Targeting Fruit Size With A Weather Model
February 1, 2008
The problem with Ted DeJong’s weather model to predict stone fruit size is that, at least at first glance, it appears to defy common sense. When the University of California (UC)-Davis pomologist gives one of his frequent grower talks, he’ll survey the audience, asking what type of weather produces a great crop. “Almost invariably they say a nice warm spring is great,” says DeJong. “And temperatures in the 60s and low 70s are great, but when you get up into the high 70s and 80s, you end up with smaller fruit.”
What growers need to understand is that higher temperatures do push the development rate, but not necessarily the fruit growth rate. In other words, the fruit matures faster than it has a chance to grow. If it gets really warm after bloom, the fruit starts the development process before the tree is really ready to supply the resources needed for growth. “You’re asking the tree and fruit to do the same amount of work in 65 days that it would normally do in 90 days,” he says.
DeJong compares the process to raising a family. “If you have kids and don’t feed them, they won’t stop developing, but you’ll have smaller kids,” he says. “They may have had the genetic potential to be six feet tall, but they’ll only get to five and a half. And even if you feed them after they’re 21, they won’t grow any more.”
That’s Cool
DeJong discovered the relationship in analyzing weather data to predict harvest dates. When he looked at the data — he and his colleagues analyzed temperatures from the California clingstone peach industry going back to 1984 — he was surprised to find that the fruit at reference date (10 days after the beginning of pit hardening) strongly correlated with the number of growing degree hours in the first 30 days after peach full bloom date.
The numbers are eye-popping. If the number of growing degree hours in the 30 days after bloom were about 6,000, growers got a pretty normal crop (see chart.) But if the number got too high, such as it did last year, 8,095, or in 2004, 9,001, growers got generally smaller fruit. And when it was markedly cooler following bloom, such as in 2006 when there were less than 4,000 growing degree hours in the critical 30-day period, growers harvested a bumper crop of big fruit. And, as most growers know, the bigger the fruit, the more it pays. They don’t call it “target fruit” for nothing.
DeJong doesn’t pretend that this is the only factor affecting fruit size. Individual growers’ cultural practices, including standard fruit thinning, nutrition, irrigation, etc., will certainly have a large impact. However, if all those other factors are equal, there’s no question that cool spring weather can quite literally pay off for growers of peaches, plums, nectarines, and prunes. DeJong has yet to see if the principle applies to pome fruits such as apples and pears, though he suspects it does. He also believes it will apply in other parts of the country. His colleague, Scott Johnson, has looked at data from the federal NC-140 project, and while he has yet to analyze the data, he has found a clear relationship between post-bloom temperatures and fruit size, says DeJong.
Don’t Be Late
DeJong recommends that growers use the information to modify fruit thinning practices. Currently, most growers just go out in late April or early May and start thinning. “But if they’re using a traditional approach, they could be too late,” he says. “Especially today when there’s such a premium on large fruit.”
Growers with large amounts of acreage, especially if they’re expecting it tough to find workers, are going to want to start thinning much earlier in a warm year, says DeJong. Thirty days after bloom, growers should know exactly how many degree hours have accumulated in the period. “If it’s 8,000, they’d better get started with large (thinning) crews 10 to 20 days earlier than they might normally,” he says.
To find out the growing degree hours in the first 30 days after bloom, growers in much of the country might have to do a little figuring, but those in the Golden State have it easy. Get on the Web and point your browser to the UC-Davis Fruit & Nut Research Center: http://fruitsandnuts.ucdavis.edu. Click on “Weather Services,” then “Harvest Prediction Module,” then on the weather station closest to your orchards. Then all you have to do is enter the date of full bloom, which is the date that half of the flowers in your orchard are open.
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