Q. I never thought I would be emailing you with a question, but here it is. We use white oak quite a bit and recently have had a sawmill offer us some post oak that he says is a white oak. Is this a reasonable substitute? What is post oak?
A. I think my educator hat is on this morning as I write the answer. So, rather than just a one word answer, here is some technical, yet practical info.
There is a group of oak species (about 20 commercial species) called “white oak.” Trees in the white oak group have rounded leaves (vs. red oaks with pointed ends), have sweet acorns (red oak acorns are bitter), and are often considered to be water tight (for most species within this group).
White oaks have a band of large vessels within each annual growth ring, like red oaks, but in white, the vessels are typically plugged with chemical junk that makes the wood waterproof. The plugs are called tyloses. (One species in the white oak group that has very few tyloses is chestnut oak; this species would not be used for wine barrels.)
Note that within the lumber market, the name "white oak" refers to the group and not to the specific species.
Within the white oak group is a species called white oak. This white oak species is very common and widespread; it is often the species we get when buying white oak lumber. This is indeed a topnotch species and is likely what you are used to processing.
Post oak, another species in the white oak group, is the same density as white oak, but is about 10 percent weaker and 20 percent more bendable. The hardness is the same in both. Processing for both is the same.
Post oak got its name because it is widely used for fence posts and has excellent natural decay resistance so it lasts for decades. Post oak lumber for furniture and cabinets often has a negative preference is because of its generally poor lumber quality.
What makes this species more difficult to talk about is that it has hybridized naturally with a dozen other species in the white oak group. The properties of this mixed breed wood are somewhat difficult to predict.
Contributing to the poor quality is the slow growth, plus the fact that branches, when they die, are so decay resistant that they can persist on the stem for decades. This means a lot of pin knots in the lumber; the knots will have included bark plus the knot itself is not well attached to the surrounding wood. (In softwoods, we call these black knots). Grading post oak lumber is difficult using the standard grading rules. Then there is the distorted grain around the knots that contributes to poor quality.
The best post oak grows in bottomlands in eastern Texas and in the Mississippi River valley in western Mississippi, southeastern Arkansas, and Louisiana. This variation of post oak is called Delta post oak and does find its way into furniture and cabinet manufacturing. Another fairly good variety is sand post oak that occurs from southeastern Virginia south to central Florida and west to eastern Oklahoma, and south and central Texas. It is most common on coastal plains and is scattered in the Piedmont. Post oak elsewhere is not too good in quality.
In answer to your question, using the above information, I think Delta post oak could be considered as a reasonable substitute for white oak.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.