This is the illustration that Jeff’s guest article refers to - see below

Off The Vine had occasional guest-written articles. This is the first - written by founder of Southern Exposure Seed Exchange, Dr. Jeff McCormack. It’s a great read and provides lots of things to consider for tomato seed savers that strive to prevent bee-induced crossing in their gardens.

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Isolation Distances for Tomatoes (by Jeff McCormack, PhD)

Most seed saving guides lack specific information about the minimum isolation distance for predominantly self-pollinated crops such as tomatoes. Published isolation distances for tomatoes vary considerably. Some sources say no isolation is necessary while other sources recommend distances up to 150 feet. Some seed saving guides omit this information entirely because of its controversial nature. The matter is controversial because isolation distance depends on a number of variables. In addition, the importance of each variable is modified by conditions specific to each growing site.

Tomato isolation distance requirements depend on the intended use of the seed. Thus isolation requirements are different for seed companies, breeders, seed savers exchanging seed, and for seed savers saving seed for only their own use. Tomato breeders may separate modern tomato varieties by as little as 10 feet, partly to avid mechanical mixing of the seed crop. Large commercial plantings are often made in areas that are bee-poor due to pesticide use of lack of suitable habitat. Therefore modern varieties planted in a bee-impoverished environment may require only 10 feet isolation. Most seed companies isolate tomatoes anywhere from 10 feet to 150 feet or more. The isolation distance also depends on the purity of the seed grown (certified seed, stock seed, etc.) Other factors affecting isolation distance are the variety and number of plants being grown.

I have heard reports that tomatoes being exchanged by members of the Seed Savers Exchange (SSE) have significant purity problems. One experienced member of the SSE reports that 10 to 15% of the tomato varieties show evidence of crossing to some degree. Clearly, many seed savers are not isolating their tomato varieties adequately. How can a seed saver determine the required isolation distance for tomatoes without making a study of the subject? Some general guidelines are needed.

Natural cross-pollination (NCP) of tomatoes frequently occurs when two or more varieties are grown in close proximity in a garden under certain conditions. Even a small percentage of NCP over a number of years could eventually cause the loss of one or more characteristics that are unique to a particular open-pollinated variety. If there is a 5% crossing each year, what will be the compounded effect of this crossing in 10 years?

Prompted by these concerns, I am suggesting some guidelines for isolation distances specifically suited to seed savers who wish to exchange seed with others. In preparing these guidelines I reviewed the scientific literature on tomato pollination, talked with tomato breeders in different areas of the country, and talked with gardeners who have saved their family heirloom seed for many years. In addition, I have made my own observations and photographed several different species of bee pollinators active on tomato blossoms.

Most tomato seed results from self-pollination, a process that is enhanced by gentle breezes that vibrate the blossom. Although cross pollination by the wind is possible, the actual amount of such cross pollination is of little significance. I have observed tomato pollen being blown for a distance of at least 8 feet. Similar observations have also been made by several of our seed saving customers, but the chances of wind pollination are so small as to be inconsequential.

Home gardeners tend to plant row crops of many varieties in a small space. These crops are frequently visited by wild bees (halictid bees, such as sweat bees) and bumblebees in search of pollen. These insects may contribute to a high frequency of NCP in bee-rich areas in crops that are primarily self-pollinated. The amount of NCP tomatoes is a function of a number of variables: (1) wind movement; (2) variety characteristics such as style length; (3) environmental variables affecting style length such as light intensity, day length and carbon-nitrogen ratio; (4) type of bee pollinators; (5) pollinator behavior on the flower; (6) isolation distance; and (7) the presence of other pollen-producing plants in the area of the seed crop.

Tomato varieties having long style (pollen-receptive organs) are more likely to be cross-pollinated by bees than varieties with short styles. If the length of the style exceeds the length of the anther cone (pollen producing organ), NCP be bees is more probable, and probability increases as the style length increases. (see Fig 1). Gardeners attempting to preserve old varieties need to be aware of this point because many older varieties have generally longer styles than modern varieties. Most modern varieties have styles equal in length, or shorter than, the anther cone (see Fig. 2). Our modern varieties were derived originally from wild tomato ancestors (primarily from Ecuador and Peru) which relied on bee pollination to a large degree. As these wild types were transported out of their center of origin to new geographic areas, the absence of their usual bee pollinators resulted in selection for variants that had shorter styles and an increased capacity for self-fertilization. Although style length is genetically determined, environmental conditions may cause style length to increase, thereby affecting the probability of cross-pollination. Generally, small fruited cherry type varieties, and current tomatoes have a long style that extends beyond the anther cone (exserted stigma). Large fruited Ponderosa type varieties have a large stigmatic surface which in some older varieties extends at least 1-2 mm beyond the anther cone. Some seed savers claim that potato leaf varieties cross more readily than normal leaf varieties; however, I have not observed this to be the case. Many potato leaf varieties do bear large Ponderosa type fruits that develop from a flower structure that is more susceptible to crossing. To my knowledge there is no correlation between foliage type and NCP>

Another factor affecting NCP is insect activity. Generally tomato flowers are not attractive to bees if other pollen sources are available; however, in some bioclimatic regions of the US, bee visitation of tomato flowers may be quite common even in the presence of other pollen sources. Such a situation exists, for example, in regions of California and parts of the mid-Atlantic region. In parts of Virginia I have observed and photographed bumblebees and sweat halictid bees such as sweat bees collecting pollen from tomato flowers. Bumblebees tend to vibrate the flowers while halictid bees appear to chew the anthers to get at the pollen. In terms of their behavior and position on the flower, halictid bees seem more likely to cause cross-pollination than the bumblebees, but this has not been fully investigated.

Controlled studies of cross pollination in inter-planted tomatoes have yielded values of 2-5% NCP; however, factors such as style length, frequent visitation of tomato flowers by bees and suitable environmental conditions may produce much higher NCP values. Various studies have reported values of 12, 15, 26, and 47% NCP values in inter-planted tomatoes. The wide range of results reflects the influence of different methods and variables used in these studies. Clearly, NCP values can be very high under the right conditions.

What does all this mean for gardeners wishing to save their own open-pollinated tomato seed where there is high bee activity on tomato blossoms? Modern tomato varieties (style length equal or less than the anther length in most cases) should be separated by a distance of approximately 10 feet to give a high degree of purity. Large fruited older varieties, small cherry type tomatoes, and currant tomatoes require 25-75 feet isolation distance (for our location in Virginia). These recommended isolation distances give average purity values of approximately 99-99.5% or better. Because occasional out-crossing may occur at large distances, plants used for stock seed may require an isolation distance of 75-150 feet or more. At the minimum, seed savers (who exchange seed) should separate tomato varieties by at least 10-12 feet. For large fruited varieties or varieties wish an exserted stigma the isolation distance should be increased to at least 20-25 feet. These are general guidelines for minimum isolation distance. Factors that call for an increase in isolation distance include: (1) an increased number of plants of each variety; (2) an increased number of varieties; (3) a large pollinator population, especially bumblebees and sweat bees. Factors that allow a decreased isolation distance include: (1) presence of alternate pollen sources actually utilized by the potential pollinators; (2) collection of seed from the center of the block planting; (3) collection of seed during the period of peak production rather than at the start of production; (4) presence of tall barrier crops; (5) isolation in time rather than space. Certain flowers are especially attractive to bumblebees and halictid bees. These include bee balm, coneflower, hollyhock, and sunflower.

The relationship between isolation distance and NCP is geometric rather than linear. Thus as isolation distance increases, the amount of NCP falls off rapidly. A study by Currence and Jenkins (1942) illustrates this point very well. It is evident that even a separation of a few feet between varieties in a small garden will greatly reduce NCP of tomatoes even though minimum isolation distances cannot be achieved. NCP can also be reduced or eliminated by taking advantage of different blooming times of early and late varieties (provided early planted tomatoes are pulled out before the late planted varieties bloom).

Seed savers should not be discouraged from saving their own seed because of the probability of NCP. Though a small amount of NCP could eventually improve a variety, it could just as easily cause the loss of quality of a variety. If you are trying to preserve a variety in its purest form, then isolation distance becomes very important. Although a small amount of NCP may not be a problem one year, its effects are additive and detrimental to preservation efforts in the long run. The goal is not just to save the variety from year to year, but for generations to come. Toward this end I’ve recommended that seed savers grow larger amounts of plants of each variety in order to produce more seed. That way it will not be necessary to grow out as many varieties each year provided your seed is properly fermented (stirred at least twice daily), dried with silica gel and stored in an airtight container (preferably frozen); you’ll have enough seed to exchange for years. I’m sure you’ll want to grow some varieties every year. Why not set aside an areas for these to be planted side by side for edible harvest only?

Selected references:

Bennet, J. 1983. A tomato blossom for all seasons. Horticulture Volume 61, page 53.

Currence, T. M., and Jenkins, J. M., 1942. Natural Crossing in Tomatoes as related to distance and direction. Proc. Am. Soc. Hort. Sci, Vol. 41, pg 273-276.

Rick, C., M. 1949. Rates of natural cross-pollination of tomatoes in various localities of California as measured by the fruits and seeds set on male-sterile plants. Proc. Am. Soc. Hort. Sci. Vol 54, pg 237-252.

Rick, C. M., 1950. Pollination relations of Lycopersicon esculentum in native and foreign regions. Evolution Vol. 4, pg. 110-122.

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I’ve been fortunate to be a friend of Jeff for many years, though it has been quite a few since we’ve had a chance to see each other or talk. I have an enormous amount of respect for his intellect and gardening knowledge. As I’ve told folks who ask about my seed saving when planting many varieties quite closely together, I use timing for my seed saving, focusing on the first few clusters of flowers, as I’ve noted bee activity is absent in my area early in the season. The one tactic that Jeff does not cover is bagging blossoms for those gardeners who wish to, or need to, guarantee that crossing cannot occur. Loosely shrouding a cluster of unopened flowers with a light, airy material, such as Reemay, creating a barrier to bee visits, works well. After the flowers open and small tomatoes are observed, that cluster can be marked and used for seed saving with complete confidence that the variety will be uncrossed. It was good to read that article again. It is amazing to think that Jeff wrote the first version of this topic back in 1984 - nearly 40 years ago.