Tomato fruit set fails when night temperatures persistently exceed ~21C (70F); high day temperatures (>29-32C) compound it.
Well established · costly if ignored → warn
How it works
Sustained high day (>29-32C / 85-90F) and night (>21C / 70F) temperatures make pollen non-viable ("tacky") and impair its release; flowers go unfertilized within their ~50-hour window and abscise.
What to do about it
Wait it out — fruit set resumes on its own once nights cool, and the plants make up the gap. Do not reach for shade cloth: it cools the day, and this failure is driven by the night temperature, which Missouri IPM says shading will not touch. If hot nights are normal where you garden, buy a heat-set cultivar next season.
The evidence
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Univ. of Missouri IPM, 'Understanding Tomato Fruit Set' (Missouri Produce Growers Bulletin), ipm.missouri.edu/MPG/2013/4/Understanding-Tomato-Fruit-Set (read 2026-07-11): 'When daytime temperatures exceed 85 degrees F or nighttime temperatures exceed 70 degrees F, pollination suffers due to pollen becoming "tacky" and non-viable.'
UMD Extension (J. Brust), 'Expect Poor Fruit Set in Tomatoes This Week', extension.umd.edu/resource/expect-poor-fruit-set-tomatoes-week (read 2026-07-11): 'Daytime highs of 90° F and above and nighttime lows only getting down to 70° F ... will cause blossom drop and fruit abortion in tomatoes.'