Roofing Tar Probably Toxic to Trees

Q. I have a Mimosa tree that is about 15 – 20 years old with four main branches forming the trunk of the tree. These four branches form a "well" about 8 inches deep which catches rainwater and remains wet for about a week or so after a rain. Someone recommended filling this "well" with roofing tar so that the tree doesn't rot at the point where the main branches come together.

Not the readers picture but a silk tree that started to yellow. It could be poor drainage but it also could be the beginning of a disease called Mimosa wilt.

A. Mimosa, or silk tree, is not a long-lived tree due to disease problems. At 15 – 20 years of age it's getting close to the end of its life. Its short life is because of a disease problem called Mimosa wilt disease that invades some of the limbs and causes limb or stem dieback. The tree usually dies in a matter of months once it gets the disease.

            The reason I’m mentioning this is because you should realize this tree may die regardless of what you do so prepare yourself for that eventuality. It may die because of nothing you did or didn’t do. You don’t see a lot of old silk trees around for a good reason.

            I think filling that well with roofing tar is a bad idea and will damage the tree. Rainfall is so infrequent in our desert climate that I don’t think it’s worth the effort or concern. If this catchment is filling with water because of frequent irrigations by sprinklers, then this situation is different.

            Secondly, any compound made from petroleum is potentially damaging to plants. Petroleum products are normally not applied to plants because of their toxicity. You run the risk of damaging parts of the trunk in contact with the roofing tar.

            I appreciate the concern you have, but in my opinion if that tree has survived this long, then leave it alone.