Good Fruit Grower

February 15

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M other Nature opened the new year with an arctic blast that drove temperatures across the Midwest and East to levels not seen since 1994. It brought some new terminology to the TV weather shows—who'd ever heard of a polar vortex?—and it revived cold hardiness as a hot topic of conversation. University fruit experts across the northeast quadrant of the United States were e-mailing each other, comparing notes. Twenty years may not seem like a long time, but much has changed in orchards and vineyards over the last two decades—new varieties, new rootstocks, and new growing systems. And, especially in vineyards, the general trend to ever-warmer winters encouraged wine grape growers to venture further into the cold-tender viniferous grapes to make better wine than they can from the French hybrids. This winter may test the results of their boldness. In only a few fruit-growing areas did temperatures fall to levels known to be damaging. One of those was southwest Michigan, where temperatures reached -20˚F on some sites. Mark Longstroth, the extension fruit educator in that area, said that in his experience minus 13 degrees F will dam- age peaches and by minus 16, damage will be so extensive growers won't bother to spray what's left. Coincidentally, he began work in southwest Michigan in 1994, so he remembers the last really cold winter. "I expect there will be a light peach crop in our area this year," he said, "especially on sites away from Lake Michigan." "Peaches are the simplest to understand," he said. "They have a single fl ower, and they're quite cold sensitive. Apples are more complex—they have a bunch with fi ve fl owers—and most apple varieties are safe to minus 40." With peaches, when temperatures reach minus 20 degrees F, the tree wood itself can be damaged, and damaged trees will show signs of water stress the next summer and begin to decline and die. It's not just temperature that makes the difference, however. Cold temperatures have different effects at different times, and how rapidly the temperature changes makes a difference, too. Bill Shane, the peach breeder at Michigan State University's Southwest Michigan Research and Extension Center, noted in an e-mail that Michigan was virtually wiped out of the peach world in 1906 when temperatures fell to 10˚F. That's not even below zero, but it occurred on October 10, before trees had hardened off, and it killed 73 percent of the state's peach trees. On January 16 and 19, 1994, temperatures fell to minus 17 to minus 22 degrees F, resulting in nearly complete crop loss and widespread damage to wood. This year looks similar. Pennsylvania In Pennsylvania, Penn State University tree fruit professor Dr. Bob Crassweller said the amount of the temperature drop and the speed of it is a greater concern than the absolute low temperature. Temperatures fell to nearly minus 9 degrees F in some areas, but not in Adams County, where most of the fruit is grown. But when temperatures falls 40 degrees or more in a day, that's a cause for concern. As Longstroth explained, cold tolerance is complicated. Basically, he said, warm temperatures in winter may not break dormancy, but they can "reset" the cold hardiness level. "If the temperature has been above freezing recently, the plants have lost cold hardiness," he wrote in an e-mail. His rule-of-thumb is that after two days with night-time lows above freezing, the plants will have lost all their acclimation to cold weather and be back to the 0-degree F damage threshold. "The zero threshold is only for cold-tender plants such as peaches, wine grapes, and blueberries. More cold hardy are cherries and European plums, with apples and pears being the most hardy. Temperatures of minus 20 to minus 25 degrees F will have an impact on Prunus, and apples and pears should be able to go to minus 25 with little damage. (Continued on page 10) 8 FEBRUARY 15, 2014 GOOD FRUIT GROWER In this satellite image, a frontal system that brought rain to the East Coast in early January is draped from north to south. Behind it lie the clearer skies that brought bitter cold air associated with the polar vortex. Image captured by NOAA's GOES East satellite on January 6. PHOTO COURTESY OF NOAA/GOES The polar vortex of 2014 will test the cold hardiness of trees and vines. by Richard Lehnert "It was so cold the brine was freezing in the pickle vats. We saw many young trees killed outright." —Mark Longstroth Winter tests HARDINESS

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