Why does pine have knots

Because the branch is dead prior to the time it is engulfed by the expanding, living tree trunk tissue, it is often only weakly connected to the surrounding tissues. As a result, when a board containing a knot is cut in the lumber mill, the weakly attached knot can fall out, creating a hole a knothole.

The wood grain weaves its way around the knot. If you have a closer look, notice how the grain pattern slopes up to 90 degrees to get around wood knots. This is why many consumers enjoy having decorative wood with knots. Wood strength in knots can be assessed in two ways: tensile strength and compressive strength. These measurements allow us to predict how much load weight wood can safely bear before collapsing. Tensile stress elongates or expands an object.

Compressive stress shortens or compresses the material. Veneers are therefore graded in terms of their colour and the presence of defects such as knots. Clear sheets will be used for the surface layers to maximise the value of the final panel.

Unfortunately, the majority of veneer sheets have one or more defects, so they must be improved to increase their value. One method which is very commonly used is patching, whereby the defect is stamped out by a die and replaced with a similarly-shaped piece of clear veneer see Figure 2.

The patches are initially held in place by friction, because the patch is the same size as the hole. Later the patch is kept in place by the glue between the veneers. This is the case in Figure 1; the knots are appreciably darker than the clear wood. Within a factory, just one size and shape of patch will be used. Some knots are formed by fungal infections, however, and can spread to other trees on your property as well. As a tree grows and increases the circumference of its trunk, the growing trunk begins to overtake the branches that grow out from it.

Knots form around these branches, building up trunk material as the tree continues to expand, advises Oregon State University Extension. Since the branches are still growing as they are overtaken by the trunk, the knot that forms is solid and contains living wood throughout. The wood of the knot is typically tougher than the surrounding wood and may form a bulge around the branch emerging from its center.

If a branch becomes injured or otherwise dies while still attached to the tree, a loose knot forms as the trunk grows larger.