Tomatoes often remain green during hot weather because elevated summer temperatures halt pigment production and interrupt pollination, according to horticulture experts. Publication Good Housekeeping reports that tomatoes require six to eight weeks from pollination to reach maturity, depending on the variety and weather conditions.
Gardener Nate Muri said multiple factors influence when fruit matures. Former farmer Frank Hyman noted that slow ripening is usually not caused by grower actions, and plants will eventually yield ripe fruit as conditions allow.
Temperature and humidity limits
Chris Enroth, a horticulture educator at the University of Illinois, said ideal daytime temperatures for fruit development range from 21°C to 29°C, while optimal nighttime temperatures for pollination, fruit setting, and fruit development range between 16°C and 21°C. Daytime temperatures above 29°C slow fruit development, and temperatures exceeding 35°C during the day and 24°C at night cause pollen to lose viability. The plant then drops its flowers, creating a lull in fruit growth.
High humidity also impairs fruit setting. Enroth explained that relative humidity above 70% causes pollen to clump, reducing pollination even when normal flowers remain visible for days. In addition, Muri said temperatures reaching 32°C or higher prevent the plant from producing lycopene and carotene, the pigments responsible for red coloring, leaving fruits green.
Methods for managing ripening
Growers cannot speed up the ripening process directly and must wait for cooler weather, Hyman said. Enroth noted that temporary heat spells generally do not cause permanent damage to the plant, and normal flowering, pollination, and fruit setting resume when temperatures moderate.
To assist ripening late in the season, Enroth recommended removing new flowers and small fruits that lack time to mature. This action allows the plant to redirect its energy into ripening established fruit. Hyman also advised picking tomatoes at the first sign of redness to prevent skin cracking from uneven soil moisture, though early picking does not accelerate ripening.
