Protecting blueberry blooms from extreme heat using overhead irrigation

Preventing fields from reaching temperatures above 90 degrees keeps the pollen viable for pollination.

The increasing frequency and intensity of high temperatures during flowering pose a potential risk to blueberry pollination. In some cases, temperatures have been above 95 degrees Fahrenheit, such as during Memorial Day weekend of 2018. This heat wave was the hottest year on record in the last 92 years, and Michigan’s blueberry harvest was 30-50% lower than previous years. despite a “snowball” bloom. Although the low yield could be caused by other factors, we investigated whether the warm conditions could limit blueberry pollen development.

Blueberry pollen was collected from plants grown in the greenhouse and then exposed for four hours in an environmental chamber to temperatures between 50 and 104 F. The pollen was then examined under the microscope for its development after 4 and 24 hours to determine the percentage of germination and the measure the length of the pollen tubes.

Pollen development was slow at 50 F, as would be expected during cool conditions, and was optimal at temperatures between 68-86 F. In Figure 1, the yellow circle shows pollen at 86 F, with the pollen tubes having germinated and growing. In red, the 104 F conditions have no pollen tubes. This shows that pollen germination is almost completely prevented by 4 hours of exposure to extreme heat.

A diagram showing pollen tube growth at different temperatures.
Figure 1. Growth of pollen tubes from blueberry pollen exposed to conditions of 86 F (left) or 104 F (right) for 4 hours, demonstrating inhibition of pollen development.

Between 86 and 104 F, we found that pollen performance steadily decreased as temperature increased, with significant decreases in pollen tube length at temperatures above 30 F (Figure 2). In a separate experiment, we also found that pollen exposed to extreme heat did not bounce back and regenerate, making the effect permanent.

A series of graphs.
Figure 2. Pollen tube length (millimeters) at 4 and 24 hours for Bluecrop, Elliott, Jersey and Liberty cultivars at temperatures between 86-104 F.

Based on these results, we developed an early warning model to integrate predicted bloom times and predicted upcoming warm weather, which has been posted on the Michigan State University Enviroweather website for the first time this season: Blueberry Bloom honey model. This provides a colour-coded warning for conditions where flowering is predicted to commence And we have forecast high temperatures above 90 F.

It is also worth noting that air temperatures can be above weed barrier fabric on sunny days 5 degrees warmer than fields with a weed-free strip, so those fields must be monitored more closely for temperature during heat waves. A thermal temperature gun can be used to easily check field conditions.

If predicted field temperatures are above 90 F, Michigan State University Extension recommends cooling fields if you have permanent above-ground irrigation. These systems are typically used for frost protection and irrigation, and they can also prevent flowering fields from reaching the extreme temperatures that hinder pollen growth. To test the effectiveness of this approach, we conducted overhead irrigation in the blueberry plantation at the Trevor Nichols Research Center in Fennville, Michigan, during flowering from May 30 to June 2, 2023, watering for 15 minutes every hour (for 5 hours ). ) once the air temperature was expected to rise above 35 degrees Celsius. Figure 3 shows that the irrigation system has reduced the air temperature by 10 degrees Celsius, with the evaporative cooling keeping the air temperature below the heat threshold where pollen damage will occur.

During the spring, water temperatures in ponds are also lower, which likely helps lower temperatures in the fields. It is also important to note that we have seen some evidence of pollen damage from high temperatures during early flower development. Therefore, we recommend overhead irrigation during extreme heat, even if the plants are not yet in full bloom.

A series of graphs.
Figure 3. Average air temperature (F) in a Bluecrop plantation that received overhead irrigation (blue) and no irrigation (red) between May 30 and June 2, 2023. The requests were made every hour for 15 minutes. Dotted, black horizontal lines on figures represent the critical temperature for blueberry pollen damage.

Our current research examines potential disadvantages of irrigation during flowering, including increased risk of disease and inhibition of bee activity. However, the potential benefits of maintaining fruit set and berry size are expected to outweigh these concerns. The reduced foraging of the bees also occurs during a small part of the pollination window and is likely to have a negligible effect as bees can still pollinate before and after irrigation on the same day.

This research was funded by the United States Department of Agriculture, Project GREEN and by the Michigan Blueberry Commission.

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