Heartwood is the dense, dark-colored wood at the center of a tree trunk. When you look at a tree stump, you'll notice the inner rings are often darker than the outer rings. That darker center is the heartwood.

While a tree is young, all its wood helps transport water and nutrients from the roots to the leaves. But as the tree grows older, the inner wood stops carrying water and becomes heartwood. The tree fills these old cells with chemicals that make the wood harder, darker, and more resistant to rot and insects. This transforms the wood from a living transport system into a strong inner skeleton that helps the tree stand tall.

Heartwood is what makes trees like oak, walnut, and cedar so valuable for building. Because it's denser and more rot-resistant than the lighter sapwood on the outside, heartwood makes excellent furniture, fence posts, and construction lumber that lasts for decades. A cedar fence made from heartwood can stand strong for twenty years or more, while sapwood might rot away in just a few.

Interestingly, heartwood is technically dead tissue, but it's far from useless. It provides the structural strength that allows massive trees to tower hundreds of feet into the air without toppling over.