Q: Is it possible to prune a 30-foot mimosa tree down to 8 feet tall?
Monday, January 4, 2016
Q: I have a very tall mimosa tree that flowers 30 to 35 feet above ground and cannot be enjoyed without straining to look straight up. The tree has three main trunks that I had to chain at about the 8-foot mark to prevent it from splitting and falling due to the weight during the bloom. There are some "shoots" off the three main trunks, but I had them "limbed" last year because of dead branches and the heavy weight on the main trunks. I would like to prune the tree down to about 8 feet, which I know would eliminate the entire bloom, but I hope to have the new bloom start at this level. When is the latest point in spring that this can be accomplished without damaging the tree all together?
A: Reducing the mimosa's height from approximately 30 feet down to about 8 feet would place a lot of stress on the tree. The leaves make food for the tree through photosynthesis in the form of carbohydrates. By pruning down to 8 feet, you would be losing a considerable amount of carbohydrate-producing foliage and would also be losing a considerable amount of wood, where the tree stores its excess carbohydrates.
The tree may survive such a loss, but there is also the possibility it might die from the shock. It would be better if the height were reduced in stages over a two-to three-year period.
Another problem is that the "limbing up" you did has removed lateral (side) branches and in effect has forced the tree to grow taller. Proper pruning technique indicates one should always reduce the height of a pruned leader back to a side branch. However, in the case of your limbed up tree, there are no side branches, so a reduction cut will in effect be a heading cut, which will stimulate many sprouts to form. These sprouts can be weakly attached around a stub that will likely begin to decay. Consequently, these sprouts can break off as they enlarge in future years.
With the current tree structure you have, you don't have any good options. Furthermore, your comment that the tree has been chained to prevent it from splitting is also an indication that the current 30-foot structure with multiple leaders is not sustainable. Plus, holding leaders together with a chain can cause the chain to girdle (strangle) the leaders, causing decline.
I suggest you contact a Certified Arborist, which you can locate in your area using ASCA's Find a Consulting Arborist. The arborist can provide both information on pruning as well as cabling your mimosa tree. Also, consider planting a replacement mimosa now so it will be flowering within the next two or three years.
Responder: John Leffingwell, RCA #442, Pleasanton, CA