Climate Viticulture Newsletter - 2020 February

Monthly average temperatures in January were within 2° Fahrenheit of the 1981-2010 average for much of Arizona and New Mexico (yellow and light green areas on map). Some parts of north-central Arizona and western New Mexico, and most of the eastern plains of New Mexico, however, were 2° to 4° Fahrenheit above normal (light orange areas on map). Like December, relatively equable temperatures were the rule. There was no increase in risk of freeze damage to dormant vines as extreme cold didn’t make any appearances, nor did any stretches of really warm temperatures send the accumulation of heat units skyward.

Monthly precipitation totals were less than half of the 1981-2010 average for almost all of the western two-thirds of Arizona and scattered parts throughout New Mexico (red areas on map). Much of the remaining area across both states received between 50% and 90% of normal (orange areas on map). A few locations such as south-central New Mexico nonetheless managed to get above-average amounts (dark green, blue, purple, and magenta areas on map). These widespread mostly dry conditions put an end, or at least a pause, to the increase in cool-season soil moisture that many vineyards had seen from precipitation in November and December.

There is a slight increase in chances for above-average temperatures across extreme northwestern Arizona (light orange area on map). Equal chances for above-, near-, or below-normal temperatures for the month exist for the rest of the region (white area on map). How February turns out in terms of temperature will have a strong influence on whether bud break is relatively early, on-time, or late this spring. More on this below.

Equal chances exist for above-, near-, or below-average precipitation totals for the entire region (white area on map). Like last month, let’s hope that this three-sided coin flip comes up either ‘near-normal’ or ‘above-normal’, especially since January turned out on the dry side for many winegrape growing areas in the region. Having good soil moisture levels in vineyards as we get closer to the end of winter helps guard against delayed spring growth, which manifests as uneven bud break, stunted growth, reduced flower clusters, and other issues.

In the December issue, we mentioned that most winegrape varieties – particularly those of Vitis vinifera – only need 150 to 200 chilling hours during dormancy to meet chilling requirements. But, these numbers are based on the conventional calculation of the cumulative sum of hours between 32° and 45° Fahrenheit starting on November 1, which doesn’t work as well in regions where daytime high temperatures frequently reach well above the chilling-temperature range. The reason why is that such daytime warmth can effectively negate some of the accumulated chilling hours. For such regions, use of a dynamic model incorporating this complexity is recommended.
 
An initial thing to note when switching our thinking from chilling requirements in the conventional way to one with more complexity is the change in terms. Chill portions replace chilling hours. Another thing is that the values of these two chill variables are different. Using hourly temperature data from the Arizona Meteorological (AZMET) Network Willcox Bench station, which is located in south-central part of the Willcox American Viticultural Area (AVA), accumulated chill portions (top graph) from this and the past three winters reached between 30 and 60 by February 1 whereas accumulated chilling hours (bottom graph) were between 600 and 1000. Think of chill portions and chilling hours both as money, but different currencies.
 
If accumulated chilling hours (bottom graph) reach the required 150 to 200 chilling hours by December 1, as during recent winters, do vines only need about 10 chill portions (top graph) to meet chilling requirements? This is not clear as we have not yet seen any publications addressing this. If it were the case, then it would be the difference between these two models that becomes clear. Accumulated chilling hours during winter 2017-2018 (bottom graph, dashed orange line) tell us that vines met chilling requirements by the beginning of December. Chill portions that winter (top graph, orange line), however, were near zero at this time and didn’t reach a value of 10 until the middle of that month. The reason for so few accumulated chill portions in November 2017 is record-setting warmth that negated any chilling at that time. We certainly weren’t feeling any chill that Thanksgiving weekend in Tucson with afternoon temperatures near 90° Fahrenheit!

We also noted in the December issue that once vines meet chilling requirements, they shift from a state of endodormancy to ecodormancy and begin losing cold hardiness while accumulating heat units that determine when bud break occurs. If a cold snap takes place during ecodormancy, freeze damage to vines can occur even though the plants have not yet leafed out. This undesirable scenario is one we suspect might have been possible in late winter of 2011 and 2013.
 
Given this overall description of vine dormancy, you might suspect that a comparison of current accumulated chill portions and growing degree days to previous years will provide an early hint as to the start of the growing season later this spring. Returning to the hourly temperature data from the Arizona Meteorological (AZMET) Network Willcox Bench station and the chill portions we just discussed, chilling this winter (top graph, pink line) is more similar to that of last year (purple line) than to chilling during winters 2017-2018 (orange line) and 2016-2017 (green line). Accumulation of growing degree days this winter (bottom graph, dotted pink line) also is more similar to last year (dotted purple line) than to winters 2017-2018 (dotted orange line) and 2016-2017 (dotted green line).
 
With the equal chances for above-, near-, or below-normal temperatures for February across most of the region that we saw above, it currently is hard to say whether or not vines continue to follow a path like last year that would suggest a relatively late start to the growing season.

Having recently been in contact on this topic with Stuart Pigott, a speaker at the fourth annual Arizona Vignerons Alliance symposium, we thought it would be good to continue the conversation here. In the newsletter last November, we noted how growing season temperature – the average temperature between April 1 and October 31 – is used as a simple way to compare different winegrape growing regions and as a first guess of which grape varieties might do well in a new area. The latter application is related to vine phenology studies showing that some varieties require more heat accumulation during the growing season to ripen fruit than do others – a reason why Grenache / Garnacha isn’t grown in Germany!
 
Based on average growing season temperatures from 1981 to 2010 that we saw last November, the proposed Verde Valley American Viticultural Area (AVA) is slightly warmer than the Willcox AVA, and both of these AVAs are warmer than the Sonoita AVA. Climate-viticulture studies have used long-term averages to categorize and compare winegrape growing regions. Using one such classification for example, both the proposed Verde Valley AVA in north-central Arizona (polygon 3 on map) and the Willcox AVA in southeastern Arizona (polygon 2 on map) fall into the ‘very hot’ viticulture category (dark peach areas on map). The Sonoita AVA in southeastern Arizona (polygon 1 on map) is in the ‘hot’ viticulture category (light peach areas on map). It’s no surprise that Grenache / Garnacha is grown in Arizona!
 
There are several other more complex and perhaps more meaningful classification variables than growing season temperature that we’ve seen in the climate-viticulture literature. We’ll take a look at them in coming newsletter issues.