Tomatoes are overwhelmingly self-pollinating, meaning a single flower can complete the entire reproductive process without needing pollen from another plant or flower. This characteristic, known as autogamy, is why tomatoes are so successfully cultivated. While they possess this biological ability, the actual movement of the pollen often requires some form of external assistance to produce fruit.
The Anatomy of Self-Pollination
The tomato flower is classified as a “perfect flower” because it contains both the male and female reproductive structures.
This arrangement is highly specialized for self-pollination. The five or more anthers (male parts) are fused together along their edges to form a hollow, pointed cone, often referred to as the anther cone. This cone completely surrounds the female style, which holds the receptive stigma at its tip.
When the flower is ready, the pollen is released inside this protective cone. Due to the flower’s downward-facing position and the close proximity of the stigma, the pollen grains simply fall from the anther walls onto the stigma below. This physical arrangement ensures that self-fertilization is the primary reproductive strategy for the plant.
Environmental Factors Limiting Fruit Set
Despite their self-pollinating nature, tomatoes frequently fail to set fruit because the pollen needs a physical trigger to be successfully released. The pollen grains must be dislodged from the anther cone to fall onto the stigma. This movement is usually provided by wind, a gentle shake from an animal, or the high-frequency vibration of a bee’s wings, known as buzz pollination.
Temperature extremes present another significant challenge to fruit development. Pollen can become non-viable or sterile when daytime temperatures exceed approximately 85°F or when nighttime temperatures remain above 70°F. Conversely, if nighttime temperatures consistently dip below 55°F, the necessary growth of the pollen tube is inhibited, which also prevents successful fertilization.
Humidity levels also play a role in limiting fruit set. Very high relative humidity, above 70%, can cause the pollen to become too moist and sticky, preventing it from shedding properly from the anthers. In contrast, very low humidity, below 40%, can cause the stigma to dry out, making it unreceptive to the pollen. The ideal conditions for pollen transfer occur when humidity is in the 40 to 70 percent range.
Practical Methods for Aiding Pollination
When growing tomatoes in sheltered areas like greenhouses or during periods of calm weather, gardeners often need to manually provide the vibration that nature usually supplies. The simplest intervention is gently shaking the entire plant or its support stake once a day during the midday hours. This action mimics a gust of wind and is often sufficient to release the pollen inside the anther cone.
A more targeted and highly effective method involves using a handheld electric device to deliver precise vibration. Many gardeners use the back of a battery-powered electric toothbrush, touching the vibrating head to the stem of the flower cluster. The gentle buzzing causes a visible puff of pollen to cloud out from the flower, confirming that the pollen has been successfully dislodged onto the stigma.
For direct hand-pollination, a cotton swab or a soft, small paintbrush can be used. The gardener gently brushes the inside of a flower to pick up the loose pollen, which appears as a fine yellow dust, and then transfers it to the stigma of the same flower. These methods are most successful when performed mid-morning on a sunny, dry day when the pollen is most viable.