Sapwood is the lighter colored outer wood of a tree. The living area of a tree is the cambium layer which is located at the outermost edge of the sapwood, just under the bark. This is where trees grow in girth. The cambium forms the xylem and phloem, which carry water (xylem) and food (phloem) up and down the trunk. When the growing season is over these wood cells harden into sapwood.

Depending on the species, the sapwood will change color as natural chemicals in the wood, called extractives, mature the wood cells into heartwood. Heartwood is darker in color and does not contain living wood cells. These extractives give heartwood color but also make the wood more resistant to rot and insect infestation. Rot and incest resistance in heartwood varies by species. For example, white oak is more rot resistant then red oak, but black locust is more rot resistant then white oak.

In species such as hard and soft maple, yellow poplar, and white ash the sapwood is more desirable. These species are normally harvested in the cooler times of the year. The white wood will sap stain quickly in the warm summer months. This is due to the abundance of sugars in the sapwood and because the trees are growing. Once the tree is cut grow stops and the fresh wood cells start to decay, which leads to staining.

Black walnut is one of the most recognizable hardwood species due to its dark chocolate brown heartwood. Sapwood in black walnut lumber is bright white and considered a defect or less desirable quality. To increase the amount of heartwood in walnut lumber, it is steamed be fore kiln drying. Once fully steamed the sapwood turns brown to match the heartwood.