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Growing in the Desert Column

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Another Record Freeze!

by Mark Dimmitt

Photos by Mark Dimmitt unless noted

(This article may only be reprinted with the author's permission.)

What? Again? Southern Arizona had catastrophic freezes in 2007 and 2011, which were both rated as 30- to 50-year events. Now we’ve had another in January 2013, the third severe freeze in six years. The 2011 freeze was the worst in most of the Southwest, largely because it was accompanied by high winds that added to the heat loss and blew away many frost covers. (It has long been assumed that it cannot freeze in Tucson if there is any wind, which prevents cold air drainage.) The lows in February 2011 at my place at the eastern base of the Tucson Mountains were two nights of 18° F, with a high of only 36 between them. It was colder on the east side of the basin.

There are numerous microclimates in the greater Tucson area. On our normally calm winter nights cold air drains downhill. Therefore the coldest areas are valley bottoms and washes that drain the mountains; these areas experience several nights in the teens almost every winter. In contrast, south-facing slopes such as in the Catalina foothills may not have any frost for a decade or more. But this general rule can be broken on windy nights and severe Arctic outbreaks, during which the coldest areas are wherever the frigid air mass descends to the ground. In 2011 the Catalina foothills were as cold as the valley bottoms, and the normally safe tropical plants there were seriously damaged.

This year's January freeze broke many temperature records, but in the absence of wind there appears to have been less damage than from the 2011 freeze in most areas. (See reports from a few colleagues below. But I wonder if the reports of less damage stems in part from the fact that so many plants perished in 2011 and weren't around to be tested in 2013?) My neighborhood seems to have taken the worst of the 2013 event. I'm normally 6 degrees colder than the "official" airport temperature. That was true for this freeze too, but the forecast was spectacularly wrong, as usual. (In my 34 years in Tucson, the weather service has not once forecast a severe freeze more than a day in advance. It appears that their models are so heavily weighted toward the mean temperature for the date that they can't predict extremes.) On the morning of the fourth day of the five-day freeze, the forecast was for a low of 23 F. But that night it fell to 17 at the airport, and 11 at my house. A thermometer at the lowest point on my property recorded 7. In 2011 I did not have the high winds that most people suffered, so consequently this was the most damaging frost I've ever had.

Some noteworthy results of a list of survivors and casualties in my collection during the past two freezes:

- Welwitschia mirabilis planted in the ground and covered with a blanket froze to the caudex, but is already regrowing (Figure 1).

- Boophone spp., including B. ernesti-ruschii, suffered only minor leaf damage under a clear plastic cover (Figure 2). Boophone disticha in the ground in the open lost nearly all foliage.

- Encephalartos horridus is unscathed (Figure 3); it was in an enclosed patio that got down to 24°.

- Zamia furfuracea next to the Encephalartos above and covered with a blanket lost all its leaves for the second time in three years. Zamia floridana in the open outdoors also lost all leaves, but another one against a wall is undamaged. Gene Joseph reported the same pattern. This is a pretty tough cycad.

- Euphorbia decaryi in the patio at 24°: undamaged.

- Aloe claviflora was untouched under a cover. My several other supposedly hardy aloes, except for A. variegata, suffered severe damage in the open.

- Trichocereus pachanoi plants were badly damaged, especially unestablished cuttings (Figure 4 foreground). Cuttings of the related T. scopulicolus (background of Figure 4) were undamaged, as was the mature plant (Figure 5).

- Ferocactus herrerae (Figure 6) were severely damaged; some will probably die. I have most of the barrel species except those from the far south of the genus' range, and none were damaged except this one.

- Haworthia truncata in the patio and under a blanket: undamaged (Figure 7). Crassula argentea (jade plant) on the same bench perished.

So have we learned anything yet about landscaping in Tucson? With each of these catastrophes, we add to our knowledge of what is reliable here, and which plants need protection at what temperature. But a bigger lesson is becoming clearer.

The events of the past few years are consistent with the models of global warming. The actual warming is barely noticeable at our latitude. A more significant effect of the warming is climate destabilization, resulting in more frequent extremes. If the models are right, we can expect both more hard freezes AND more heat waves; more droughts AND more floods. Moreover, the models predict that the waves in the jet stream will slow down, meaning that extreme events will be not only more common, but will last longer. January's freeze lasted five consecutive nights; that hasn't happened since 1971. If this is our future, we should be considering: a) choosing hardier plants for our collections; b) spending more money on frost blankets; or 3) moving south. (Hint: Aduana, near Alamos in southern Sonora, has never recorded a frost. Sounds great, huh?)

Notes from around town:
Bob Web reported from the Barry Goldwater range in western Arizona: I noticed that quite a few native plants were damaged by the freeze, which my companion said reached 15-17 F over there. Encelia farinosa, Parkinsonia microphylla, various Ambrosia (particularly A. ambrosioides) were seriously damaged, with dead or dying leaves and branches. (My palo verdes seem to be undamaged. –MAD)

Matt Johnson (far east side): It got down to 8 F in my shade house, which is a new record low. I didn't have a thermometer placed in the lower part of our lot, but it usually runs another 4 F colder in that area. For the five coldest nights, my lows were 14, 12, 12, 8 & 15 F. I don’t recall a previous freeze when the coldest night was so far into the cold spell. Too early to tell about damage on a lot of things, but the two large Agave vilmoriniana in the patio that made it through both 2007 and 2011 with relatively minor damage have extensive yellowing, and my last surviving Stenocereus alamosensis also appears to have suffered considerably more damage than in 2011. It looks like a few individual plants of at least two Thelocactus species in containers in the shade house were hit, in spite of being covered. I haven't seen that before. So far, senitas (which had stem tips covered) appear to be okay.

Michael Chamberland: I was in Sonora during the first night of this run of cold weather. In Magdalena the freeze made the front page of the local newspaper. There was a photo of someone holding up a sheet of ice. Later in the day, driving through Curcurpe, the river crossing was strewn with masses of ice - and there were organ pipes on the hills. We explored around El Bajio and I found organ pipes, Fouquieria macdougallii, and Jatropha cordata growing on rocky hills, all regrowing from damage from the 2011 freeze. Fortunately I had covered or moved indoors hundreds of plants before the trip. I returned in time to move hundreds more inside, not to trust mere covering, before the coldest night set in. At my house I had a minimum of 18F. I do not know how accurate my thermometer is, but the same device recorded a maximum low of 13F in 2011. Between the "warmer" temperature, lack of wind, and unprecedented number of potted plants I moved into the house, I expect the damage will be far less than in 2011.

Gene Joseph (west of U of A): I had 15° out back at our home property. I still haven't entirely figured out why it's so cold back there (drainage? shaded during day?), but our porch was 19°. My regular thermometer was under frost cloth, so I can’t count that. The damage this year was not as bad as 2011, I think primarily because of the lack of wind. Mark, that wind was a whole new aspect of cold damage that will be hard to pin down in a frost damage list. In 2011 I resolved not to change my protocol for severe cold, based on one year of extreme cold, supposedly a multi decade event, but now I have done a 180°. I am planning heaters for more greenhouses and setting up more frost cloth (which worked very well this year because of the lack of wind). I hate global weirding!!

David Palzkill: I had 15F at my nursery on Ina; this is the coldest I’ve ever recorded there. I’ve gotten down to 16F several times over the past 10 years. South side of Ferocactus in containers damaged.

Miles Anderson (Avra Valley): We were 14.4, the lowest we've ever have had, but without the wind, not as bad as 2 years ago (15F).

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