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Blossom drop in tomato plants

Just as many gardeners get a jump-start to the season using milk jugs around their tomato plants, commercial growers use high tunnels or greenhouses. You may not notice in your garden when tomato blossoms do not set fruit, but you can bet that farmers do.

Observing flowers that have failed to produce a tomato is very frustrating for growers. Often, this can be attributed to environmental or nutritional reasons, or a combination of both. While I’m mainly thinking about high tunnel tomatoes, most of this information can be applied to garden tomatoes as well.

• Temperature: Tomatoes will drop their flowers under extreme high or low temperatures. The optimal temperatures for tomato are between 70 and 85 degrees during the day. At night, aim to keep temperatures above 55 but below 70 degrees. Consistent temperatures above 90 degrees can cause blossom drop in tomato.

Have you ever noticed the black cloth on top of some greenhouses? Shade cloth helps to reduce heat and sunscald of greenhouse fruit. With more sunshine and spring-like weather, farmers must vent and/or roll up the sides of their tunnel. Rolling up the sides of a greenhouse will allow for air to come in and help with pollination.

In your garden, tomato flowers are pollinated mostly by wind. However, in greenhouses, farmers must allow air in, use a blower, or place a box of bumblebees inside to ensure good pollination.

• Humidity: The optimal relative humidity (RH) for tomatoes is 40-70%. RH is very important for pollen transfer. You can imagine how a too-low RH would cause the pollen to dry out and not able to stick, while a too-high RH would make it too sticky and not able to be transferred properly.

• Nitrogen: Extreme high or low nitrogen rates can cause blossom drop. High N will cause too much leafy growth at the expense of fruit production, while low N produces weak, spindly vines not able to support a crop.

• Heavy fruit set: At about the fifth flower cluster, I very often observe a failed fruit set. The nutritional load on the plant is stretched thin enlarging fruit, ripening fruit, etc. Usually once the first crop is harvested, you get over the hump and the next group of flowers will set.

• Other factors may include water stress, and insect or disease damage. Tomatoes can grow a deep root system. Keep them well-watered throughout the season to avoid stress. As with any crop, insect and disease pressure will have a negative effect on fruit set.

To reduce blossom drop in tomatoes, farmers must keep heating and ventilation systems in good working order, use the recommended nitrogen rates and consider how pollination is occurring.

For questions about this or related topics, please email the Juniata County Extension Office at juniataext@psu.edu or call (717) 436-7744.

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