Hardwood or Hard Wood?

Several times a year I will get a question that resembles something like this: "Is (insert tree species) a hardwood?" Usually, it involves a species like yellow poplar which is a species that tends to be less dense than many other species. I almost always have them clarify what they really want to know by asking "Do you want to know if it is a hardwood or do you want to know if it is a hard wood?"

The terms hardwood and softwood actually have little to do with the hardness of the wood itself. Hardwood refers to a category of tree species (angiosperm) that have broad leaves and true flowers with the seed being enclosed. Softwood trees (not to be confused with soft wood) are gymnosperms. The term gymnosperm means naked seed so their seeds do not have a covering like an angiosperm. Another difference between hardwoods and softwoods can be found in the anatomy of the wood itself. Hardwoods have a much more complex structure than softwoods but the main the difference is hardwood contains pores or vessels while softwoods do not. A simple way to think about it is this: hardwoods have broad leaves and fruit, where softwoods have cones and needles.

The term hard wood (as opposed to hardwood) is referring to a physical property of the wood itself: hardness. Hardness refers to the ability of the wood to resist indentation. It is measured by determining the load that is required to embed a steel ball with an 11.28 mm diameter to one-half its diameter. The force is usually measured in newtons.

Fortunately, testing to determine the hardness of many species has already been done and put into tables that can easily be referenced. One reference that can be used is Wood Handbook: Wood as an Engineered Material (PDF) which is a publication offered by the USDA Forest Products Laboratory.

Below are hardness values for various hardwood and softwood species.

If we compare the hardness of yellow poplar to the hardness of the softwoods listed we find that it is harder than some, but not as hard as others. Aspen and basswood are also softer than most of the softwoods. Given that the hardness property for yellow poplar is only 2400 N, it would not hold up well in a flooring situation. It would be very prone to scratches and dents. Maple, ash, or oak would be a much better choice for something like flooring. Certain pine species (softwoods) would perform better than yellow poplar as flooring. Yellow poplar is much better suited for something like moulding or drawer sides because it weighs less than other species.

Many hardwood species have a hardness that is greater than many softwood species; however, that is not true for every species. Is yellow poplar (or basswood and aspen) a hardwood? Yes, it is. Is it a hard wood? That depends on what the individual considers to be a hard wood and perhaps the application it will be used in.