Tomato plants are self-pollinating, meaning a single flower has the necessary parts to fertilize itself and produce fruit. This reproductive strategy ensures fruit set even in the absence of external factors like wind or insect activity. Self-pollination is defined as the transfer of pollen from the anther (male part) to the stigma (female part) within the same flower or to another flower on the same plant. Tomato plants are able to consistently produce fruit with a high degree of genetic stability.
The Anatomy of a Tomato Flower
The tomato blossom is classified as a “perfect flower” because it contains both male and female reproductive structures. The male parts, known as the stamens, are fused together to create a distinctive cone-shaped structure that surrounds the female part, the pistil.
The stamen is composed of anthers, which produce pollen, encircling the pistil’s style and receptive tip, the stigma. This tightly packed, internal configuration ensures that when the pollen is released, it is already in close proximity to the stigma of the same flower. The stigma may become receptive two days before the pollen is released, and remains receptive for several days after the flower opens. This architecture facilitates the direct and reliable transfer of pollen from the anther to the stigma.
The Process of Self-Pollination
While the flower structure makes self-pollination possible, the process often requires a physical trigger to move the pollen. Tomato pollen must be dislodged from the anther cone to fall onto the stigma. Outdoors, natural wind movement provides the necessary vibration to shake the pollen loose for transfer.
In sheltered environments, such as greenhouses, this physical agitation is often missing. Growers must manually intervene, either by gently shaking the plants or using a vibrating tool near the flower clusters. Successful pollination is dependent on environmental factors. Extreme daytime temperatures above 90°F or nighttime temperatures below 55°F can cause the pollen to become sterile or prevent proper development. High humidity can also cause the pollen to clump, making it difficult to release and transfer.
When Cross-Pollination Can Occur
Although tomatoes are self-pollinating, cross-pollination can occur. This outcrossing happens when pollen is transferred from one tomato plant variety to the flower of a different variety, typically facilitated by insects that bypass the flower’s protective cone structure. The primary natural vector for this is the bumblebee, which uses a specialized technique called “buzz pollination.”
Bumblebees grasp the flower and vibrate their wing muscles at a specific frequency, forcing the pollen to release from the anther cone. This vibration can dislodge enough pollen that some may be transferred to a different plant, resulting in a cross-pollination event.
Open-Pollinated vs. Hybrid Varieties
The distinction between open-pollinated (OP) and hybrid (F1) tomatoes is important in the context of cross-pollination and seed saving. Open-pollinated and heirloom varieties are genetically stable, meaning their self-pollinated seeds will produce plants that are “true to type.” If a cross-pollination event occurs with an OP variety, the resulting seeds will produce a mix of traits in the next generation, which is a concern for gardeners aiming to preserve a specific lineage. Hybrid tomatoes, on the other hand, are the result of an intentional, controlled cross between two parent lines, and their saved seeds will not grow true to the parent plant regardless of cross-pollination.