DO NOT FILL TREE CAVITIES WITH CONCRETE

GARDEN PATCH POINTERS
By Robert and Hoberley Schuler
UC Davis & OSU Master Gardeners and NGC & CGCI
Garden Study and Landscaping Design Master Consultant

DO NOT FILL TREE CAVITIES WITH CONCRETE

Q: I have a multi trunk Albizia Julibrissin that is 7-8 years old. It started out with 5 trunks but one died off the first year and another was growing out of the center and slowly got choked out over the next few years and I eventually had to cut it out. The tree has flourished and I love the pretty silky flowers and aroma, but hate the annual fallout over my patio area. The tree has given me the shade that I was seeking on my patio. The problem is that after removing the center trunk, the tree has developed a cavity in the center which fills up with flower debris and sprinkler water fills it to a standing depth of 8-10 inches.

I am concerned that with this constant bowl of water and debris it will start to rot the tree and eventually end up as its demise. I am thinking that I should somehow fill the conical cavity which is 14 inches deep and 6-7 inches at the top of the inverted cone.

My thought was to fill the cavity with cement so as to allow the water to spill over and the tree to grow around it. The base of the tree is 16 inches in diameter. The remaining three trunks are 7 to 9 inches in diameter each and are as one at the base up to 14 inches above ground level where the trunks branch off. Cliff Straight, Fontana

A: Your tree is a “Mimosa” or “silk tree”. It is native to Iran and Japan. It likes the warmth of the Inland Valleys, especially Fontana. FIRST, do not fill tree cavities with concrete!! In the “old Days” cavities were filled with concrete in hopes that this would strengthen the trunk. The University of Florida has researched this problem and finds that this could really cause problems in Fontana because of the winds! When the tree moves normally it can cause tree abrasion problems and when the wind in Fontana really causes the tree to become abrasive against the concrete, we have more problems.

Cavities in trees do not need to be filled with concrete. Spray insulation has been used to keep children and animals out. The University of Florida recommends the use of “spray insulation”. Do not attempt to clean out the tree cavity or hollow. Do not drill holes in the bottom of the cavity to remove water. This is likely to spread decay to healthy living portions of the tree.

Any tree that has limbs that form a crotch with several limbs is susceptible to developing a weak structure where the wind and/or children will cause the limb to separate and fall from the trunk.

I would strongly suggest that you contact a local tree arborist regarding the condition of your tree. I really admired my silk trees when I lived in Rialto and Redlands. Yes, they were messy at times but many good trees are!

Q: My second question. I had a flowering red plum in my front yard. All of a sudden within one year the tree died. I noted that there were a lot of toadstools, 3 to 4 feet from the trunk. Several others in the neighborhood suffered a similar fate. About 35 feet away from where the plum tree is located, I had a clump of three birch trees. Slowly one died off one branch at a time. I tried pruning back all the dying branches but it continued declining over the next couple years and another tree this year is starting to die. There are more toadstools. The septic tank is in the area, but I have no problems with it.

A: The toadstools are a result of dead wood or roots. I removed a Locust tree from my front lawn. Since the roots were not removed, I kept mowing toadstools for months, waiting for the roots to die! A flowering plum tree will normally not last more than 25 years. They are also susceptible to termites.

I suspect that while your house was being built, sub-contractors dumped miscellaneous paint liquids, drywall or plaster pieces etc. around the house. Depending on the exterior grades of your house and the amount of fill, you could have construction materials and liquids 2- 4 feet below grade causing your tree roots not to carry proper nutrition to your trees.

Do you have a garden question? E-mail giving your name, city where you live to:
rhschuler@frontier.com
Robert H. Schuler, M.G.