Pollination is the biological process that allows plants to reproduce, leading to the development of fruit and seed. It involves the transfer of pollen, which contains the male genetic material, from the stamen to the stigma. For a tomato plant to produce a mature fruit, successful pollination must occur within the flower. Understanding how this process functions is necessary to determine if different varieties can exchange genetic material through cross-pollination.
The Tomato’s Self-Pollinating Design
Tomatoes have “perfect” flowers, meaning each blossom contains both the male reproductive organs (stamens) and the female reproductive organ (pistil). The male stamens are fused together, forming a protective, hollow structure known as the anther cone. This cone completely surrounds the female stigma and style, creating a self-contained environment. This physical arrangement makes self-pollination the dominant mode of reproduction for tomatoes.
When the pollen matures, it is released from the anthers through small slits inside the cone structure. Since the stigma is positioned just inside the tip of the anther cone, the pollen naturally falls directly onto the receptive surface of the same flower. This self-pollination process is often enhanced by simple movement.
In an outdoor garden, wind or gentle shaking can provide the necessary agitation to release the pollen. Commercial growers often use electric vibrators or introduce bumblebees to create low-level vibration. This mechanical movement ensures the pollen grains are effectively transferred from the anther to the stigma.
Conditions That Facilitate Crossing
While self-pollination is the rule for tomatoes, cross-pollination can occur when external factors bypass the protective anther cone. This event requires an agent, typically an insect, to physically move pollen from one tomato variety to the stigma of a different variety. The most effective insect vector for this is the bumblebee.
Bumblebees engage in a specialized behavior called “buzz pollination.” The bee grasps the flower and rapidly vibrates its wing muscles, causing the anther cone to shake vigorously and release a cloud of pollen. If the bee moves to a different variety, it can deposit this foreign pollen on the new flower’s stigma.
Environmental conditions also increase the likelihood of crossing by altering the flower’s structure. High heat, with temperatures consistently above 90°F, or periods of high humidity can cause the female style to elongate, resulting in an “exserted stigma.” When the stigma protrudes significantly past the tip of the anther cone, it is openly exposed to pollen carried by insects or wind from neighboring plants. This structural change facilitates crossing.
The rate of natural cross-pollination (NCP) is low, typically less than 5% in garden settings. However, it can be higher in areas with high bumblebee activity or where varieties with naturally exserted stigmas are grown. Currant and cherry tomato varieties, for example, cross more frequently due to their flower characteristics.
Protecting Varietal Purity in the Garden
For most home gardeners who simply want to grow and eat tomatoes, the low rate of cross-pollination is not a concern, as it does not affect the fruit grown in the current season. However, for those interested in saving seeds, cross-pollination can introduce unwanted genetic material. This contamination affects the variety grown in the next generation. Therefore, preserving varietal purity requires intentional action.
Seed savers must consider “isolation distance” to minimize the risk of insect-mediated crossing. Modern tomato varieties require planting different types approximately 10 to 25 feet apart to maintain purity. Older or wilder varieties, such as cherry tomatoes, may require a greater separation distance, sometimes up to 75 feet or more.
For serious seed savers with limited space, a reliable method involves using physical barriers. Individual flowers can be covered with small cloth or paper bags before they open, preventing insect access to the stigma. Once the flower has been successfully self-pollinated and the small fruit begins to swell, the barrier can be removed. This technique ensures the seeds saved from that specific fruit will be genetically pure.