January 1, 2011

1 - Prune tree to remove codominant trunk and prevent trunk splitting.

Yard and Garden - January 1, 2011


I am hoping you can tell my husband and me what we can do for the desert willow we have in our backyard. We moved to our home about 5 years ago and it was already in the garden. It has grown quite a bit and we have trimmed it as necessary. The trunk is about 4 inches in diameter. We live in the foothills, east of Tramway, so there are a lot of heavy winds and it has been wrenched around countless times, but it blooms and is green every year. It sits next to a courtyard wall that is about 8 feet tall and the tree is well over that wall by maybe another 5 or 6 feet. The main trunk has two main branches that form the canopy. One of those branches has another growing off it that is equal in size. Both branches are about 2 inches in diameter. Those two connected branches have recently developed a deep fissure between them, threatening to split the whole thing in half. I am afraid that it would also break off a huge portion of the main trunk, as well. As soon as I noticed it, about two weeks ago, we taped the whole thing together with silver duct tape, but I am sure that is not a solution!

The tree is in an area of the garden that has many annual bulbs and plants around it, gets lots of watering on a regular basis (during growing season) and also gets a new layer of eucalyptus mulch each spring. I do not know if those conditions are the best for this tree, either! I would very much appreciate any information you could provide for me. I hate the thought of losing any living thing if there is something I can do.

- Patty Mc



The situation you have described is called codominance, two branches trying to be trunks, in addition to a separate trunk from ground level. The branches that are co-equal in size and forming a fissure can be a problem as you are anticipating. If wind, ice, or snow loads cause spreading of the branches, the trunk below the branches can be split. You will want to do something to avoid this.

One option is to use bolts to hold the branches together. However, this can create problems later if the tree is cut and a chainsaw is used in the area with the bolts, especially if the tree has overgrown and concealed the bolts. A second, and I think, better approach is to prune the codominant trunks to subordinate or remove one. Without seeing the tree, it is impossible to tell you exactly how to prune. Your goal will be to remove much of the weight of branches and foliage from one of the competing trunks. It is sometimes possible to train the pruned branch to become more wide spreading and to develop a branch collar (natural protective mechanism where the branch joins the trunk). However, sometimes the best choice is removal of one of the branches before the trunk can split.

The use of any material that completely encircles a branch can girdle that branch, but if I am envisioning your use of the duct tape correctly, you have not completely circled the branches, so it can be used temporarily while you work to reduce the problem by pruning. The duct tape should be removed (and then replaced if necessary) at least once a year to accommodate growth of the branch in diameter.

Regarding the care the tree is currently receiving, it sounds like the tree is growing well, so those conditions can persist. Be careful about digging around the base of the tree (under the branch canopy) where you are growing bulbs. Digging can damage the roots and cause problems.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


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