“Fruit set” in tomatoes is the successful transformation of a fertilized flower into a developing fruit, which dictates the final yield of the plant. Temperature is the most important environmental factor governing whether a flower develops or drops off, a phenomenon known as “blossom drop.” The tomato plant maintains a relatively narrow optimal temperature range for this process, possessing distinct high and low temperature limits for successful fruit production.
The Upper Temperature Threshold for Fruit Set
Tomato plants cease setting fruit when temperatures consistently exceed their upper tolerance limits, a common concern during mid-summer heat waves. The critical failure point begins when daytime high temperatures consistently rise above 85°F to 90°F (30°C to 32°C). Nighttime temperature is often the more significant constraint; when temperatures remain consistently above 70°F to 75°F (21°C to 24°C), the plant’s ability to set fruit is severely diminished. This prolonged heat stress causes the plant to prioritize survival over reproduction. A short, acute period of extreme heat, soaring above 104°F (40°C) for just a few hours, can also immediately cause flowers to abort and drop. The failure to set fruit under high heat is primarily a reproductive failure, not a lack of general plant health. The plant may still appear vigorous, producing new blossoms that fail to be fertilized if temperature thresholds are continuously breached, resulting in a temporary cessation of fruit development.
The Lower Temperature Threshold for Fruit Set
While heat stress frequently causes blossom drop, fruit set is also inhibited by exposure to temperatures that are too low, common in early spring or late autumn gardening. Tomato plants require warmth to initiate the reproductive process successfully. Fruit set becomes significantly inhibited when daytime temperatures fall below 65°F (18°C). The lower limit is more clearly defined by nighttime temperatures; consistent lows below 55°F (13°C) prevent fertilization and fruit development. Unlike the rapid blossom drop associated with high-temperature failure, cold stress manifests as a significant slowing of growth. Flowers may remain on the plant but fail to progress into fruit, waiting for warmer conditions. This cold inhibition delays or halts the reproductive cycle until the thermal environment improves.
How Temperature Impacts Pollen and Fertilization
Temperature thresholds cause fruit set failure by directly compromising the function of the flower’s reproductive components. High heat has a devastating effect on the pollen. At elevated temperatures, pollen grains become less viable, often turning sticky or non-functional, preventing them from being released or adhering properly to the stigma. Heat stress also negatively affects the female stigma, which may lose its receptivity. Furthermore, high temperatures can trigger “stigma exertion,” a physical deformity where the stigma pushes out too far past the cone of male anthers. This separation prevents the self-pollination that tomatoes rely on, even if the pollen remains viable. Conversely, cold temperatures impact the process after the pollen lands on the stigma. The pollen grain must germinate and grow a pollen tube down the style to reach the ovule for fertilization. Low temperatures significantly slow the growth rate of this pollen tube, causing it to fail to reach the ovule within the required time window. If the growth is too slow, the flower aborts before fertilization is completed.
Managing Temperature Stress to Encourage Fruit Set
Gardeners can implement practical measures to mitigate temperature extremes and encourage successful fruit set. During intense summer heat, providing afternoon shade is an effective strategy to lower the plant canopy temperature. Suspending a light-colored shade cloth (blocking 30% to 50% of sunlight) over the plants during the hottest part of the day helps keep the plant within its optimal range. Maintaining consistent soil moisture is also beneficial, as the plant uses transpiration for evaporative cooling. A deep watering in the morning helps the plant manage the heat load throughout the day. For small-scale applications, gently misting the flowers during the hottest hours can momentarily lower the surrounding air temperature and increase local humidity, which aids proper pollen shedding. To avoid the low-temperature threshold, strategic planting is the primary tool, ensuring the fruit-setting period does not coincide with unseasonably cold weather. If unexpected cold snaps occur, using temporary row covers or cloches at night can trap enough residual heat to keep the nighttime temperature above the critical 55°F limit. These management techniques focus on maintaining the microclimate around the plant, allowing the reproductive processes to proceed normally.