How Trees Grow and How That Becomes Wood

Every board you’ve ever held was once a layer of living cells, quietly added to a tree trunk during a single growing season.

Wood is not manufactured. It is grown.

This guide explains how trees grow in height vs width, how growth rings form (earlywood vs latewood), and why sapwood and heartwood behave differently in real timber.

How Trees Grow: Height and Width

Trees grow in two distinct ways:

• Height growth occurs at the tips of branches and the main stem, where specialised tissues called apical meristems add new length. Once a branch forms at a certain height, it stays at that height — the trunk doesn't push it upward.
• Width growth occurs through a thin layer of living tissue just under the bark called the cambium. This is where most of the wood in a tree is produced.

The cambium produces new cells in two directions:

• inward — forming new wood (xylem)
• outward — forming new bark (phloem)

Each growing season, the cambium wraps a new layer of wood around the outside of the existing trunk. Over decades, these layers accumulate to form the bulk of the tree.

Growth Rings

If you look at the end grain of a board, you’ll see a series of concentric rings. In many temperate species, each ring usually represents one growing season (roughly one year).

Rings form because trees grow at different rates throughout the year:

• In spring, when water and nutrients are abundant, the tree grows rapidly. The cells produced are larger and lighter in colour. This is called earlywood.
• Later in the season, growth slows. The cells produced are smaller and denser. This darker band is called latewood.

Together, one band of earlywood and one band of latewood form a single annual growth ring.

What rings tell you

Growth rings are more than just decoration. They carry information that matters in the workshop:

• Tight rings (slow growth) often correlate with denser, stronger timber in many softwoods, but it varies by species
• Wide rings (fast growth) can correlate with lighter wood in some species, but ring width alone is not a reliable predictor of stability
• Uneven ring widths suggest the tree experienced changing conditions — drought, competition, damage, or other stress

Learning to read growth rings is one of the most practical skills a woodworker can develop.

Sapwood and Heartwood

As a tree grows older, the inner wood gradually stops transporting water. The tree fills these older cells with chemical extractives and effectively retires them from active duty.

This inner wood becomes heartwood. The outer, younger wood that still transports water is called sapwood.

	Sapwood	Heartwood
Colour	Usually lighter	Usually darker
Function	Actively transports water	No longer active
Durability	Generally less durable	Often more naturally durable
Typical use	Interior applications	Often preferred for outdoor use

In many species, heartwood is significantly more resistant to decay, which is why it’s preferred for outdoor applications like fencing, decking, and cladding.

Why This Matters

When you cut a piece of timber, you’re not just cutting a material — you’re cutting through the growth history of a living organism.

The rings you see reveal how the tree grew. The colour differences tell you whether you’re looking at sapwood or heartwood. The density you feel in your hands is a direct product of how fast or slow that tree added each layer.

Every board carries the story of the tree it came from. Learning to read that story is one of the foundations of working well with timber.