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Why are hardwood and softwood not reliable descriptions of how hard a tree's wood is?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

The terms "hardwood" and "softwood" are often misunderstood as descriptions of a wood's physical hardness, but they are actually botanical classifications based on the type of tree from which the wood is derived.12

Hardwood refers to the wood of angiospermous trees, which are typically broad-leaved dicots.23
Softwood refers to the wood of coniferous trees, such as pines or firs.1

These labels are not reliable indicators of physical density or hardness because there is significant overlap in the mechanical properties of these groups. Physical hardness and strength are primarily determined by a wood's density, which is influenced by factors like cell wall thickness and the proportion of latewood.45

For example, some softwoods can be quite dense and hard, while certain hardwoods are relatively soft and lightweight (such as balsa, which is technically a hardwood despite its very low density). Therefore, density—rather than the botanical classification—remains the most accurate index for assessing the actual strength and hardness of wood.45