Beastmaster,California: “Yesterday we braced a Modesto ash that had split. Another company that specializes in [the] diagnosis of tree diseases and defects recommended us to do the repair. The tree had several cavities, including one large one in the lower trunk where the condiment limb (half the tree) had started to split. They had filled the hollow with foam. I thought that filling hollows or cavities with anything was no longer practiced, and that moisture trapped in there could do more harm then good. I’m wondering if others ever use foam or what your opinion on this would be.”

Treeseer,Southeast U.S.A.: “ANSI says no way does foam add support. I’ve read where Shigo says it can. I don’t know enough about how much it traps moisture and how much it lessens twisting one way or the other. I’ve used it a few times, but never went back to check.”

M.D.Vaden,Oregon: “I decided to start using expanding foam to seal cavities, like when bees are known to build hives. My preference would be summer application when the cavity is drier. The plan is to cover the hole with a template, punch a hole for injection and drape a plastic skirt to catch foam that oozes out so it does not stick to the outside of the tree. I’ll probably be using the black foam that is used for water features/ponds, as if it’s safe for fish, like the label says, it’s probably extremely low for toxicity. Once firmed the next day, it slices right off with a machete, saw or blade. The foams are also flexible.”

SilentElk,Colorado: “If support is want you want, I hear concrete with a few sticks of rebar are the ticket. Although it doesn’t flex worth a darn.”

Justice,NewJersey: “I have used foam several times. Not to add structure, but to keep moisture out and water from ponding in a hollow. Idea is to seal out air and moisture and, thus, slow decay. I have been monitoring them for a year now, and they appear to be okay. I also put galvanized mesh inside the opening and foam out, covering the mesh with an inch or two to keep squirrels out. They go right through the foam otherwise.”

Highasatree,Ont.,Canada: “I’ve heard if you shape the foam and then apply a layer of Bondo (car body filler), you can make the wound look inconspicuous on the tree.”

Upnorth,Calgary,Canada: “Wouldn’t filling with foam also trap moisture in the cavity, as well as reduce air circulation?”

Urbicide,Ohio: “I have a number of trees on my woodlot that have hollow cavities that seem to collect all sorts of debris, as well as water. My thinking would be to clean out the cavities, dry them out, then fill the cavity with something to keep the stuff out of them.”

Highasatree,Ontario,Canada:“My thinking is to leave the cavity open so that there is air circulation and to drill a hole so any water in the cavity can drain out.”

Justice,NewJersey: “If the cavity is in the side of a tree, and you use foam when it is dry, and you apply it correctly, it seals itself against the tree, thus sealing out moisture and air. The right foam is waterproof. Otherwise, the hollows just keep collecting water. This way the water runs down the side of the tree and over the foam and continues down the tree instead of filling the cavity with water again.

“This is different if the hollow is in a codominant situation, or where the decay is in a vertical hollow, or if you used a filler that is porous.”

Lxt,Pennsylvania: “I don’t know about air circulation and all that, seems to me a cavity, if left untreated, will grow larger (ask your dentist). I fill with foam after making sure the cavity is clean. I don’t use cement and definitely not rebar, if you ever have to take that tree down, it will be a pain!”

FallenAngel,Pennsylvania: “I was taught to use foam and have done it for years. It doesn’t stop the decay, but slows the process. After the foam has expanded and hardened, you can use a technique called bark tracing to permanently seal the wound. As far as support to the heartwood and cambium layers, [I] don’t think it will add any, but it is better than nothing at all. Please don’t use concrete and rebar. Chains are too expensive, and it makes for a hell of a time removing the tree in the future.”

Lawmart,Ont.,Canada: “I have been filling cavities with foam for over 12 years now, and it is not meant for any type of structural strength, but as a barrier for insects and little critters from increasing the cavity size. It helps if the cavity is dry, and to clean out the cavity first and, if you can, to spray to kill any insects that are possibly there. I use metal screening over the face of the cavity to stop any big critters from reentering the hole. You can spread Bondo/fiberglass over the face and paint it to match the tree color if you so desire. I have clients [that] paint faces on it; whatever floats your boat. For whoever disagrees with the practice, it does not hurt the tree, from my experience. If you leave the cavity open, there is a greater chance that the walls will be breached and decay will increase.”

“In Your Own Words” is contributed from the Tree Services forums on Visit them, and join in the discussions!