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A: There are those who suspect Wildflower Center volunteers are the culpable and capable culprits. Yet, others think staff members play some, albeit small, role. You can torture us with your plant questions, but we will never reveal the Green Guru's secret identity.

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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Wednesday - December 02, 2009

From: Marble Falls, TX
Region: Southwest
Topic: Cacti and Succulents
Title: Potted crown of thorns cactus cold tolerance in Marble Falls, TX
Answered by: Barbara Medford

QUESTION:

I have a large potted crown-of-thorns cactus. Can it remain outside this winter in the Texas Hill Country?

ANSWER:

When we originally received this question, we went to our Native Plant Database and found Koeberlinia spinosa (crown of thorns), which grows natively in far South Texas and the Big Bend area, where temperatures are warmer than they are in Central Texas; however, this plant is known to be hardy to 0 deg. F. So, we replied that we thought the potted plant would be all right outside. Turns out we let the common name curse keep us from finding the plant actually referred to in the question. This old-fashioned houseplant, Euphorbia milii, crown of thorns, is more likely the plant about which the inquiry was sent. 

The Lady Bird Johnson Wildflower Center is dedicated to the use, care and propagation of plants native not only to North America but to the area in which it is being grown. This puts this native of Madagascar our of our realm of expertise. We can tell you that while we said the Koeberlinia spinosa could be wintered over outside in a pot in the Hill Country of Texas, we find that the Euphorbia milii is only hardy from Zones 9b to 11. Central Texas is generally Zone 8a. The plant itself would be threatened by a hard frost in this area, and certainly a plant with its roots exposed in a pot would be even more likely to be damaged by cold weather. Euphorbia milii should be treated as an indoor potted plant over the winter. Here is more information on this plant from Floridata and pictures from Google

One further note: neither the Koeberlinia spinosa nor the Euphorbia milii are cacti. Koeberlinia spinosa is a member of the Capparaceae or Caper family, while Euphorbia milii is a member of the Euphorbiaceae or Spurge family.

 

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