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Monday - April 12, 2010

From: Austin, TX
Region: Southwest
Topic: Transplants, Trees
Title: Mexicana Redbud failing to bloom in Austin
Answered by: Barbara Medford

QUESTION:

I have a Mexican Redbud that I bought last fall. It is now April (I'm in Austin), and the leaves have emerged, but the tree did not flower. So . . . what should I do to get it to flower? (The tree is now approximately 7-8 ft tall and in a 30 gallon container, but I would like to plant it in the ground and was hoping to use it as an understory tree near some Shumard oaks.)

ANSWER:

This is not the first time this year that we have been asked why their redbuds didn't bloom before they began to leaf out, which is the normal procedure. Cercis canadensis var. mexicana (Mexican redbud), although it grows in Austin, is native to the Big Bend area according to this USDA Plant Profile. The Cercis canadensis var. texensis (Texas redbud) around us did, indeed, bloom out and now the blooms are gone and the leaves are all out. We have been unable to find any explanation of why some redbuds bloomed and some did not. We thought it might be that the tree was not mature enough to bloom, but a plant in a 30-gallon container must be pretty big. We are theorizing, but the tree may be suffering from transplant shock because it was dug up and put in a container and left in it over the winter. We had some pretty sudden, fast freezes earlier in the Winter, and a pot in a container is always more susceptible to cold damage than one in the warm ground. Another possibility is that you were trying to be too kind to the new tree and put a lot of high nitrogen fertilizer in the pot, where it was more or less trapped with the roots. Plants love nitrogen fertilizer, but it often causes them to make more leaves (which is what nitrogen-rich fertilizers do, as for grass) and fewer flowers. And yet another possibility is that one of our late freezes just nipped the flower buds at the right (or wrong) time. The tree will not attempt to bloom again before early next Spring; once the leaves are out, the blooming is over. If you get the tree planted (see below), we believe it will bloom normally next Spring, and be fine.

Our first suggestion is to get that tree in the ground right away, before it gets any hotter. Ordinarily, woody plants should be transplanted in the late Fall or Winter, but this one has already been out of the ground too long. Don't fertilize, native plants are adapted to the soils and environment they have grown in for millions of years, and do not need artificial fertilizers. When you remove the rootball from the pot, check the roots and make sure they are not wrapping around inside the container, or rootbound. If they are, clip some of them; it may seem ruthless, but it will force the tree to put new rootlets out into the soil, instead of strangling itself. Another suggestion is to reconsider using it as an understory for the Quercus shumardii (Shumard's oak). An oak tree is a valuable landscape asset, so your first question should probably be: "What will not harm the tree?" Oak trees do not like competition. Their heavy shade in summer, and the year-round presence of the root system are disadvantages for plants beneath the oak. Furthermore, the mature oak is known to have the quality of allelopathy, which means they emit substances that will inhibit the growth of plants beneath them; these substances can come from roots, twigs, or leaves. The Mexican Redbud prefers full sun, although it can tolerate some light shade.

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