Hardwoods and Softwoods
Most of the tree species in Tennessee are hardwoods. But often people will refer to some species as “soft hardwoods” or “hard hardwoods.” To make matters more confusing, pines can be grouped as “hard pines” or “soft pines”, even though all pines are softwoods. So how can wood be hard and soft at the same time?
The classification of tree species as “hardwoods” or “softwoods” is based on a botanical distinction. Softwoods (or “gymnosperms”) are the conifers, the needle-bearing trees that are often evergreen. Softwoods that grow in Tennessee include the pines, eastern redcedar and cypress. Hardwoods (or “angiosperms”) are broad-leaved, mostly deciduous trees. Oaks, poplar, maples and cherry are all hardwoods. Magnolias are also hardwoods, even though some of them keep their leaves in the winter.
The botanical distinction between hardwoods and softwoods is also reflected in their wood anatomy. Softwoods have a simple anatomy, with only a few different cell types. Hardwoods have more cell types, including large vessels than can be seen as “pores” with the naked eye.
The woods of both hardwood and softwoods vary in their hardness, and this is the source of confusion: hard and soft wood occurs in different species of both hardwood and softwood tree species. Hardness is a function of the heaviness, or density, of the wood: the amount of wood material compared to air spaces. Hard woods are denser, soft woods contain more air.
On average, the wood of softwoods is softer than hardwoods. Hickory (a hardwood) wood is harder than cypress (a softwood) wood and oak is harder than cedar. However, there are many exceptions to this trend. The Southern pines are softwoods but they are denser than yellow poplar, which is a hardwood. An extreme example is balsa wood, the very soft wood that is used to make model airplanes, which comes from a hardwood tree. Because of the wide range of wood density in the hardwood tree species, people will often distinguish between “hard hardwoods” (eg. hickory, oak, ash) and “soft hardwoods” (e.g. yellow poplar, gum, willow). Likewise the pines, which are softwoods, can be organized into “hard” (e.g. Southern, Virginia ) and “soft” (white, sugar) groupings.
In summary, trees can be labeled as hardwoods or softwoods based on their biology and wood anatomy. Wood is rated as hard or soft based on its density.