Black locust belongs to the group of ring-porous deciduous tree species.
The thin sapwood (about 2-6 annual rings) is bright yellow, the colour of the heartwood varies from yellowish-brown to greenish-grey. There are no extractives and extraneous materials in the sapwood, therefore its durability and mechanical properties fall behind those of the heartwood. The heartwood formation takes place in black locust in two manners: heart wood forming substances get deposited on the one hand, and tyloses penetrate into the vessels and clog them on the other. The most important of these substances are tannins, resins, pigments, gum and robinetins. Above all, tannins and robinetins are essential for the durability of black locust heartwood. Sapwood contains much moisture and many simple organic compounds, and so fungi and insects are attracted.
The wood of black locust, like that of ring-porous trees in general, has distinct annual rings and latewood/earlywood boundaries. According to our measurements, the ratio of latewood in trees from a 30-35 year old black locust stand amounts to 77%, and the average annual ring width is 3.0-3.5 mm.