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Robinia service life outdoors: what the studies and the field show

Writer: Ferdinand Terlinden
Ferdinand Terlinden
Sep 24
6 min read

Updated: 5 days ago

Robinia series · Service life guide

Updated 9 October 2026.

A robinia stake can last for decades in the ground without any treatment. A small round pole full of sapwood can be finished within a few years. It is the same tree: the difference lies in the part of the wood used, the application and the installation. Here is what the standards and the studies say, followed by what we see in the field.

The short answer

  • Natural durability: robinia heartwood is rated durability class 1 to 2 under EN 350:2016, i.e. "very durable" to "durable" against fungi. It is the most durable timber of European origin, ahead of oak and sweet chestnut (LWF Bayern).

  • In the ground: it is used untreated in use class 4 (EN 335: contact with the ground or fresh water).

  • In years: in our experience, up to 30 years in the ground, depending on the soil, and up to 60 years above ground. The only published study that puts a figure in years on service life points the same way: in New Zealand, round robinia posts showed no failures after 30 years in the ground (Scion, 2013).

  • Sapwood (the pale layer under the bark) is not durable: it rots within a few years. It must be removed, or you accept that it will disappear.

Two standards not to confuse

EN 350 — natural durability of wood. It rates the heartwood of a species from 1 (very durable) to 5 (not durable), based on standardised tests against fungi. It tells you what the wood is worth.

EN 335 — use class. It describes the situation of the wood: from 1 (dry interior) to 5 (sea water). Use class 4 covers contact with the ground or fresh water: stakes, posts, fencing.

A class 1-2 timber such as robinia can go into use class 4 without preservative. A low-durability timber such as pine only goes there after pressure treatment.

What the studies say

Belgium — Université de Liège, Gembloux (Pollet, Jourez and Hébert, 2008). 25 robinia trees sampled from 5 sites in Wallonia, tested in the laboratory against two decay fungi. Result: heartwood class 1, "very durable"; the wood close to the pith (juvenile wood) is rated 1 to 2. No significant difference between sites, but large differences from one tree to another.

Germany — LWF Bayern (Risse and Richter, LWF Wissen 84). Review by the Bavarian forestry institute: heartwood class 1-2 under EN 350:2016, the most durable of European timbers, ahead of oak and sweet chestnut; suitable for direct contact with soil and water without preservative. Two caveats: juvenile wood (the first 7 to 15 growth rings) is markedly less durable, and the wood has poor resistance to shipworms in sea water.

Germany — Thünen-Institut and University of Göttingen (Brischke, Stolze et al., 2024). Seven provenances from Europe and America tested to EN 113-2 and classified under EN 350:2016. Most mature heartwoods are class 1. Differences between provenances are mainly explained by extractive content (the natural compounds in the wood). The inner heartwood of certain provenances drops to class 2, or even 3.

New Zealand — Scion (Page, Ledgard and Singh, 2013). This is the only source we have found that gives years in the ground:

  • debarked round posts from 40-year-old trees, installed in 1982: no failures after 30 years, only 5 to 15 mm of surface decay; the authors estimate an average service life probably over 40 years;

  • small test stakes cut from young trees (9 to 11 years old): 4 to 10 years depending on the clone;

  • sapwood: less than 5 years.

What makes service life vary

  • Sapwood. It is not durable. On a round stake, it is 1 to 2 cm thick. Once debarked and pointed, a stake loses part of its sapwood; a sanded post has none left at all.

  • Juvenile wood. The core of young trees, and the centre of all trees, is less durable than mature heartwood. A small diameter cut from a young tree will last less long than a large diameter cut from a mature tree.

  • Diameter. The larger the section, the more mature heartwood it contains, and the more negligible surface decay becomes. That is why the 6-8 cm round stake is the least durable in our range.

  • Soil and water. The main enemy is not rain, but moisture that lingers at ground level, where air and water meet. Free-draining soil extends a stake's life; heavy, waterlogged soil shortens it.

  • Sea water. Robinia does not rot when permanently submerged (fungi need oxygen), but it has poor resistance to shipworms at sea.

Split robinia stake fence at ground level, the critical moisture zone for service life
The critical zone of a stake is neither in the air nor underground: it is at ground level, where moisture lingers.

Robinia, oak, sweet chestnut, treated pine

Robinia

Oak

Sweet chestnut

Pressure-treated pine

EN 350 durability (heartwood)

1-2

2

2

sapwood 5 (not durable)

In the ground, untreated

Yes

Yes

Yes

No

Service life in the ground, in our experience

up to 30 years, depending on the soil

10 to 15 years

15 to 20 years

about 10 years

EN 350 classes: LWF Bayern and ATIBT. Service lives: what we observe in the field, with comparable soil and installation; no published study compares these species in years.

In practice: how to get the longest service life

  1. Choose heartwood. Debarked and sapwood-free for demanding uses: our sanded posts have no sapwood left.

  2. Choose the right diameter. For a long-lasting fence, avoid 6-8 cm on posts that carry load; see our stakes and our large posts.

  3. Drive deep enough. One third of the length in the ground: see stake height and spacing.

  4. Drain the foot. Gravel at the bottom of the hole; for a post set in concrete, slope the top of the concrete so it sheds water, never dish it.

  5. Ventilate above ground. Decking and cladding: spaced boards, a 1 to 2% fall, air under the joists.

  6. Fix with stainless steel. Robinia makes ordinary steel rust; drill pilot holes.

No treatment is needed. A penetrating oil only serves to keep the honey colour; without it, the wood turns grey, which takes nothing away from its durability.

Robinia posts and rails fence greyed by UV in a pasture, a surface patina with no loss of strength
Posts & rails fence after several seasons: the grey is a surface patina, not wear.

Frequently asked questions

How long does a robinia stake last in the ground?

In our experience, up to 30 years, depending on the soil. In New Zealand, round robinia posts showed no failures after 30 years in the ground (Scion, 2013).

Does robinia need to be treated?

No. Its heartwood is rated 1-2 under EN 350: it is used in contact with the ground (use class 4) without preservative.

Natural durability and use class: what is the difference?

Durability (EN 350) describes the wood; use class (EN 335) describes the situation. Robinia is durable enough for use class 4 without treatment.

Why do some robinia stakes rot quickly?

Most often because of sapwood, a small diameter cut from a young tree, or a foot standing in water. Mature heartwood, on the other hand, holds up.

Does permanently submerged wood rot?

No: without oxygen, fungi cannot develop. It is the zone where the water level rises and falls that wears. In sea water, watch out for shipworms.

Robinia beams submerged in a pond for a jetty, durable timber in a permanently wet environment
Jetty structure set in a pond: permanently under water, the timber does not rot – it is the fluctuation zone, between two water levels, that takes the strain.

How can you tell when a stake is at the end of its life?

Push the tip of a knife into the wood at ground level: if the wood is soft and stringy, it is rotten. Only the affected stakes need replacing.

Sources

The service lives given are orders of magnitude: they depend on soil, climate and installation. They do not constitute a guarantee.

Further reading

Contact: Unsure about the service life of your project? Describe the use, the soil and the exposure: the sawmill will answer in years, not slogans.

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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