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.
Often, tomato blossoms pollenize prior to being visited by insects. This means that it is “possible” to get pure seed even when another tomato is directly alongside. Under normal conditions, most tomatoes have a natural cross-pollination rate of about 2 to 5%. Under some conditions though, this can be much higher – maybe as high as 50%. The incidence depends on the types of insects active in the area, the existence and types of inter-planted crops, the wind, the blossom structure, and the blossom timing of the varieties involved.
The tomato ovary at the base of a blossom contains many ovules. The ovules can be fertilized by either pollen from that blossom or with pollen from the blossom of a different variety if there are unfertilized ovules available. Fertilized ovules become the seeds inside of fruits, so a single fruit could have both pure (pollenized seed) and also impure (cross-pollinated seed) within the same fruit.
To maintain a “pure” variety, the grower must prevent the introduction of pollen from unrelated varieties. There are three basic techniques to prevent cross-pollination (1) protect the blossoms with a physical barrier, (2) grow plants at a time when other tomatoes are not blooming or insects are not present, and (3) maintain isolation distance between the varieties.
Using Physical Barriers to Prevent Cross-pollination
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.
Using Timing to Prevent Cross-pollination
If you can grow plants very early or late in the season so that tomatoes flowers are not available for cross-pollination, pure seed can be saved without any special efforts. This requires careful timing and growing only one variety to ensure pollenization (self-pollination).
Aside from blossom timing. You can also gain a benefit from saving fruit based on the population of your particular pollinating insects. This requires a good knowledge of the insects in your locality.
Using Isolation Distance to Prevent Cross-pollination
A general rule is that the further apart varieties are grown, the lower the probability of cross-pollinating events. Experts disagree on the distances required. For most gardeners with limited area to grow plants, distance is NOT an effective means of obtaining 100% purity. Inadequate isolation, season after season, will result in some natural cross-pollination and may eventually produce undesired changes in the characteristics of a variety.
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 20 to 25 feet, 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 a area where pesticides and tilling have been extensively used.
To obtain 100% seed purity by isolation distance, very large separations are required, possibly a 1/4 mile 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.
Many varieties that are potato leaved and have large pink fruits often show very large double and triple blossoms early in the season which are quite attractive to insect pollinators, and some other varieties do the same. And sometimes the result is fused fruits. So it’s best to not use fused fruits for seed saving since one or the other of the fused fruits could be cross-pollinated.
Special Concerns regarding the Currant Tomato
Home growers are growing more and more of what are called currant tomatoes, which are in a different species from our garden tomato. All currant tomatoes (L. pimpinellifolium) have exerted stigmas. Some folks highly recommend that you don’t plant currant tomatoes near garden tomatoes ( L.lycopersicon) since they might serve as a source of foreign pollen in terms of cross-pollination. Others say that they’d worry more about regular garden tomatoes cross-pollinating currant tomatoes, and a third group might say that not all currant tomato varieties are of concern as regards cross-pollination.
Recognizing a “crossed” tomato
Depending on the varieties involved, it may be quite difficult to realize when one variety has crossed with another. Sometimes it’s easy to tell that something went wrong because the tomatoes are far from your expectations. Other times, you may not notice any problems until you begin seeing off-types in the F2 generation. Just remember, growing your seed out the first year is not always a 100% verification that your seeds are pure.