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.
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.
If you want to be absolutely sure that your tomato seed line remains pure, then you will want to provide a physical barrier to prevent foreign pollen from being introduced. The technique most often used by home growers is called “bagging.” It is quite simple but it also is limited with regard to seed production.
To “bag” a tomato means to cover the blossoms before they open. Various materials can be used. Some use floating row cover, others use tulle (bridal veil fabric), pieces of nylon stockings, sheer tricot or other lightweight fabric, or bridal favor bags. Depending on the size of the bags used, the bags must be monitored and removed after pollenization so that the tomato can grow to full size without restriction. After removing the bag, mark the fruit with yarn or a string to identify it when it has reached proper maturity for saving seeds.
It is difficult to collect large quantities of seed using bagging. Fruits do not always form inside the bag. High temperatures and the lack of mechanical movement can hamper pollenization. Lack of mechanical movement is easily corrected by shaking the bagged trusses.
If you are really serious, and you want a large amount of seed that is 100% pure, you could build isolation/screening cages as large as required to house the number of plants you desire.
There are no hard and fast rules to follow with regard to isolation. If you are knowledgeable about the pollinating insects in your locality, you may be able to design a system that reduces natural cross-pollination to a very low level with a small amount of isolation. If you lack specific knowledge about your locality, the following guidelines may be useful.
Generally, tomato varieties should be isolated 6 to 10 meters, and they should have a pollen-producing crop planted between. The objective of the inter planted crop is to divert insects away from the tomatoes. The amount of natural cross-pollination will depend on the factors previously discussed. Generally, organic gardening methods result in many more pollinating insects than would be present in an area where pesticides and tilling have been extensively used.
To obtain 100% seed purity by isolation distance, very large separations are required, possibly 400 meters or more. Obviously, these resources and geography are difficult to achieve. Also, tomato volunteers from previous seasons could remain undetected within the isolation perimeter. Again, if you desire 100% seed purity, look to the physical isolation as provided by bagging or caging.
If you rely on isolation distances, it’s best to grow several plants of the same variety, and if in a row, harvest fruits from the inner plants and if in a square area, from the interior plants. If only a few plants, it’s best to harvest several fruits from each of the two or three plants for seed saving so as to minimize the chance of getting nothing but crossed seed if you chose only one or two fruits.
Heirloom seeds are the gardeners’ choice for seed-saving from year-to-year. Learning to save seeds is easy and fun. Before you harvest, consider which varieties you might want to save seeds from so that your harvesting practice includes plants chosen for seed saving.