Warm Winters Threaten Nut Trees. Can Science Help Them Chill Out? : The Salt Many fruit and nut trees need cold weather to bloom, which is becoming less common in a warming climate. So, farmers and scientists are teaming up to find ways to help orchards chill out and cope.

As Warm Winters Mess With Nut Trees' Sex Lives, Farmers Help Them 'Netflix And Chill'

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All right. So spring is on the way. It's the season of birds and bees and pollination. But as the climate warms, winters are not as cold as they used to be, and that's throwing off these springtime cycles in places like California's Central Valley. NPR's Lauren Sommer takes us there.

LAUREN SOMMER, BYLINE: They say in love, timing is everything. Jonathan Battig knows something about that - at least when it comes to his pistachio trees.

JONATHAN BATTIG: In the pistachios, the females need to be pollinated by the male trees. Ideally, you'd like the males to be pushing out the pollen as the females are receptive.

SOMMER: We're standing in a pistachio orchard that Battig manages for Strain Farming Company in Arbuckle, Calif. Around 99% of the country's pistachios are grown in California.

How do you know which ones are female?

BATTIG: I know by just looking at them. The buds on the males are usually more swollen.

SOMMER: Right now, the trees are bare and dormant, basically sleeping for the winter. But in a few weeks, if all goes well, both the males and females will bloom in sync for pollination.

BATTIG: And for that to happen, the timing needs to line up pretty close.

SOMMER: Several times in the last decade, it's been off. The male trees haven't bloomed at the same time as the females because the winters were too warm. Pistachio trees and a lot of other trees need chill hours - a certain number of hours below 45 degrees. Without that, they don't wake up normally, and they bloom late or erratically.

BATTIG: Now, with the weather changing, it's becoming more of a challenge to get those chill hours.

SOMMER: Plus, if the days are really warm, that can cancel out the chill hours the trees have already gotten. So Battig is doing an experiment with scientists on some of his trees to trick them into thinking they've been colder.

GURREET BRAR: So what we're doing is we are spraying at different chill accumulation...

SOMMER: With him is Gurreet Brar, a professor at California State University, Fresno.


SOMMER: He's clipping pistachio buds to take back to his lab because these trees have been oiled - covered in an oil-based horticultural spray.

BRAR: It's supposed to help the tree and the buds wake up normally and have a normal bloom.

SOMMER: The hope is, in warm winters, farmers could use the oil to wake the trees up on time. But so far, it's not very predictable.

BRAR: The oil sprays work in some cases. In some cases, they don't. So it has been hit or miss.

SOMMER: Brar is researching this oil and other chemicals that could give farmers like Battig a way to cope - because according to one study, winter chill could decline as much as 60% by the end of the century. Battig says not all of his neighbors talk about that.

BATTIG: But me personally, I do feel something's going on and the climate is getting warmer. There are other people that choose to ignore it, but I don't.

KATHERINE JARVIS-SHEAN: We're on this march. And it's really just a matter of how bad it's going to be, not whether it's happening or not.

SOMMER: Katherine Jarvis-Shean helps farmers adapt to climate change with the University of California Cooperative Extension. She says it's more than pistachios at risk. It's walnuts, pears, apples and peaches. Spraying seems to be helping some of those trees. But in the long run, she says, farmers will need better-adapted trees, especially Bing cherries.

JARVIS-SHEAN: So we'll need to be breeding new varieties that still have that rich ruby flesh and that juicy flavor that can still do well under these low chill conditions.

SOMMER: But breeding new trees can take 20 years. And farmers will need those trees soon, she says. Orchards live for three to four decades, so the trees being planted today will have to cope with a very different climate.

Lauren Sommer, NPR News.


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