Tomato blossom drop, technically known as flower abortion, occurs when the plant sheds its flowers before they can develop into fruit. This is a physiological response, not a disease or pest issue, indicating the plant is under environmental stress. The plant prioritizes survival over reproduction, leading it to prematurely shed the developing flower structure.
Managing Environmental Stressors (Temperature and Humidity)
The primary atmospheric trigger for blossom drop is extreme temperature fluctuation, particularly heat exposure. Tomato plants generally thrive in daytime temperatures between 70°F and 85°F, but once temperatures consistently exceed 85°F, reproductive processes begin to fail. High heat causes the ovules to become non-viable or prevents the pollen from properly germinating on the stigma. This physiological failure means fertilization cannot occur, and the plant signals the flower structure to drop, preserving energy.
Cooler night temperatures can also cause flowers to abort, especially below 55°F. Nighttime temperatures affect the plant’s respiration rate and the viability of the pollen produced the next day. If the plant experiences a significant drop, the energy required for fruit set is diverted away from the flower structure, leading to shedding.
Gardeners can mitigate intense heat by deploying shade cloths that block 30% to 50% of direct sunlight during the day. Placing these covers over the plants effectively lowers the localized ambient temperature around the foliage and flowers. Some growers utilize evaporative cooling, such as misting the undersides of the leaves or the surrounding air early in the morning, which lowers the localized air temperature as the water quickly evaporates.
Humidity extremes also play a role in the plant’s decision to drop flowers. When air humidity is too low (typically below 40%), the pollen can become desiccated and fail to adhere to the stigma. High humidity (above 80%) can cause the pollen to become overly sticky or prevent anthers from releasing pollen effectively. Improving air circulation in enclosed spaces, like greenhouses, helps balance these conditions and ensures optimal pollen dispersal.
Ensuring Successful Pollination
Even when environmental conditions are ideal, successful fruit set depends on the physical transfer of pollen within the flower. Tomatoes are self-pollinating, meaning each flower contains both male anthers and female stigma parts and does not strictly require external pollen. However, the pollen must physically move from the anthers to the stigma, a process that requires physical movement or agitation.
In natural settings, this agitation is typically provided by wind or the specialized buzzing action of insects, such as bumblebees. Without this movement, the pollen often remains trapped within the flower structure, preventing fertilization. In protected or still environments, such as inside a screen cage or greenhouse, the plant may drop the unfertilized flower regardless of viability.
Gardeners can substitute for natural movement by manually vibrating the flowers during peak pollen release hours, usually mid-morning. Using a small electric toothbrush to gently touch the flower stems provides the necessary high-frequency vibration to dislodge the pollen. Alternatively, lightly tapping the main support stakes a few times each day achieves a similar result.
Some growers employ specific flower-set sprays, which contain plant hormones, to encourage fruit development without successful fertilization. These products typically use auxins or gibberellins to chemically induce the ovary to swell, even if pollen transfer failed. While these can rescue a crop by inducing parthenocarpy, they do not address the underlying issue of poor pollen movement or temperature-related viability problems. Relying on these sprays should be a last resort after optimizing environmental controls.
Optimizing Soil Moisture and Nutrition
The consistency of water delivery to the root zone is just as important as the atmospheric conditions surrounding the plant. Blossom drop is a frequent symptom of inconsistent watering, where the plant experiences cycles of drought stress followed by waterlogging. When the plant encounters water stress, it immediately sheds energy-intensive parts like flowers to conserve resources.
To avoid this stress, establish a consistent, deep watering schedule rather than frequent, shallow applications. Deep watering encourages the development of a robust root system, which helps the plant better manage minor fluctuations in surface moisture. Allowing the top inch of soil to dry out between watering sessions is good practice, but the subsoil should remain consistently moist.
Nutritional imbalances, particularly an overabundance of nitrogen, contribute to flower abortion. High nitrogen levels encourage prolific vegetative growth, resulting in dense, leafy plants with few flowers, a phenomenon called “going to vine.” The plant directs its energy toward foliage rather than reproductive structures, neglecting the blooming stage.
To support healthy flowering and fruit set, ensure the fertilizer provides adequate levels of phosphorus and potassium. Phosphorus supports the development of strong roots and flower buds, while potassium is involved in regulating water use. Switching to a fertilizer formulation that is lower in nitrogen and higher in these other two elements during the blooming stage can help retain flowers.