Two Parts of the Same Tree
The differences between heartwood and sapwood are related to how a tree grows. If you were to crosscut the trunk of a mature hardwood tree and remove the bark and outer cambium layer (which eventually becomes new bark), you’d notice two distinct sections of the trunk. The outer, lighter-colored wood is the sapwood. This is the “working” part of the tree, since water and sap flow through the sapwood much like blood through your arteries, veins, and capillaries. While this part of the trunk is vital to the tree when it is living, it doesn’t make for very good stock for woodworking. Because sapwood contains a lot of moisture, it shrinks considerably when it dries, and it is much more susceptible to fungus. The inner, darker section of the trunk is the heartwood. Heartwood is formed from old, “retired” sapwood and becomes the strong spine of the tree. Heartwood is preferred for woodworking, as it is far less susceptible to fungus and contains much less moisture than sapwood, which means it will shrink less when it dries. Once the tree has “promoted” some of its sapwood to heartwood status, the sap will stop flowing through that part of the wood and the converting material essentially dies. As part of the conversion process, the pores begin to plug up with organic matter, which causes the cell walls to change color due to the presence of chemicals often generically called extractives. The extractives typically are responsible for the rich character and colors found in heartwoods.
Decay-Resistance
Some wood species, such as redwood and cedar, are touted as naturally decay-resistant materials that are less susceptible to rot and insects than other softwood, like pine or spruce. While this claim has some validity, it’s important to note that natural decay-resistance often applies primarily to the heartwood. So if you’re planning a woodworking project for outdoor use and you really want it to last, you should probably pony up for “all-heartwood” or “all-heart” grade when buying lumber like redwood and cedar. Also, bear in mind that all wood weathers and turns gray outdoors if it’s not stained or otherwise protected from UV degradation.
Should Sapwood Be Discarded?
Many experienced woodworkers actually remove the sapwood and use only heartwood for their furniture projects. But this doesn’t mean that the trimmed sapwood is junk that’s good only for the woodpile. While the sapwood will never be as strong, rich, or beautiful as the heartwood, it still has its uses. Just be certain to dry the sapwood thoroughly and to use it in projects where a little bit of movement will not cause problems. Also plan to seal it thoroughly, with paint or polyurethane, to prevent it from soaking up environmental moisture.
