Heartwood is the dense, often darker core of a tree trunk that no longer carries water. Sapwood is the lighter outer ring of wood that actively transports water from the roots to the leaves. Every tree has both zones, and the difference between them matters for everything from how a tree survives to which boards you pick for a building project.
How Sapwood Becomes Heartwood
A tree grows new wood just beneath its bark each year. That fresh wood is sapwood, and it’s alive in the sense that it contains living cells and functions as the tree’s plumbing system, pulling water upward through the trunk. As the tree adds newer rings on the outside, the older inner rings of sapwood gradually retire from duty.
During this transition, the living cells in the inner sapwood die. The tiny tubes that once carried water get blocked by balloon-like growths called tyloses, which form when cell walls bulge inward and seal off the passageways. At the same time, the wood absorbs a mix of chemical compounds, often called extractives, that act as natural preservatives. These extractives are also what give heartwood its characteristic color. The result is a hard inner cylinder of dead cells saturated with protective chemistry, surrounded by an active outer sleeve of working sapwood.
Color and Appearance
The most obvious difference is color. In many species, heartwood is noticeably darker. Black walnut heartwood is rich chocolate brown, while its sapwood is pale cream. Cherry heartwood deepens to reddish-brown; cedar heartwood ranges from reddish to pinkish tones. The extractive compounds deposited during heartwood formation are responsible for these hues, and they vary dramatically from one species to the next.
Not every tree follows this pattern, though. Hemlock, most spruces, true firs, aspen, cottonwood, basswood, and holly show very little color difference between heartwood and sapwood. These are sometimes called “sapwood trees” because you can’t easily tell where one zone ends and the other begins just by looking at a cross-section. For these species, chemical staining tests are sometimes needed to locate the boundary.
What Each Zone Does in a Living Tree
Sapwood is the working tissue. It moves water and dissolved minerals from the roots up to the canopy, sometimes tens of meters in a tall tree. It also stores sugars and starch in its living cells. A tree can’t survive without functional sapwood.
Heartwood, by contrast, is structural. It forms the central pillar that keeps the tree upright and rigid. It no longer transports water or stores energy, but the extractives packed into its cells resist fungi and insects, helping protect the tree’s interior from decay. You can occasionally find a hollow tree still standing and alive because it lost its heartwood to rot but retained enough sapwood to function. It’s structurally compromised, but biologically still viable.
Moisture Content
Because sapwood actively carries water, you might assume it’s always wetter than heartwood. That’s true in many species, but not all. In red oak, green sapwood runs about 71% moisture content compared to roughly 42% for heartwood. But in yellow-poplar, the relationship flips: heartwood measured around 96% moisture content while sapwood came in near 54%. The pattern depends on species-specific anatomy and how water fills or drains from different cell types.
These differences matter practically. Wetter wood takes longer to dry in a kiln, and uneven moisture between sapwood and heartwood boards can cause warping, checking, or other defects during the drying process. Knowing the moisture profile of a given species helps sawyers and woodworkers plan for drying time and predict how boards will behave.
Decay Resistance
This is where heartwood and sapwood diverge most sharply in practical terms. Sapwood of nearly all species has no natural durability against decay. It doesn’t matter if you’re working with cedar, redwood, or white oak: the sapwood portion will rot when exposed to moisture and fungi, roughly on par with any other non-durable wood.
Heartwood durability, on the other hand, varies enormously by species. Researchers rate it on a four-point scale: very resistant, resistant, moderately resistant, or nonresistant. Species like black locust, old-growth redwood, and certain tropical hardwoods fall into the “very resistant” category, meaning their heartwood can last decades in ground contact without chemical treatment. Heartwood from poplar, birch, or most spruces offers almost no resistance at all. The protective extractives deposited during heartwood formation are the key variable, and different species produce very different chemical cocktails.
Permeability and Treatment
Sapwood is far more permeable than heartwood. Its open, unblocked tubes readily absorb liquids, which makes it easy to pressure-treat with chemical preservatives. If you’re buying treated lumber for a deck or fence, the preservative has mostly penetrated the sapwood portion of each board.
Heartwood resists liquid penetration because its tubes are sealed with tyloses and filled with extractives. This is a double-edged quality. For species with naturally durable heartwood (like cedar or white oak), the impermeability is a feature: the wood shrugs off water on its own. For species with non-durable heartwood, it’s a problem, because the wood won’t accept preservative treatment easily and will still rot when exposed to moisture.
This permeability difference also explains why white oak makes excellent barrel wood (its heartwood is watertight) while red oak doesn’t hold liquid nearly as well (its heartwood has fewer tyloses blocking the pores).
Strength and Weight
In most species, heartwood and sapwood are comparable in structural strength. The basic cell structure is the same; heartwood is simply older sapwood with blocked tubes and added chemistry. The chemical deposits can add some weight, particularly in dense tropical species like ebony and rosewood, where the pores become heavily filled with soluble compounds. But for common construction timber, you won’t find a meaningful strength difference between a heartwood board and a sapwood board from the same tree, assuming they grew at the same rate and dried to the same moisture content.
Which to Choose for Projects
Your choice depends on the application. For outdoor use where the wood will face rain, soil contact, or humidity, heartwood from a naturally durable species is the better pick. Cedar, redwood, white oak, and black locust heartwood can handle exposure without chemical treatment. If you’re using a less durable species outdoors, sapwood actually has an advantage: it absorbs preservative treatment far more effectively.
For interior work and furniture, the choice is mostly aesthetic. Heartwood is generally preferred for its richer color and visual consistency, and it’s considered more ornamental in most species. But there are exceptions. Yellow pine interior trim and maple flooring are two cases where the paler sapwood is traditionally favored over the darker heartwood.
For woodturning, carving, or any project that involves staining, keep in mind that sapwood absorbs stain and finish more readily than heartwood. Mixing the two in a single visible surface can produce blotchy, uneven color unless you plan for it deliberately as a design feature.