Black Locust — Density, Movement, Workability & Cost

Fence posts cut from black locust (Robinia pseudoacacia) in Appalachia a century ago are still standing in some pastures today, doing the job with no chemical treatment of any kind. That's not folklore — the USDA Forest Products Laboratory rates this species' heartwood among the most decay-resistant temperate hardwoods tested, in the same conversation as teak, at a fraction of teak's price.

Rot resistance that rivals the tropics, grown domestically

At 48 lb/ft³ and 1,700 lbf on the Janka scale, black locust is hard, springy, and hard on cutting edges — interlocked sections tear under a plane the way elm's do. The payoff for fighting through that is a wood that shrugs off ground contact for decades without preservative treatment, a genuinely rare trait among species native to the eastern United States and one that has kept demand for locust fence posts and decking steady regardless of fashion.

Exceptional stability for a wood this dense

Volumetric shrinkage of just 10.2% at 48 lb/ft³ is remarkable — one of the most stable dense hardwoods grown in North America, meaningfully better-behaved than oak or hickory at a comparable weight. A tangential:radial ratio of 1.6 (7.2% tangential against 4.6% radial) keeps it well clear of this site's 2.0 movement-prone line. An 8"-wide deck board, seasoned outdoors from a wet 16% moisture content down to a drier 10%, loses about 0.115" (2.9mm) across its width by locust's own tangential coefficient; the wood movement calculator handles other widths and moisture swings. A 1"-thick, 6"-wide, 8-foot deck board like that one weighs about 16 lb; check other board sizes on the wood weight calculator.

Working it green, and a genuine safety note

Green locust machines far more easily than dry, and many turners deliberately work it wet rather than fighting the fully seasoned wood at the lathe. Gluing is adequate — the natural extractives that give locust its rot resistance are less troublesome for adhesion than teak's oils, though a freshly machined surface still helps. One point worth taking seriously: the bark, leaves, and seeds are genuinely poisonous, and the sawdust is a recorded skin and respiratory irritant, so dust extraction matters even for a species with no exotic reputation.

An odd colour trick, and cost

Black locust fluoresces green under UV light and shifts from a fresh yellow-green toward russet brown as it weathers in sunlight — a colour change that reads as aging rather than damage. At roughly $8 per board foot it's priced in this site's moderate tier, remarkably affordable given the durability it delivers. Fence posts, outdoor furniture, boat frames, decking, and tool handles are its main uses, all of which lean directly on the rot resistance.

Locust versus locust, and locust versus the tropics

Against its own genus-mate honey locust, black locust trades some of that species' pink lustre and lower price for genuinely superior outdoor durability — the choice is straightforward once you know whether the piece will see weather. Against imported decking species like ipe or teak, black locust gives up density and hardness but not the essential property that matters for a post or a deck board: it survives ground contact and weather for decades, at a fraction of the exotic import price and without the shipping distance.

What people actually ask about black locust

Does black locust need any chemical treatment at all for a ground-contact post, or is the natural durability really enough on its own? No treatment is needed — that's the entire commercial case for the species; adding a preservative wouldn't meaningfully extend a heartwood post's already multi-decade service life and most suppliers don't offer treated black locust at all.

Are the seeds specifically more toxic than the bark, or is the whole tree equally hazardous? The seeds and inner bark carry the highest concentration of the toxic compounds, which is why livestock poisoning cases trace mainly to grazing on fresh bark or fallen seed pods rather than to any exposure a woodworker handling cured lumber would realistically encounter.

Does black locust's UV colour shift stop once it reaches russet brown, or does it keep darkening indefinitely? It effectively stabilizes once the surface has fully weathered to that russet tone — further sun exposure at that point mostly affects surface texture and sheen rather than driving the colour meaningfully darker.

Black locust's density and Janka hardness above are The Wood Database's 12%-moisture-content measurements; its shrinkage traces to Table 4-3 of the USDA Forest Products Laboratory's Wood Handbook (FPL-GTR-190, 2010) — the same handbook whose durability ratings back up this page's rot-resistance claim.