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.'
Plants this fires on
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- corpus: another rule here says what to do, and names the wrong move (W-NOREMEDY -> 0)
- promote to verified — tomato fruit set fails at high night temps
- cite tomato heat sources; correct night threshold 24C->21C, add day-temp driver
Cite this page
Milpa Gardens. "Tomato fruit set fails when night temperatures persistently exceed ~21C (70F); high day temperatures (>29-32C) compound it." Evidence grade: Well established. https://milpa.garden/rules/tomato-fruit-set-fails-when-night-temperatures/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.