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Tuesday - November 27, 2007

From: Round Rock, TX
Region: Southwest
Topic: Pruning, Trees
Title: Mimosa shape
Answered by: Barbara Medford


I planted a summer chocolate mimosa, and although it has bloomed lovely foliage, it has two main branches growing in a vee shape. Is this normal? Do I need to do anything to spur the growth in a more upright manner?


The first thing we do when we are asked about a specific plant is check to see if it is a native of North America. We do this by searching on our Native Plant Database. At the Lady Bird Johnson Wildflower Center we are dedicated to encouraging the use and preservation of plants native to North America and, more especially, native or well adapted to the area in which they are being grown.

First the good news: Desmanthus illinoensis (Illinois bundleflower) , also called a mimosa, is a native of North America and does appear on the Edwards Plateau. Now, the bad news: The Albizia julibrissin, c. "Summer Chocolate" isn't it. Both are legumes, members of the Fabaceae family, but the native is a shrub, and what is commonly referred to as a mimosa or silk tree is a relatively small, fast growing tree. The mimosa tree is a native of southern and eastern Asia and the burgundy-leaved cultivar "Summer Chocolate" was recently developed in Japan.

So, we'll just talk about mimosas, in general, because the culture of the "Summer Chocolate" seems to be no different from that of other mimosas. It has a beautiful burgundy leaf, turning that color after a green spring. In answer to your question about the "V" shape, yes, this is fairly common. Not necessarily a good thing, but common. The mimosa often, but not always, develops multiple trunks. Trimming off the smaller of the two branches or trunks would not solve your problem and would only leave the tree lopsided. The mimosa has tender bark which is easily damaged, but it requires pruning to help develop a strong structure. It is susceptible to breakage either at the crotch due to poor collar formation or because the wood itself is weak and tends to break. Pruning excess top growth, evenly on both trunks would help to lighten the load while the trunks increase in girth with growth and perhaps become less liable to snap. This is the right time of year to be pruning, while the tree is semi-dormant. Mimosa trees tend to develop a draping, hanging shape, with stems growing toward the ground. These are good candidates for pruning, as that is extra weight and unbalance pulling on the crotch of the tree. Don't prune off more than a third of the top growth, concentrating on shape and relieving the strain on the crotch. For more information on the mimosa, see this University of Florida Extension website for details on culture, disease, etc.



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