Heart pine, defined
Forum contributors define heart pine discuss differences between heartwood, sapwood and pith. August 12, 2001
Exactly what is heart pine?
Heart pine is the center of a pine tree where the wood has "died", hardened and ceased to pass nutrients up the tree. It is not only harder than the sap wood, but generally doesn't move as much in drying. It dries to a much darker color than the sapwood.
Heart pine is noted for its use in pine flooring, where it is generally cut with vertical grain. It wears very well.
Heart pine, in the south, is often associated with long leaf pine, which is a slow growing and dense wood. Slash and loblolly will both produce dense wood, under certain growing conditions, that will make for good heart lumber.
What is the difference between heart pine and pith?
Pith is that line you see in the middle of the tree if you split it in two. It is formed by the growth bud and could be considered ring #0. The pith isn't pretty, isn't strong and most folks will try to find some way to discard or hide it. To hide it, it must be "boxed" in the board or timber. Timbers that are boxed have more of a tendency to develop longitudinal cracks, but because of the equal stress on either side of the pith, tend to remain straight. A board with the pith in it may develop cracks, which would ruin it for any work but rough carpentry. Exposed pith in a fine pine heartwood floor is a no-no. An exception being a floor of wide boards, and even then it's better hidden.
Heart pine is the name given to the heartwood of the four southern pines. Heartwood does not typically develop until the tree is 30+ years old, so it is not common in most of the trees cut today. The conversion from sapwood to heartwood involves the deposition of materials in the empty spots (the cell openings or the lumens), which increases the density and hardness, and also provides the red color and decay resistance. The chemicals also tend to make the wood move less when the humidity changes, compared to sapwood.
Gene Wengert, forum technical advisor
So the heart is what puts the red in redwood, etc.
The dark wood in the middle of the log--walnut: black on the inside, white on the outside; Douglas fir: red or pink in, yellow out; cedar: pink or red in, tan out.