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Frost tolerance of Sonoran Native Plants

© Jared R. Shortman

So as it is snowing now in Tucson I think about cold hardiness of Sonoran plants. I have tested out many plants and have been both surprised and also confused by my finds. I get pretty cold at my place which means last winter I got down into the teens (19 degrees F.) and this is unusually cold for most of Tucson. I am the first open space in a long way and my hypothesis is that all the cold flows down Park avenue here in Tucson and flows and lingers into my space. I think you all are growing an array of different things and many of you are in different climates so take this information with a grain of salt.

Jatropha vernicosa may prove to have a little hardiness. It is a winter grower from Baja California supposedly (though I have my doubts) close to J. cordata. I find it may be closer to J. malacophylla though they are all in the same subgenus. I have not had plants freeze at temps nearing mid thirties F. That in fact happened the other morning when I left a few large plants out (previously spending time in warm greenhouse). We will see tonight if I can resist the urge to put all Jatrophas back in the greenhouse.

You all of course know by now that the limberbush (Jatropha cardiophylla) is very hardy. My plants have not frozen at high teens. This is not true about new cuttings or seedlings or plants with fresh new growth. But mature plants are very hardy and every home in arid regions of USDA zone 9 and above should have at least 3 plants in the landscape. They are great!

Acacia willardiana (palo blanco, white barked acacia) has done excellent in the cold. I had a few plants spend all winter outside (some in one gallons and some in 5 gallons). All that happened was defoliation. The phyllodes (succulent petioles) have not dropped and not a bit of tip damage (just little leaflets dropped). I am impressed by this because they survived temperatures in the mid twenties F so far. They had absolutely no cover or sheltering of any kind, just open sky.

Sebastiania biloculare (=Sapium biloculare) Arizona jumping bean (obviously an Arizona native as well) also has fared well with the slightest protection provided there is not fresh new growth on plants. I harden plants off before the winter comes. Seedlings may not fare as well. I think from my experiences plants can take at lease high twenties (around 28 degrees) but only further testing will prove them tough.