Question
A live oak with a ~5 foot diam trunk, which branches out into three 2 foot diam limbs, which in turn branch into smaller limbs. One of the 2 foot limbs branched into two ~1.5 ft limbs. 30 years ago one of these 1.5 ft limbs broke off close to the "Wye", leaving a "stump" with a horizontal surface. The "stump" has never closed over with bark, and has a ~5" diam vertical cavity which looks deep--going down into the 2 foot diam limb. The bark on the 2 ft diam limb below the "stump" now has a ~4 ft long vertical "crack" in it, which I'm told indicates progression of rot. How should the "stump" be covered--with an "umbrella' to keep out water but admit air; or other treatment??

Answer
Benefits from filling the cavity are questionable at best. Since the fill material will expand and contract at a different rate than tree wood it can create problems of its own. The strength of a hollow tree comes from the new wood produced after the injury, not from material used to fill a cavity. Remove hollow trees that appear weak and are likely to fall.

Sometimes the cavity will contain water. The old recommendation was to drill holes below the cavity so the water could drain. However, drilling holes will break the barrier that keeps the decay from invading healthy wood. If water has been present for one or more growing seasons, the tree has already adjusted.
When a tree is wounded, it uses its natural defense of compartmentalization to create a barrier between the wound and the rest of the tree. Recent research shows that it is better to leave the cavity open and 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.

Filling of hollow trees, a process called "cavity filling," was practiced by arborists for many years. Thanks to modern research, it has been discovered that cavity filling is not needed to support or improve the health of hollow trees.

Tree experts have found that cavity filling with cement can actually damage a hollow tree. According to Bob Rouse, Staff Arborist at the National Arborist Association, "The column of cement created in the tree by a cavity fill doesn抰 move, just like a column on a building, but the tree is always moving. It sways with the wind constantly. The rubbing created by the swaying tree and the solid column of cement further damages the tree."

Decay organisms, such as rot fungi, that created the hollow in the first place are able to take advantage of the new injuries created by the rubbing and invade the healthy tissue of the tree. Rouse adds, "If that wasn抰 bad enough, the cement holds moisture, creating a favorable environment in the filled cavity for the decay organisms!" Tree experts explain that it is much the same as when carpenters place a vapor barrier between a house抯 foundation and the wooden sills. If they put the sills directly on the concrete foundation, the wood will rot rapidly. If you place cement in a tree cavity, it will speed the wood decay! If cavity filling is desired for aesthetic reasons, there are some new synthetic foams that can be sprayed into the cavity by professional arborists. These materials will bend with the swaying tree, but Rouse warns, "There is really no reason to fill a cavity; it doesn抰 improve the tree抯 health and doesn抰 offer any added support. If structural support of a tree is required, a professional arborist will recommend cables, braces, or tree guys, not cavity filling." Source: National Arborist Association  


I would Leave the area as is and spray the area with an insecticide for wood borers Merit is one of these type insecticides.


No need to cover the hole the fungi has already begun the decaying process and it will get enough moisture with or without the cover --the cover will not effect the amount of decay and in my opinion do nothing but cost you. I would do nothing --you could fill the hollow with potting soil and use as a planter for flowers.