One of the greatest miracles in nature is that plants turn flowers into fruits. Petals morph into pears; blooms become blackberries. That only happens if conditions are right, though.

This is especially true for blueberries. Blueberry pollination is sensitive to extreme heat – which growers are seeing more and more of these days. During flower bud development and bloom, exposure to high temperatures can reduce pollen viability and fertilization success, which ultimately ends up negatively impacting yields.

Can overhead irrigation protect blueberry blooms from heat stress? This was the question posed by researchers from the Department of Entomology at Michigan State University (Jenna Walters, Steven Van Timmeren and Rufus Isaacs). They tested their theory and presented their findings on a poster at the most recent Great Lakes Expo.

The MSU team found that blueberry pollen does best between 68º and 86º F. Viability is reduced above 90º. And even brief periods of heat exposure (as short as four hours) can cause permanent and irreversible damage to blueberry pollen viability.

“Protecting blueberry pollen viability is critical for high yields and high-quality fruit so heat mitigation strategies are needed,” they wrote.

For protection, they wanted to test the effectiveness of overhead irrigation to cool blueberry fields during bloom, since evaporative cooling with overhead irrigation is a reliable method of reducing air temperatures during fruit ripening. They simply wanted to begin that cooling strategy sooner, if the need arose.

Using overhead irrigation during hot periods while bushes are blooming can help blueberries better withstand the heat. Photo courtesy of Juan Campa, MGAP, Bugwood.org

Bushes of the ‘Bluecrop’ blueberry variety at the Trevor Nichols Research Center in Fennville, MI, were exposed to irrigation-based cooling while blooming when the air temperature was predicted to exceed 90º. When it got that hot, overhead irrigation came on for 15 minutes of every hour for five hours.

Air temperatures were recorded every five minutes in the middle of the bush, mid-canopy and in the middle of each plot. The bushes were at about 50% bloom when the experiment took place. Standard solid set overhead irrigation was used in this study.

To see how the cooled bushes fared, they were compared to a control plot where irrigation was only turned on in the evening to ensure all plants received equal amounts of water.

The good news is the water worked. The researchers reported that overhead irrigation reduced air temperatures by five to 10 degrees in the 2023 trial. Using water to reduce air temps can actually help protect pollen viability and vigor.

The MSU team suggests growers turn on overhead irrigation when air temperatures are expected to rise above 90º during bloom to protect blueberry pollen from heat damage. As many commercial blueberry growers already have these systems, it makes this heat mitigation strategy “affordable, readily available and reliable.”

There are other questions that arise from this research, though – namely, how this cooling will affect fruit set, ripening and quality; if the irrigation during bloom increases disease prevalence and bee activity; and if the mulch substrate (such as black weed fabric) affects canopy-level air temperatures (and if so, at what temperatures cooling should be initiated for different mulch systems).

by Courtney Llewellyn