Nakamoto Zourin foresters working in the mountains of Hiroshima, Japan.

Understanding Tree Anatomy in Wood Siding: How Composition Affects Appearance

When looking at a cross-section of a tree trunk, you’ll notice several distinct sections that make up the wood. These sections have different properties and uses when it comes to lumber. In this post, we’ll take a closer look at four key parts of wood anatomy, how they appear in sugi cypress specifically, and how they contribute to the overall appearance of finished yakisugi products. Every piece of wood is unique, and their color depends on the specific board’s makeup of heartwood, sapwood, earlywood, and latewood.

Heartwood vs. Sapwood

Heartwood is the dark, inner core of the trunk. Sapwood is the light, outer section.

Heartwood 

The heartwood is the innermost part of the trunk, composed of dead wood cells that no longer transport nutrients. It often has a darker color than the younger sapwood. Heartwood tends to be more brittle but also more durable and resistant to decay. Its high proportion of extractives (oils, resins, etc.) also make it less permeable, helping prevent rot. Sugi cypress has a dark, reddish brown heartwood that contrasts heavily with the lighter sapwood.  

Sapwood 

The sapwood is the younger, outer section of wood that actively transports water and dissolved nutrients in the tree. It has lighter coloring due to lower extractive content. Freshly cut sapwood has high moisture levels but is also vulnerable to insects and fungi unless quickly dried. Over time, sapwood transitions into heartwood as cells die and become filled with extractives. All wood starts out as sapwood, but as the tree grows the heartwood must expand to provide structural support. 

A sugi stump several months after the tree was harvested. Note the moss and mildew growing on the nutrient rich, less-dense sapwood section, but not the inert heartwood core of the trunk.

Effects on Board Appearance

In terms of how they will affect color variation, sapwood is lighter and more receptive to oil pigments than heartwood. It soaks up the oil and remains closer to the wood’s original color. Heartwood is less receptive to the oil pigment than sapwood. Pigment will dry on top of the surface of the wood, and more closely resemble the selected oil color. This is the greatest factor contributing to color variation from one end of the board to the other, as well as between boards from the same tree. We often recommend that after opening up your yakisugi siding for installation you lay all of the boards out and sort them to match your aesthetic preference for the wall, illustrated in the video below.

Four different finished boards from the same bunk of Pika-Pika® after allowing the same oil finish to dry on each of them. The variation in color is caused by different ratios of heartwood and sapwood.

Earlywood vs. Latewood

The dense latewood growth rings of a tree become the visible dark grain we see in milled lumber, shown above. This contrasts heavily with the blonde earlywood sections of sugi cypress.

Earlywood 

In temperate climates where seasons alternate between periods of rapid growth and dormancy, wood cells form differently at different times of the year. Earlywood, the light part of the rings, refers to large-diameter wood cells produced early in the growing season when warm conditions are optimal for fast growth. The lower density of earlywood makes it weaker mechanically. Sugi cypress, the species of wood we use for all of our yakisugi products, has porous earlywood that allows for fantastic heat treatment penetration. 

Latewood 

Latewood, the dark part of the rings, consists of smaller, thicker-walled wood cells formed later in the growing season when cold winter conditions slow down growth. Denser latewood has more structural strength than earlywood. After being milled, the varying proportions of earlywood and latewood rings create the distinctive grain patterns we see in processed lumber, shown above. Sugi cypress has a hard latewood that develops a very durable soot layer. This gives the siding an additional 30-40 years longevity without any additional maintenance 

Effects on Board Appearance

In yakisugi, the ratio of earlywood to latewood impacts the appearance of the surface differently depending on the type of surface selected. In Suyaki, where sugi cypress has been charred black and left unbrushed, there is no visible contrast between the light earlywood and the dark latewood grain. However, when brushed to remove the topmost layer of soot, the hard, blackened latewood rings remain while the soft, lighter earlywood layer below becomes exposed. This layer of wood has still been heat treated and retains the beneficial properties of yakisugi, while also allowing for more oil finish colors to be applied to further alter the appearance of the siding. Compared to unburned sugi cypress, the grain is far darker and more pronounced. Since it is also more dense, pigment will roll off of the latewood grain, leaving it mostly black regardless of oil color.

Suyaki®, sugi cypress that has been burned, but not brushed. Note that both heartwood/sapwood and earlywood/latewood ratios are no longer apparent.
Pika-Pika®, sugi cypress that has been burned and then brushed twice to fully remove the soot layer, leaving the latewood growth rings as a prominent black grain pattern. The difference in heartwood/sapwood ratio is also visible between boards.
Unoiled Pika-Pika® showcasing the increased contrast in grain after burning & brushing the surface of the boards. The difference in heartwood/sapwood content is also visually apparent.
Unburned sugi cypress boards showcasing pronounced difference between heartwood/sapwood, but less contrast between earlywood/latewood rings than charred & brushed surfaces.

Resaw Pattern, Board Orientation and Feng Shui

We use a simple-sawn resaw pattern used for our products. Due to this, the visible heartwood/sapwood ratio varies between boards from the same tree. After a board has been milled it may contain primarily sapwood or heartwood. Most boards contain a mixture of both. Visible grain on the surface of the board is the result of latewood proximity and angle in relation to the saw, creating unique grain patterns that help identify the natural top and bottom orientation of the board.

Learn more about utilizing natural board orientation for proper feng shui here.

Learn more useful tips for yakisugi installation and maintenance on our YouTube channel

Putting it All Together

It is challenging to specialize in natural building materials since they are more inconsistent than engineered materials. Every single tree will have wood with different density, grain, and color characteristics. Each stick of wood from that tree will be primarily heartwood at one end and sapwood at the other. This makes the sampling process incredibly challenging. Small samples rarely paint a full visual picture of the product spanning an entire wall. Engineered materials are processed and more visually consistent, but they are not as beautiful or wholesome as natural materials.

Color and tone variation in our natural wood products is one of their most desirable characteristics, but it’s also a subject most people have a lot of questions about. We hope that by sharing our knowledge of how wood anatomy factors into the unique appearance of each final product it will help garner greater appreciation for the beauty and visual play of wood products across the board. The most important thing to remember is that change over time is considered beautiful with natural materials. For those who prefer a uniform siding that will stay the same color throughout its lifetime, painted wood products are probably a better fit. However, if beauty and natural textures are your drivers in choosing a cladding then natural wood siding can’t be beat.

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