Hot Temps Affect Prunes At Bloom

High temperatures at bloom present a huge economic challenge to California prune growers. Heat at bloom (greater than 80°F) in 2004 resulted in the smallest California prune crop in almost a century. Temperatures above 80°F at full bloom in 2005 and 2007 produced prune crop failures in Sutter and Yuba counties. Full bloom timing differences of only 24 to 48 hours in 2005 or 2007 resulted in crop load differences of 200% to 300%. In 2008, temperatures above 75°F, but less than 80°F, appeared to reduce set. If bloom temperatures head for 80°F in 2009, the crop may again be at risk.

What exactly is the relationship between heat at bloom and low crop set? Recent research by Dr. Vito Polito (Plant Sciences Department, University of California-Davis) showed that pollen germination and pollen tube growth decline rapidly above 75°F. At temperatures above 80°F, Dr. Polito believes that flower parts are severely damaged or killed.

Researchers have not yet determined exactly what temperatures for how long will affect fruit set. However, experience shows that it doesn’t take much heat above 75°F to damage flowers. The 2005 Sutter/Yuba prune crop was essentially destroyed when temperatures at full bloom were at or above 80°F for a total of 11 hours over a three-day period.

What can growers do if the temperatures of 75+°F are predicted for bloom? While research has not yet developed rock solid recommendations for growers to follow, here’s what has been learned over the past few years.

Cool The Orchard

Evaporative cooling may reduce temperatures enough to help set a crop. Impact sprinklers or micro-jet irrigation systems have an advantage over flood irrigation systems for orchard cooling. There are reports of good crops in 2005 after running water, while other growers ran water with no benefit. Here are some key points to consider when using irrigation water to try to reduce temperatures in an orchard:

• The top one foot of soil should be moist (not saturated) when warm weather hits. 

• If you can only irrigate part of the orchard per set, run water long enough to wet the soil and then shift flow to another part of the orchard. “Flash” irrigation water across irrigation checks and move on to others when using flood irrigation. If the soil surface dries and isn’t rewet, the potential for evaporative cooling decreases significantly.

• Concentrate irrigation/cooling efforts on the upwind side of the orchard. Let the wind move the cooled air through the orchard.

• If I were a grower, I’d start running water (impact sprinklers or microjet sprinklers) when the temperature gets above 70°F. If I only had flood irrigation to work with, I’d try to wet the soil surface in advance of predicted warm (above 70°F to 75°F) weather. If the warm weather stayed and the soil surface dried, I’d irrigate again.

Get Bees In The Orchard

This means renting bees, as native bee populations have weakened due to bee mites and poor food availability. Experience suggests better fruit set in 2005 and 2007 on trees close to hives, and poor fruit set away from the hives. It may be beneficial to spread hives throughout the orchard. In almond orchards beehives are distributed at one-tenth to one-quarter-mile intervals for optimum pollination.

Leave grass long in the orchard if heat at bloom is predicted. Tall, well-irrigated vegetation should be cooler compared to short mowed vegetation on the orchard floor. If frost is a threat at bloom, keep the orchard ground cover as short as possible. Delay the orchard floor management decision as long as possible so that a better forecast of bloom weather is available and can be included in the final decision.

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