Help, tomatoes are dropping their blossoms! This problem can be frustrating and difficult to manage as the problem usually is caused by high heat and low humidity. That’s a hard problem to deal with in southwest Idaho.

Some blossom drop is normal because plants generally produce more flowers than the amount of fruit the plant can support. However, as we move into August, we’re beginning to wonder if we’ll ever get ripe tomatoes, knowing September will leave us with an abundance of green tomatoes and not enough time for them to ripen.

Tomato flowers are a complete flower, meaning they have male and female parts within the same flower, making them self-pollinating. The pollen does not have to move very far, so what’s the problem?

Blossom drop can be caused by a few different factors. It might be nutritional, environmental or a combination of the two.


According to research, tomato flowers must be pollinated within approximately 50 hours or they will abort and drop. Anything that interferes with the pollination and fertilization process may cause flower loss. This condition can affect tomatoes, peppers, snap beans, and other fruiting vegetables.

Tomatoes, like us, prefer a nice temperate growing season, between 70-85 degrees. They will drop blossoms if daytime temperatures get above 85 degrees, nighttime temperatures stay above 70 degrees or nighttime temperatures drop below 55 degrees. Tomatoes can tolerate short periods of extreme temperatures, but several days outside the optimal range can cause problems. For example, temperatures over 104 degrees for only four hours can cause blossoms to drop. Why is that?

The pollen becomes tacky and nonviable, preventing pollination. Sustained high temperatures, especially at night, deplete food reserves that are produced in the tomato during the day. This changes the viability of the pollen. It also causes morphological changes as parts of the flower dry out.

Humidity also plays a role in pollination. Ideal relative humidity for pollination is between 40%-70%. Outside this range, the pollen is likely to dry out and be unable to stick to the stigma. If by chance the relative humidity is higher than ideal, the pollen will not shed properly.

Nutrition may also play a part in blossom drop. High nitrogen rates encourage the plant to produce lots of vegetation at the expense of fruit set. Too little nitrogen and the vine will have low food reserves and not be able to support a tomato crop.

How to help prevent blossom drop? Try growing varieties suited for our climate with better heat-setting capabilities.


You might try watering, or misting, the plants during the day when daytime temperatures go above 90 degrees. The evaporation will help lower the temperature a little and raise the humidity. This might be helpful, however, if daytime temperatures are above 100 and the night temperatures are above 75, this isn’t going to help at all.

Outside of heat and humidity, low or high soil moisture can be a cause of blossom drop, so always check to see that your soil is moist, but not soggy or dry. A layer of mulch will help regulate soil temperature and moisture. Lack of sun could be another reason for blossom drop. Optimally, your fruiting crops should be getting 8-10 hours a day of direct sun but need a minimum of six hours.

What’s the solution? Patience. As soon as the weather cools down, tomatoes will go nuts. Oh, and learn some good recipes for green tomatoes!

Easy Roasted Tomato Sauce by Gretchen Anderson
How to Extend Your Harvest: Tomatoes by Gretchen Anderson

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