Until then, the trees must undertake their most perilous journey of the year.
Growing fruit trees in Appalachia has always been a high-wire act as peaches, plums, apples and pears race to flower in early spring while dodging a killing frost.
But researchers across the United States say the milder winters of a changing climate are inducing earlier flowering of temperate tree fruits, exposing the blooms and nascent fruit to increasingly erratic frosts, hail and other adverse weather.
The problem is not obvious to consumers, in part because a harvest collapse in one region can be masked by a bumper crop in another. But unless breeders can produce more climate-resilient varieties, fruit-growing regions of the United States will be seriously disrupted by future warming scenarios, scientists say.
Breeders are working to develop new varieties, said Katherine Jarvis-Shean, a University of California plant physiologist and farm adviser. But new trees typically take two decades of methodical breeding to create, exposing existing varieties to the vagaries of shifting winters and springs. “The consumer will begin to know it’s happening in the coming 10 to 20 years,” she said, and got a foretaste five years ago when the sweet cherry crop in California crashed after a winter that was too mild.
Even the problem-solvers are having problems. Last year, Demuth and his colleagues at the U.S. Agriculture Department’s Appalachian Fruit Research Station in Kearneysville, W.Va., were able to conduct the controlled pollination they needed to keep their breeding program going. However, in each of the preceding four years freezes killed virtually all the blossoms and brought years of hybridization work to a halt. As a backup, they moved some of their breeding stock to a commercial nursery in Adams County, Pa., where the season is later. The peach blossoms at both locations are poised to bloom on schedule, and Demuth and his colleagues are hoping for a frost-free April.
Spring blossom freezes in Kearneysville “would happen before, occasionally,” said Demuth, a research technician. “But to have it happen in back-to-back-to-back years, it’s never happened before in the 23 years I’ve been doing this here. To lose four years in a row? Yeah, heartbreaking.”
“There’s no question that the winters have become warmer and the spring weather more erratic,” said Michael Wisniewski, a plant pathologist at the research station, which is part of the Agricultural Research Service. “There’s great concern about the suitability of cultivars and adapting them to climate change.”
“These are real things happening on the ground right now,” said Albert Abbott, a plant geneticist in Cape Vincent, N.Y., who works with breeders around the world.
Two years ago, peach growers in South Carolina saw catastrophic losses from an unusually warm winter capped in March with freezes in the low 20s.
In 2002, tart cherry growers in Michigan suffered large freeze-related losses to their crop but considered the disaster a once-in-a-century event. In 2012, trees began breaking dormancy more than five weeks early, followed by a freeze. The state’s cherry harvest that year tanked to just under 12 million pounds, compared with a more normal 158 million pounds in 2011, according to Agriculture Department statistics. Michigan produces two-thirds of the nation’s tart cherries. The same year and for the same reason, the state’s apple crop dropped from 980 million pounds to 115 million. In the pome-fruit orchards of Upstate New York, the apple harvest that year fell by almost half and the pear crop by three quarters.
The vulnerability of hardy tree fruits — and nuts such as almonds, walnuts and pistachios — to climate change is linked to the way they have evolved to enter hibernation and to awake from it.
In late summer, tiny buds emerge along fresh twigs, which grow until falling autumn temperatures prod the tree into dormancy. At that point, the tree counts the days when temperatures stay between freezing and 45 degrees. Once a tree has accumulated its required chilling hours over the course of a winter, it signals its buds to count warming temperatures. As those needs are met, the buds enlarge, drop their protective outer scales, bloom and form embryonic fruitlets. The further along the process they are, the more vulnerable they are to freeze damage.
The obvious peril is of killing frosts to blossoms that have awaked too soon. Another major concern is fruit and nut trees with high chilling needs will not get enough accumulated cold to thrive after the winter. This is the greater worry in California’s Central Valley, the nation’s principal fruit and nut producing region.
Trees that are insufficiently chilled bloom poorly and sporadically. The fruit quality is compromised, and trees may become so stressed that they die, scientists say. Some long-winter varieties in South Carolina did not leaf out last year after two mild winters. “That was alarming to us and to growers because we hadn’t seen it before,” said Ksenija Gasic, a peach hybridizer at Clemson University.
A staggered harvest linked to inadequate chilling can create nightmares for commercial orchardists, including increased pest and disease risks. Growers of peaches for processing operate on such slim margins that the labor costs of multiple harvests from the same tree are economically unfeasible, said Thomas Gradziel, of the University of California at Davis.
“We are seeing industries that may not survive if we don’t find a solution, and we are only just seeing the consequences of climate change,” he said.
Chilling requirements vary among species and among their cultivated varieties. Apples typically need between 500 and 1,000 chilling hours (that’s about three to six weeks continuously below 45 degrees), almonds 400 to 700 hours, walnuts 400 to 1,500 hours, pistachios 800 to 1,000 hours, sweet cherries 600 to 1,400 hours.
A 2009 study funded by the California Department of Food and Agriculture determined that in 1950, growers in the Central Valley could rely on between 700 and 1,200 chilling hours, but by 2000 that had declined by up to 30 percent in some areas.
Even using a more sophisticated chilling model that predicts a lesser impact of climate change, the authors still anticipated a decrease in winter chilling of between 30 and 60 percent by the end of this century. “For some crops,” they wrote, “production might no longer be possible.”
“A 30 percent decrease, that’s something we can breed our way out of,” said Jarvis-Shean. “Once it gets out beyond that 30 percent realm then it gets darker.”
She said an insufficiently cold winter used to occur every 15 years or so, but California has seen two in the past five winters, Jarvis-Shean said. In 2014 the sweet cherry crop fell by 63 percent as a result.
Some researchers say growers in the Central Valley eventually may have to consider moving farther north or to higher elevations to produce the same crops.
Lack of winter chilling and spring blossom freezes are obvious problems, but climate change is bringing a range of other difficulties for fruit growers. Pests and diseases may spread, warm nighttime temperatures in fall reduce fruit quality, hot days in winter cause sunburn to plant tissue, and extreme weather events such as floods pose other risks.
In the recent low-chill winters in California, male, pollen-bearing pistachio trees bloomed out of sync with the female nut-bearing varieties, reducing the crop, Jarvis-Shean said.
“A lot of growers or consumers or political leaders might say there’s always been climate change, but never have we lost our crop as in 2012,” said Susan Brown, a veteran apple breeder at Cornell’s Geneva, N.Y., research station. “Two years later we had hail like I have never seen in my life.”
In 2012, the apples had met their chilling needs when the late winter saw a relative heat wave. “The blooms progressed much earlier than we had ever seen before,” she said.
Peaches typically bloom in South Carolina in mid-March, but in 2017 “we had the earliest bloom recorded, some varieties in bloom in February,” said Chalmers R. Carr III, whose Titan Farms has 6,200 acres of peach orchards. The subsequent freeze then killed vulnerable fruitlets, and Carr suffered 87 percent losses. As for a long-term solution, he said, “you’re pretty much waiting for breeders at this point.”
Traditionally, breeding programs have been focused on fruit size and quality, disease resistance and more upright trees for denser planting. The unusual winters are forcing hybridizers to add climate resiliency to the suite of traits.
That means, for example, developing trees that will not wake up early if it hits 80 degrees in February, said Abbott, a professor emeritus at Clemson.
An underlying worry, he said, is that fruit breeding programs will not be able to keep pace with rapid climate change. It can take 15 to 30 years to create a new commercial apple or peach variety from scratch.
In New York, Brown said, the challenge facing breeders is not lack of winter chilling but how to create varieties that can deal with the roller-coaster warming period in the weeks leading up to bloom.
Gasic said peach growers in the Southeast need varieties with reduced chilling requirements, to account for the warmer winters, but with longer heating requirement to delay flowering until after the last frosts.
South Carolina growers have used varieties requiring as much as 1,100 chilling hours, but Gasic has been working since 2008 on creating varieties needing 600 to 800 hours. More recently, she has dropped the range to between 550 and 750 hours in response to the milder winters. Growers are trialing two of her varieties and will add 10 more this year, she said.
The challenge, said the Agriculture Department’s Wisniewski, is low-chill varieties tend to bloom early naturally where “you set yourself up for late-spring frosts.”
His colleague and peach hybridizer Christopher Dardick said he expects to generate hundreds of new seedlings a year for evaluation as part of his breeding program at the Kearneysville research station, but the frosts this decade have reduced the tally to just 50 or so over a seven-year span. “It’s a huge problem,” he said.
The amount of frenetic work involved in preparing each tree for controlled crosses adds to the frustration. Demuth, Dardick and their colleagues must wait until a bud is a day from opening to remove the petals and pollen-bearing stamens, leaving the exposed pistil to receive the harvested pollen from the desired other parent. Each tree, with thousands of blossoms, takes hours to prepare. Emasculated flowers are more exposed and prone to frost injury than regular blossoms, though the frosts in some of those years killed intact blossoms as well, Demuth said.
This winter so far seems fairly normal, he said, but the trees still face the prospect of a false-spring March followed by a freeze at just the wrong time. “I always assume everything is going to work out just fine,” Demuth said. “I have got to be an optimist, otherwise I probably wouldn’t be in agriculture.”