Question
Near the base (5ft dia) of my willow a large knot has rotting to a cavity of about a sq. foot or more.  Can it be filling in with something.

Answer
A tree cavity is similar to a cavity in your tooth. Without proper treatment, the situation will only get worse. Unfortunately, cutting down on sweets and brushing regularly will not help to prevent a tree cavity.

In simple terms, a cavity in a tree is a neglected bark injury that can be the result of many factors. The most common are improper pruning, mechanical injury and storm damage. Storm damage and injury from gnawing animals and landscape equipment (lawnmowers and string trimmers) contribute to bark injury by tearing the bark from the tree trunk or branch. When bark injury has occurred, the exposed sapwood or heartwood is more susceptible to attack by fungi that initiate the decay process. Insects and animals such as raccoons, woodpeckers and squirrels that inhabit tree cavities utilize the tree wound as the front door to their new home. Carpenter ants, in particular, will excavate tunnels throughout the decayed portion of the tree and excrete wood preserving enzymes as they do so. While these preservatives are beneficial to the tree, the tunnels the ants create allow water to accumulate. Excess water simply facilitates the wood rotting process.

In the past, tree cavities were filled with many different materials including cement, asphalt, masonry and even rocks and gravel. These cavity-filling techniques came to the United States from Europe after the turn of the century. In fact, in Europe having cavities filled in trees was sign of affluence. Only the very wealthy could afford such service for their trees. Today we know that these materials are very abrasive. Natural tree movements, such as swaying and twisting, rub the inside surface of the tree cavity against the filling, further weakening the tree's defensive walls and allowing decay to expand. In addition, we know that these materials do not allow the tree to bend and therefore renders the tree more susceptible to storm damage. Also, because of these tree movements the cavity-filling materials do not bond with wood. Gaps are often the result and these gaps frequently trap water. This dark and moist environment allows decay fungi to proliferate.

Many early "tree surgeons" used to drill holes in the tree into the bottom of the cavity in order to facilitate water removal. We now know that any cut, drilled hole or tube installed will cause more damage to the protective walls, which leads to further decay. Biologically, there is no reason to drain water from the cavity. Draining a cavity allows fast-growing, oxygen-requiring fungi to invade. Water-saturated wood has little oxygen present and is inhabited by slow-growing organisms.

Another more recent cavity treatment involved scraping out as much of the decayed wood as possible and then filling the cavity with a urethane or polyurethane foam to fill the hole. It was believed that when the foam expanded it would eliminate all the air in the cavity and prevent the accumulation of water. Besides, the foam was supposed to be flexible and move with the natural movement of the tree. What the advocates of this practice failed to remember is that the scraping action destroyed any protective barriers that were already in place further weakening the tree's defenses and allowing decay to expand.

So how is it some trees seems to flourish in spite of their cavity? When a tree is wounded, its uses a natural defense mechanism called "compartmentalization" to create both a physical and a chemical barrier between the wound wood and the rest of the tree. The exact nature of this barrier is not yet fully understood but is apparently is related to toxic phenols that are produced by the tree at the site of injury. These chemicals help the tree to establish boundaries that reduce the spread of pathogens to uninjured parts of the tree. Compartmentalization is under moderate to strong genetic control. And, the ability of microorganisms to compete successfully with others and to spread within the compartments is also under genetic control. As a result there is a lot of variation between different species of trees in how well they compartmentalize wounds. Trees like American basswood and littleleaf linden, boxelder and silver maple, aspen and birch, red and northern pin oak, and hackberry and willows are know as "poor" compartmentalizers. Rapid and extensive decay is common in these species.

How should trees with cavities be treated? Recent research shows that it is better to leave the cavity open - remember no type of drainage, sterilization, fill material, wound paint, or scraping treatment stops decay - and simply take the necessary measures required to improve the overall health of the tree. A healthy tree has the strength to compartmentalize and wall-off decay.