How Do I Prevent Tomatoes from Cross-pollinating?
If tomato varieties are planted in close proximity, pollen from one variety can land on the female part of a blossom, the stigma, of a different variety and lead to some or all hybrid seeds being formed in that fruit. This is commonly referred to as a "cross-pollination" or simply as a "cross." When cross-pollination occurs, the fruit will look perfectly normal in the current season; however, the resulting seeds carry genes from each parent and will produce varying progeny in subsequent generations.
The word pollinator is usually used to describe the ways that pollen can be transferred from the blossom of one variety to the blossom of another variety. Pollinators can include wind, insects, mechanical vibration of the blossom, etc. Thus pollination describes the transfer of pollen between varieties, commonly known as cross-pollination.
The word "pollenize" is usually used to describe pollen from a single blossom that fertilizes that blossom, thus this process is often called pollenization, to distinguish it from pollination, the transfer of pollen between the blossoms of two different tomato varieties.
If you are not interested in saving seeds, then you can safely ignore cross-pollination issues. Tomato varieties will produce fruit consistent with the varieties planted. Again, any crossing in the current season affects the seeds within the fruit, not the fruit flavor or structure.
If you are attempting to save seeds and maintain a pure tomato variety, some efforts must be taken to avoid cross-pollination. The extent and seriousness of your efforts will depend on the importance of the variety and its intended usage. If the variety is typical, widely available, or intended for home use, then you may welcome a cross as an interesting diversion. However, if the variety is a rare family heirloom, or intended for distribution as a specific named variety, then crosses must be actively avoided.
Keep in mind that if a rare or one-of-a-kind variety is crossed, it will be lost forever. There is no way to fully reverse a cross.
Tomato Reproduction Details
Tomato blossoms are perfect, meaning they have both male and female reproductive structures. They are capable of pollenization without the aid of pollinating insects and without pollen from other blossoms.
Tomatoes are an inbreeding plant but they do not suffer from inbreeding depression (loss of vigor in subsequent generations). Although not generally recommended, a tomato variety can be maintained by saving seed from a single plant.
Some tomato varieties have exerted stigmas which means that the stigma is positioned outside of the anther cone and it is more susceptible to foreign pollen. Modern varieties generally have shorter styles so they are much more likely to pollenize. Style length is genetically determined but it can vary based on environmental conditions.