Monday, January 08, 2018

Winter variations

The extremes of climate can really be madness-inducing. The area I live has long been classified as in USDA Plant Hardiness Zone 8b, but over the past decade I've seen new maps that place me just within the borders of Zone 9a, due to progressively warmer temperatures. For those of you confused by this "Zone" talk, let me offer this summation: Some fruiting plants need specific amounts of cold weather in which to go dormant and build up energy reserves to flower and produce fruit. Other types of plants cannot tolerate temperatures that drop below a certain point. Hardiness zones are a general attempt to classify the temperature environment of a region to aid in selecting appropriate plants to grow. It's not perfect, as it doesn't take into account annual rainfall, maximum summer temperatures, etc., but it's a good starting place. Got that? Good.

Several years ago, we moved from the south side of New Braunfels to the north side. The move was approximately 10 miles north, as the crow (or grackle, around here) flies. It also involved an increase in elevation of close to 100 feet, as we went from Blackland Prairie to the edge of the Texas Hill Country. That doesn't seem like much of a change on a global scale, but I've noticed that during the winter, our lows are consistently 4-5F degrees lower than those predicted for New Braunfels once the mercury dips below 40F or so. That means predictions of 38F or thereabouts can result in freezes for us. What's more, predictions for hard freezes can mean we're in store for some seriously cold weather. Case in point--last week, January 1-2, our overnight low was predicted to drop to 23F. When I woke up, the actual temperature was 16F! That is the lowest temperature I've recorded at the new house in the three-plus years we've lived here.

It also offers an opportunity to contrast this winter with last year's. The 2016-17 winter was unusually warm for us. We had one hard freeze in December and a bare handful of light freezes the rest of the season. We ended up recording in the neighborhood of 390 chill hours, that is, 390 hours in which the recorded winter temperature was below 45F. By any measure, that's a very low number, sufficient only for extremely low-chill cultivars of peaches and other fruit to produce. That kind of winter is more common to the Rio Grande Valley or Florida than Central Texas. Contrast that to this winter, where I've already logged 514 chill hours, with the traditional heart of winter--January and February--still to come (at this rate, we could easily exceed 1,000 chill hours, which is plenty more than any plant I grow needs in order to fruit). Needless to say, I'm hoping for some nice plum, pear and even apple production this spring.

But the overall amount of cold isn't the whole story--it's how the cold is delivered that has an equally powerful impact. Last year, after an unusually warm autumn, an arctic blast hit us, driving temperatures down to 22F. That cold weather killed all the fronds on the mature Mexican fan palm out front, killed the satsumas in my backyard down almost to the roots (despite their being covered by frost cloth), killed an Austin pomegranate down to the roots, killed a fig tree down to the roots, froze the buds and fruits off my Loquats, etc. My banana plants survived, but I'd wrapped them in C9 lights and frost blankets. That cold just hammered everything I had. Because of the damage done by 22F, one might expect this year's 16F to be much worse, right? Wrong. I know--I'm surprised myself. The Mexican fan palm took no damage. My pomegranates lost their leaves, but had no stem die-back. Ditto the figs. The re-grown satsumas are fine. I even had in-ground Bird-of-Paradise and plumeria survive unscathed (with cover and lights). What gives? Acclimation, that's what. The run-up to that 16F plunge consisted of a week of 40F weather, then another week of light freezes progressing to generally harder freezes. Essentially, the plants had plenty of time to adapt and prepare themselves for the cold, and weathered the low temperatures surprisingly well. The previous year, the hard freeze came out of nowhere--the weather was warm and mild, and as far as the plants were concerned, they were still enjoying the last days of summer (we were swimming in the pool until mid-October, it was so unusually warm).

The takeaway is that while plants may not have intelligence, they've still evolved ways to prepare for extremes of weather. The catch is that they need environmental signals to trigger those protective, physiological changes. The trouble is that climate change fosters more extreme weather--higher highs, lower lows, and unpredictability in between. That wreaks havoc on plants that've evolved over the centuries to thrive in a particular type of climate, or been bred for high cold or heat tolerances. If low-chill peaches are breaking bud in early February because temperatures have hit 80F for the previous two weeks, a sudden freeze in March is devastating to that year's crop. This kind of wild weather has manifested occasionally in the past, but I fear it's now well on its way to becoming the new norm.

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