Native Plants

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Thursday - January 17, 2008

From: Austin, TX
Region: Southwest
Topic: Invasive Plants
Title: Viability of Ehretia anacua tree for Austin, TX
Answered by: Barbara Medford

QUESTION:

I live in Austin, TX and I see you've recommended the Ehretia anacua tree for someone living here. Also, my arborist mentioned it as a replacement for non-native, invasive trees. However, the database says that it might freeze back in Austin. How tall can this tree get in Austin and if it freezes will that kill the tree? It was also listed at the nursery as "fast" grower. What's the meaning of fast growth with respect to tree growth per year?

ANSWER:

We found (we think) the previous question to which you referred, in which several replacements were listed for non-native, possibly invasive trees in Austin. One of these was the Ehretia anacua (knockaway). We agree that the mention of freeze was a little vague as to whether they were talking about freezing in Travis County or farther north, to Dallas. It is mainly native to South Texas, but according to this Texas A&M Education site, it can be found as far north as Austin. The USDA map shows it distributed all over Texas, with the county distribution map showing it in Travis County and some other counties north of that. Other information we found said that as far north as Dallas, it could freeze, but that it was freeze-deciduous, which meant it would be nearly evergreen farther south, and probably would survive freezing anyway. And another cool thing: It is considered deer resistant, a big selling point in the Austin area. Not deer proof, there is no such thing, but resistant is good.

And in reference to your question about the meaning of fast growth with respect to tree growth, we found this quote from the Manual of Woody Landscape Plants by Michael Dirr:

"Rate of growth refers to the vertical increase in growth unless specified differently. Rate, as is true for size, is influenced by numerous variables such as soil, drainage, water, fertility, light, exposure, ad infinitum.

"The designation slow means the plant grows 12" or less per year; medium refers to 13 to 24" of growth; and fast to 25" or greater."

Our database shows the estimated mature size as being between 36' and 72'.