Adapted from the articles “Heat at Bloom Update” by Franz Niederholzer, UC Farm Advisor, Colusa/Sutter/Yuba Counties and “High Temperatures at Bloom Reduce Fruit Set” by Carolyn DeBuse, UC Farm Advisor, Solano/Yolo Counties and Franz Niederholzer, UC Farm Advisor, Sutter/Yuba Counties in the Sacramento Valley Prune News, February 2015 and February 2011, respectively.

Excessive heat, sustained temperatures above 80^oF at bloom, can cause virtual crop failure in prunes. In Sutter County, average prune yield is in the range of 2.5 dry tons per acre over the last decade in good years. In 2004, 2005, and 2007, when temperatures reached above 80^oF for 5-10 hours over several days at bloom, the Sutter County crop was 0.5, 0.7, and 0.6 dry ton per acre, respectively. In 2014, west of I-5 up and down the Sacramento Valley, max temperatures were above 80^oF for several days, contributing to the low crop yield in many orchards in that region.

How Hot is Too Hot for Prune Bloom?

A two year UC Davis study showed the optimum temperatures for pollen germination and pollen tube growth are between 72-76°F. When temperatures are higher than 76°F the pollen viability and the pollen tube growth decline rapidly. A Sacramento Valley-wide study showed that the duration of high temperatures is also important. Our working threshold for increased risk of crop loss is exposure to 10 hours or more of max temperatures > 80°F. Extreme heat (85-87^oF max) right around full bloom seems to do the most damage. Extreme heat early in bloom followed by cooler temperatures doesn’t appear to harm the crop as much as heat at full bloom or shortly thereafter.

 What To Do If Temperatures Above 80°F Are Expected

Cool the orchard with irrigation water. Running water does not dropped orchard temperatures dramatically but the degree (^oF) or two lower temperatures can make a difference. Evaporative cooling may reduce temperatures enough to help set a crop. Key points for this approach:

• The top one foot of soil should be moist (not saturated) when warm weather hits. Deeper soil does not need to be wetted.
• If you can only irrigate part of the orchard per set, run water long enough to wet the soil and then shift to another section. If the soil surface dries and isn’t rewet, the potential for evaporative cooling decreases significantly.
• If limited, concentrate irrigation on the upwind side of the orchard. Let the wind move the cooled air through the orchard.
• Start running sprinklers when the temperature gets over 70^oF, at the latest. For flood systems, try to wet the soil surface in advance of predicted warm weather. If the soil surface dries and temperatures are still high or threaten to be high, rewet the soil.
• The goal of running water is to drop orchard temperatures by evaporation. Evaporation is greatest when it is hot. When temperatures cool off in the evening, there is no need to keep running water.

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. If the orchard is smaller than 40 acres, hives can be distributed around the perimeter.

 Leave grass long in the orchard if heat at bloom is predicted. Tall, well irrigated vegetation should be 1-2^oF 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 possible so that a better forecast of bloom weather is available and can be included in the final decision.