High heat during blueberry bloom: Great for pollination, as long as it doesn’t get too hot

Warm, sunny days promote blueberry pollinator activity but beware of temperatures approaching 95 degrees Fahrenheit in the next week.

Graph of June 2020 air temperatures.
Graph showing air temperatures during June 2020 at the Fennville Enviroweather station at the Trevor Nichols Research Center (green line) and at the blueberry planting that is established on black weed barrier fabric (black line).

This week, the weather has switched to cool to hot and sunny days. Many blueberry fields are in bloom. Temperatures from 70-80 degrees Fahrenheit are great for bees, flying from flower to flower. The bees are moving pollen, getting pollen onto the stigmas of flowers. The pollen germinates on the stigma and a pollen tube grows down to fertilize the ovules in the ovaries. Pollination and fertilization are necessary to set fruit and the more seeds there are, the larger the berry.

As long as temperatures are about 65-85 F, fields are well irrigated, stocked with enough managed bee colonies and have abundant wild bee populations, there should be excellent blueberry fruit set this year. That is a lot of things that need to go right!

Hot air is coming into Michigan this week. The hottest predicted temperatures will be on Thursday afternoon, May 20, with air temperatures climbing up into the high 80s and approaching 90 F. These predicted temperatures are based on standard Michigan State University Enviroweather station locations (a shaded thermometer at 5 feet).

It is important to be aware of local conditions in specific fields that could make temperatures even hotter. This can happen with dark bare soil or if a planting is established on black weed fabric. The dark surface absorbs light energy (heat) and increases the field temperatures the blueberry bushes are experiencing. We saw this in June of 2020 at our blueberry planting in Fennville, Michigan. The figure above shows how temperatures in the blueberry planting were up to 5 F hotter than the MSU Enviroweather station at the same farm.

If temperatures reach into the 90s on Thursday, there is a risk of pollen germination and pollen growth being disrupted. We have found that pollen germination and pollen tube growth are inhibited when air temperatures exceed 95 F.

If blooming fields approach this temperature in the middle of the day, running the overhead irrigation for a short period before the temperature exceeds 90 F can cool the bushes through evaporative cooling so they don’t reach upper critical temperatures. Recent research at Oregon State University in David Bryla’s group showed that misting for 15 minutes every hour during the hottest hours in the middle of the day cooled the bushes by more than 10 F. This needs to be done for only a few hours during the afternoon to have the desired effect. Air temperatures will cool down again later in the afternoon.

There are some risks with this approach using overhead irrigation, as it will disrupt bee flight for a short period and it may also increase the risk of anthracnose infection. Growers need to balance this against the potential for limited pollination and the implications for berry set. Locations that have high temperatures and a black weed fabric system should be monitored for high temperatures this week. Overhead cooling would be worth the effort to maintain pollination and fruit set.

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