbanks, wetlands, dune stabilisation (submerged wood does not rot).
Street furniture and public landscaping for local councils.
Frequently asked questions
Are acacia and robinia the same wood?
Yes. “Acacia” is the common trade name for robinia (Robinia pseudoacacia). The true genus Acacia refers to unrelated tropical trees. Beware, too, of garden furniture sold as “acacia” in garden centres: it is usually Asian Acacia mangium, a wood that has nothing in common with European robinia and weathers far less well outdoors.
Does robinia need treating outdoors?
No – and it cannot be treated right through anyway: its heartwood does not take impregnation. Its protection is natural and internal (flavonoids). No stain or pressure treatment is needed for durability.
Why do you talk about “sapwood removed” timber?
Because only the core (heartwood) is durable. The sapwood, the pale ring under the bark, rots: it is removed on exposed products such as sanded posts.
Does robinia turn grey over time?
Yes. Like all exposed timber, it takes on a silvery-grey patina on the surface – without losing its strength. You can leave it natural or restore the colour with a penetrating oil.
How long does it last?
It is credited with around 30 years in ground contact, compared with about 15 years for sweet chestnut and 10 years for oak. Above ground it lasts much longer; permanently submerged, it can last for centuries. The details – what lengthens or shortens these figures – are in our guide “The real service life of robinia outdoors”.
Density and hardness – what is the difference?
Density is the weight of the wood (robinia weighs ~750 kg/m³, almost as much as a small car). Hardness is the resistance of its surface to impacts, scratches and dents. Two woods of the same density can have very different hardness: oak and robinia weigh the same, yet on the Janka scale robinia is about 50 % harder.
What is Monnin hardness – and what does Janka measure?
The Monnin test (NF B 51-013) is the French reference method for measuring the hardness of wood: a steel cylinder is pressed into the wood under a fixed load and the depth of the indentation is measured. The higher the figure, the more resistant the wood. Robinia reaches 9.5, against 4.2 for oak. English-speaking countries more often use the Janka test, which measures the force needed to press a steel ball into the wood: robinia around 7,560 N, oak about 4,980 N, sweet chestnut about 3,010 N. That makes robinia about 50 % harder than oak and about two and a half times as hard as sweet chestnut – a “very hard” wood.
Which screws and fixings should I use with robinia?
Stainless steel (A2, or A4 near the sea) or hot-dip galvanised. Robinia is slightly acidic (pH ~4.5) and corrodes ordinary steel – leading to black stains and, eventually, fixings working loose.
Do I need to pre-drill before screwing or nailing?
Yes, every time. Robinia is very dense and hard: nailed or screwed without a pilot hole, it splits. A pilot hole and stainless or galvanised fixings solve the problem.
Can robinia be painted, oiled or stained?
Yes, but only for appearance – never for durability, which is already at its maximum without treatment. A penetrating oil keeps the honey colour; otherwise, let the grey patina develop.
Where does your robinia come from, and is it certified?
From sustainably managed Romanian forests, FSC® certified (chain of custody from forest to finished product). Our production is certified to ISO 9001:2015.
Is Eastern European robinia as durable as French robinia?
Yes – and it is often harder. Durability comes from the extractives in the heartwood (natural durability 1-2 under EN 350, use class 4 without treatment) and is retained wherever the species grows in Europe: a Belgian study confirmed this outside its native range. Hardness, on the other hand, depends on how the forest is managed – Romania’s dense stands make the trees grow slowly, with tight growth rings, which gives a hard, dense wood, whereas silviculture with frequent thinnings speeds up growth and gives a softer wood. Our trees are harvested at maturity, at 25-30 years with a diameter of 15 to 30 cm.
Further reading
Related articles: Robinia fencing and cleft palings – choosing by use · Robinia pergolas – the complete guide from choice to installation · Finger-jointed robinia decking and cladding – stable and durable · Playgrounds and school grounds in robinia (EN 1176) · Soil bioengineering – riverbanks, fascines and retaining structures · Is robinia an ecological timber? · FSC and ISO 9001 – Acacia Bois certifications · Traceability of Romanian timber – SUMAL, APV and the EUDR.
Sources: Inventaire Forestier National (IGN, 2022); CIRAD-Tropix (robinia data sheet); Forêt.Nature / ULiège-Gembloux; CNPF-IFC; EN 335 & EN 350.
Contact: Planning a robinia project? Describe the use and the quantities, and we will come back to you with suitable sections and a quotation.
Disclaimer. Non-contractual article, provided for information only (photos and drawings included). Robinia is a natural material: the values given are orders of magnitude and may change without notice; in case of discrepancy, only the product’s technical data sheet and the general terms of sale apply. Always check the regulations that apply to your project.
© Xylo Impact SRL — all rights reserved, products marketed under the Acacia Poles brand. Any reproduction, even partial, is prohibited without prior authorisation.
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