Specialists: Warm winter leaves fruit trees, plants at risk

February 15, 2012

Fruit trees and plants that have accumulated necessary winter chilling hours set buds when temperatures turn springlike. An apple tree bud (top) emerges, while a budding strawberry plant (bottom) shows the effects of a hard frost - the flower on the left has died. (Purdue Agricultural Communication photo/Peter Hirst and Bruce Bordelon)

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WEST LAFAYETTE, Ind. - An unusually warm winter in Indiana could be setting up fruit trees, berry plants and vineyards for crop damage if frost occurs this spring, say Purdue University Extension fruit production specialists.

Mild temperatures in much of December, January and February have caused fruit-bearing species to move through winter dormancy weeks ahead of schedule, said Peter Hirst and Bruce Bordelon. Already, most Indiana peach trees and grapevines have accumulated enough hours of winter rest that buds will begin growing with the next extended period of 50-plus degree Fahrenheit weather. And apple trees and blueberry plants, which produce two of Indiana's largest fruit crops, are not far behind, they said.

"That's bad news because we don't want them to start growing yet," said Hirst, a commercial tree fruit specialist. "If these fruit plants bud out too early and we get a frost that kills buds, there aren't going to be more flower buds. You get one shot at this and that's your crop for the year."

Fruit production is an important part of Indiana's agricultural industry. According to the Purdue-based Indiana Agricultural Statistics Service, Indiana ranks 10th in blueberry and 20th in apple production in the United States, at 3.6 million pounds and 26 million pounds, respectively. Together, Indiana blueberry and apple crops have a combined value of more than $13 million.

In addition, Indiana produces about 3 million pounds of peaches each year and the state's wine grape industry is growing. At last count there were nearly 70 wineries and vineyards statewide.

Hirst and Bordelon, a viticulture and small fruit specialist, said it would be best for fruit production if temperatures the final five weeks of winter stayed in the 40s or below.

Except for a return to colder temperatures in the last several days, the trend the past three months has been for above-normal temperatures. Half the days in January were at least 10 degrees above the state's average of 26 degrees, with temperatures reaching the mid-60s on Jan. 17. Conditions were even super for Super Bowl week, as temperatures stayed in the 40s and 50s.

While great for people, those mild conditions have played physiological tricks on fruit trees and plants. During winter dormancy, fruit-bearing species accumulate "chilling hours" when temperatures are between 34 and 54 degrees. Once a tree or plant has accumulated its necessary hours, it is ready to blossom when temperatures turn warmer.

Optimum chilling hour accumulation takes place at 42 degrees. Three weeks of optimum temperatures can produce 500 chilling hours.

Using a common chilling hours model, Hirst calculated that in Tippecanoe County from Nov. 1 to Feb. 1, fruit trees and plants accumulated 915 hours of winter rest. In the same period one year ago, those same species accumulated 373 chilling hours.

"At 915 hours we will have satisfied chilling for crops like grapes and most peaches," Hirst said. "Apples and blueberries have a requirement of about 1,000 to 1,200 chilling hours.

"If you think of chilling as getting the runners to the starting block for budding and warm temperatures as the starting pistol, right now plants are at the starting block. They aren't going to pull back from the starting block - they'll just stay there."

Even if warm temperatures don't return until spring, fruit trees and plants aren't necessarily out of the woods for crop damage.

"The frost-free date for central Indiana is around the first week of May," Bordelon said. "That's a concern because we've still got about three months to go."

There's nothing fruit growers can do to slow down the accumulation of chilling hours or prevent trees and plants from breaking buds after dormancy. The Purdue specialists said research on growth regulator products has shown that the small amount of time it can buy producers isn't worth the expense.

"We're in a wait-and-see mode," Bordelon said. "Hopefully, the groundhog was right and we'll have six more weeks of winter. Then we'll keep our fingers crossed that we avoid a hard frost this spring."

Indiana weather information is available through the Purdue-based Indiana State Climate Office at https://climate.agry.purdue.edu/climate/index.asp. Additional information about fruit production is available on the Purdue Department of Horticulture and Landscape Architecture's Fruit and Vegetable Connection website at http://www.hort.purdue.edu/fruitveg/

Writer: Steve Leer, 765-494-8415, sleer@purdue.edu

Sources: Peter Hirst, 765-494-1323, hirst@purdue.edu

                 Bruce Bordelon, 765-494-8212, bordelon@purdue.edu

Ag Communications: (765) 494-2722;
Keith Robinson, robins89@purdue.edu
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