A lupia[1]​ or lobanillo[2]​ is a rounded bulge formed in the trunk "Trunk (botany)") or the branches of a tree, generating deformations whose wood is full of small knots produced by latent buds. The wood of these excretions, known commercially as root wood,[3]​ is highly appreciated in cabinetmaking for its beautiful appearance.

A lupia is a type of gall that affects the woody tissue of plants, the result of some disturbance suffered by a tree. It can be caused by an injury through which microorganisms are introduced into the plant that generate the disordered growth of woody fibers. Most nodules develop below the ground, attached to the roots "Root (botany)") as a type of tumors that are usually not discovered until the tree dies or falls. These nodes sometimes appear as clusters of bulbous protuberances connected by a rope-like root system. Almost all of the wood in the nodule is covered by bark, even if it is underground. The most common causes that generate these nodules are insect attacks, pruning or accidental fracture of branches, which lead to infections caused by fungi, molds, bacteria or viruses.[4]​.

In some tree species, knots can grow to large sizes. Among the largest are those formed in coastal redwoods (Sequoia sempervirens), in which they reach sizes of up to 26 feet (8 m), and can surround the entire trunk. When there is moisture, these nodes can grow new redwood trees. Another place where some of the largest lupias in the world have been located is Port McNeill (British Columbia)&action=edit&redlink=1 "Port McNeill (British Columbia) (not yet redacted)").[5]​[6]​.

One of the largest known knots was found around 1984 in the small town of Tamworth, New South Wales. It was situated 6.4 feet (2 m) high, with a strange shape resembling a trombone. In January 2009, this node was controversially removed from its original location and moved to a public school in the central New South Wales town of Dubbo.

Lupias produce root wood, a peculiar type of wood highly appreciated for its beauty and rarity, characterized by the swirling appearance of its fibers. It is used by furniture makers, artists and wood sculptors. There are several well-known types of rootwood (each from a particular species). They are highly valued, and are cut into veneers for furniture, door inlays, picture frames, household objects, panels and moldings, automobile interiors, musical instruments, and wood turning.