It's cold out there, and we're filling our wood stoves and fireplaces with kindling and logs. There are tens of thousands of species of trees in the world, many of which burn very differently. So what's in your woodpile?
One of the most notable differences between wood is whether it's a hardwood or a softwood. When it comes to firewood, hardwood generally refers to any wood that's hard and dense, and will burn more efficiently. Softwoods are, yes, less dense, not as hard, and burn more quickly. Botanically speaking, however, hardwoods and softwoods are categorized not by their density, but by the way they reproduce.
Hardwood trees are angiosperms—plants whose seeds develop in an ovary that develops into a protective vessel. Think of stone fruits or acorns. Softwoods are gymnosperms—plants whose seeds develop in the the scale of a cone, or at the end of short stalks. Think of pine cones or ginkgo seeds. The rate of growth is also a determining factor—in general, the faster a tree grows, the softer the wood it produces will be. This makes sense if you consider wood as a solar energy repository. When wood is burned, the sun's stored energy produces heat and light. The slower it grows, the moer energy it will have stored.
This means that a hardwood (angiosperm) is not necessarily harder, and a softwood (gymnosperm), not always softer. A good example of the discrepancy is balsa wood (Ochroma pyramidale), one of the lightest, least dense woods, considered a hardwood. And some fast-growing deciduous species, such as aspen and cottonwood, produce softer wood than slower growing conifers like jack pine.
Popular softwoods are pine, spruce, cedar, and fir. They are evergreen. Popular hardwoods are mahogany, ash, teak, walnut, oak, elm, aspen, birch, and maple. They are mostly deciduous, the evergreen exceptions being holly, boxwood, and holm oak.
All wood has about the same amount of energy per pound; it's the density that varies. The more dense the wood, the more energy it will have per volume. Not that softwood doesn’t have its place at the hearth; easy to ignite and quick to burn, it’s good for kindling.