Hot weather causing rapid blueberry bloom

Warm temperatures are making flowers open and age rapidly.

Blueberries in bloom
Blueberry flowers are opening quickly due to the current hot spring weather. Photo by Rufus Isaacs, MSU Entomology.

The Memorial Day weekend was hot across Michigan, with temperatures up to 85 degrees Fahrenheit. This hot weather will continue for the next week, making this blueberry bloom season unusually warm. Extreme spring weather is becoming more common, as highlighted by the Memorial Day weekend two years ago when some Michigan blueberry farms reached well over 95 F while they were in bloom. This article addresses how the current hot weather could affect blueberry pollination this year and some points to consider. For more details on blueberry pollination, read this previous Michigan State University Extension article, “Invest in pollination for success with highbush blueberries.”

First, the good news. The recent hot temperatures are probably not hot enough to damage pollination. The 2018 extreme heat event was followed by poor yields in some fields, so we have recently been exploring how heat affects pollen development. Germination and pollen tube growth increase with temperature, and at temperatures in the 70s and 80s that we are experiencing this year, pollen germinates well. However, above 95 F pollen germination and growth are inhibited and this will prevent pollen tubes reaching and fertilizing the ovary before it degrades. This warm weather means that the ovules in the flowers will also age quickly. So while the warm weather is great for bee activity, the pollination window is shorter for each open flower. Instead of four days to pollinate the ovule, we may only have two days during this warm weather.

There are other considerations for blueberry pollination during hot weather. Honey bees are visiting blueberry flowers mostly to get nectar, so it will be important to keep soil moisture from drying out to ensure a strong nectar flow. This is also critical for plant growth, as this time of year is when the blueberry plant is growing shoots and producing lots of new leaves. This growth needs good soil moisture, so monitor soil moisture during this hot weather and consider watering if soils are drying out. With drip irrigation short bursts every day to wet the plant row is best. When using overhead sprinkler systems, irrigate in the early morning so plants get a chance to fully dry off during the day when bees are flying.

With flowers opening so quickly, there will be a high number to be visited by bees. Open flowers will only be receptive for a day or two with temperatures in the high 70s and 80s. Blueberry flowers drop off the plant white when the flower is pollinated, so if you see a carpet of white under the bushes, that’s a good sign. If flowers are turning dull and brown on the bush, that can be a sign of poor pollination indicating that the bees couldn’t get to all the flowers while they were receptive to pollen. This highlights the importance of stocking fields with sufficient healthy and active honeybee colonies and conserving wild bees around your farm. This will help ensure that every flower gets pollen deposited while it is still viable, and all the flowers turn into berries.

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