The premature dropping of tomato flowers, known as “blossom drop,” occurs when the plant sheds unfertilized flowers before they can develop into fruit. This action is a protective mechanism, signaling that the plant is under stress and attempting to conserve resources. This shedding ensures the plant does not commit energy to fruit development when conditions are unfavorable.
Temperature as the Primary Inhibitor
Temperature extremes are the most frequent environmental trigger for blossom drop, directly impacting the viability of reproductive material. Tomato plants set fruit most reliably when daytime temperatures are consistently below 85°F. When temperatures exceed this threshold for several days, the high heat denatures the proteins within the pollen grains, causing them to become sterile, making fertilization impossible.
Consistently warm nights above 70°F are equally important. These temperatures prevent the plant from properly metabolizing and storing sugars necessary for flower maintenance. The plant interprets these prolonged high temperatures as a signal that the environment is too harsh to support fruit development, leading to flower abortion.
Conversely, cold stress also inhibits the reproductive process. When air temperatures fall below 55°F, the plant’s overall metabolic activity slows down significantly. This reduction prevents the necessary growth of the pollen tube after landing on the stigma, thereby hindering successful fertilization.
Gardeners can mitigate high-heat stress by providing temporary shading during the hottest parts of the day using shade cloths. If an unexpected cold snap is anticipated, utilizing row covers or temporary plastic cloches can help trap ground heat around the plants overnight. Maintaining temperatures within the optimal range of 65°F to 85°F is paramount for encouraging fruit set.
Interruptions to Successful Pollination
Although tomatoes are self-pollinating, the transfer of pollen still requires physical movement. The pollen must physically shake loose from the anthers and land on the stigma of the same flower for fertilization to occur. Any atmospheric condition that impedes this physical transfer can lead directly to flower abortion.
High atmospheric humidity is a significant deterrent because it causes the microscopic pollen grains to absorb moisture. This absorption makes the pollen sticky and heavy, preventing it from easily shaking free from the anther. Environments with relative humidity consistently above 70% often experience increased blossom drop due to this clumping effect.
Conversely, extremely low humidity can also be detrimental, causing the stigma—the receptive female part of the flower—to dry out prematurely. A dry stigma is unable to properly receive the pollen grain, which prevents the subsequent growth of the pollen tube required for fertilization. The ideal range for pollen release and reception is between 40% and 70% relative humidity.
In environments lacking natural air movement, such as enclosed greenhouses or exceptionally still gardens, the necessary vibration to release the pollen is absent. Wind and visiting insects provide the natural physical agitation required. Gardeners can manually assist the process by gently tapping the flower clusters or using a low-frequency vibrating tool, such as an electric toothbrush, to simulate the buzzing of a pollinating insect directly on the flower stem.
Managing Water and Soil Nutrient Levels
Cultural practices and soil composition introduce internal stresses that can prompt the tomato plant to abort flowers as a resource management strategy. Inconsistent watering is a major cause of this stress, affecting both the physical structure and the internal chemistry of the plant. Both prolonged drought stress and excessive overwatering can lead to flower drop.
Drought conditions signal to the plant that internal moisture reserves are too low to sustain the water-intensive process of developing fruit, triggering the plant to shed flowers to conserve water. Overwatering, however, can be equally damaging by suffocating the root system, which limits the uptake of oxygen and nutrients needed for reproductive functions. Consistent, deep watering that keeps the soil evenly moist, without becoming saturated, is necessary for reliable fruit set.
Soil nutrient composition also plays a role, particularly an excess of nitrogen. High nitrogen levels promote a vegetative state, encouraging the plant to put energy into lush stem and leaf growth rather than reproductive parts. This imbalance results in the plant dropping flowers because it is prioritizing foliage production over fruit development.
To encourage flowering and fruit development, the focus must shift to nutrients that support reproductive growth, namely phosphorus and potassium. Phosphorus aids in flower formation and energy transfer, while potassium regulates water use and overall plant vigor. Gardeners should transition to fertilizers lower in nitrogen once the plant begins flowering. Applying organic mulch around the base of the plants helps regulate soil temperature and ensures a more consistent moisture level, reducing the stress caused by rapid fluctuations in water availability.