The sight of tomato flowers turning yellow and falling off, known as blossom drop, is a common frustration for gardeners. This loss of potential fruit is not a disease or insect problem, but the plant’s natural response to stress. When conditions are not right for the flower to successfully set and mature fruit, the plant aborts the blossom to conserve energy. The flower’s stem will typically yellow before the bloom drops completely, signaling that fertilization has failed.
Atmospheric Stressors
Temperature extremes and inconsistent humidity levels are frequently the primary culprits behind blossom drop. Tomato plants thrive and produce viable pollen within a relatively narrow temperature range. The ideal daytime temperature is between 70°F and 85°F, with nighttime temperatures ideally staying between 55°F and 70°F.
When daytime temperatures consistently climb above 90°F or nighttime temperatures remain above 72°F, the pollen inside the flower can become sterilized and nonviable. This heat stress causes the plant to burn through its carbohydrate reserves, leading to fertilization failure. Similarly, cold temperatures below 55°F interfere with the growth of the pollen tube necessary for fertilization.
Humidity also affects pollen viability and dispersal, with an ideal range being 40% to 70%. If the air is too dry, the pollen can become desiccated and unable to stick to the stigma. Conversely, if humidity is too high, the pollen grains clump together, preventing them from shedding properly from the anthers. In both extremes, the failure of the pollen to transfer or germinate leads directly to the flower dropping.
Hydration and Nutritional Imbalances
The health of the root zone dictates a flower’s survival. Inconsistent watering is a major source of plant stress, whether it is underwatering or overwatering. The plant enters a defense mode when it experiences moisture stress, prioritizing survival over reproduction by shedding its blossoms.
The balance of nutrients, especially nitrogen, profoundly affects flowering. Nitrogen is important for vegetative growth, meaning the development of stems and leaves. However, an overabundance of nitrogen encourages the plant to put energy into creating lush foliage at the expense of flowers and fruit. This is often described as having a “leafy plant” with few or no tomatoes.
For successful fruit set, the plant needs a balanced nutrient profile that includes adequate phosphorus and potassium. Phosphorus supports the development of flowers and fruit, while potassium is involved in overall plant health and sugar movement. Too much nitrogen can also interfere with the uptake of calcium, which contributes to physiological disorders.
Mechanical Pollination Failure
Tomato flowers are considered self-pollinating, meaning each flower contains both male and female reproductive parts. However, the pollen within the flower is heavy and sticky, and it does not simply fall onto the stigma on its own. Successful pollination requires physical agitation to shake the pollen loose from the anthers and onto the stigma.
In an outdoor garden, this agitation is typically provided by wind or by “buzz pollination” from bees. When plants are grown in protected environments like greenhouses or high tunnels, this natural movement is often absent. Without the necessary vibration, the pollen remains trapped, fertilization does not occur, and the flower will drop.
Immediate Corrective Actions
Addressing blossom drop requires mitigating environmental stress. During periods when daytime temperatures exceed 90°F, provide temporary shade, such as a 30% shade cloth, during the hottest part of the afternoon. This adjustment helps keep the air temperature within the range where pollen remains viable.
Establish a deep and consistent watering schedule, especially during dry spells, to prevent sudden moisture swings. Soak the soil deeply every few days instead of daily shallow watering to encourage deep root growth. If you suspect a fertility issue, immediately cease using high-nitrogen fertilizers. Switch to a fertilizer formulated for tomatoes or one with a lower Nitrogen number and higher Phosphorus and Potassium numbers.
To overcome a lack of mechanical agitation, physically assist the pollination process. For plants in sheltered areas, gently tap the support stake or cage several times each day during the mid-morning hours. A more direct method involves using an electric toothbrush pressed against the back of the flower stem for a few seconds; the vibration mimics a bee and releases the pollen.