The Organic Seed Grower (Chelsea Green Publishing, 2012) is the most comprehensive manual on producing quality, organic seed crops on the homestead or small commercial farm. Organic farmers are becoming aware of the ever-diminishing number of high-quality, organic vegetable seed choices available to them. Author John Navazio combines traditional know-how and the latest scientific research to place organic growing power back into the hands of local seed growers and farmers. The following excerpt, from chapter 12, "Solanaceae," covers growing tomatoes and saving tomato seeds.
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A Profile and History of Tomatoes
Common name: tomato
Crop species: Solanum lycopersicum L.
Life cycle: annual (perennial in the subtropics)
Mating system: largely self-pollinated, with increased crossing in some types due to flower structure
Mode of pollination: closed perfect flower that requires stimulation for pollen shed
Favorable temperature range for pollination/seed formation: 60–74°F (16–23°C)
Seasonal reproductive cycle: late spring through late summer or fall (4–5 months)
Within-row spacing: 1–1.5 in (2.5–4 cm)
Between-row spacing: 22–30 in (56–76 cm)
Species that will readily cross with crop: In subtropical areas there is a weedy cherry tomato (S. lycopersicum var. cerasiforme) that is fully sexually compatible.
Isolation distance between seed crops: 10–200 ft (3–61 m), depending on the crop type and barriers that may be present on the landscape
The cultivated tomato (Solanum lycopersicum) is one of the most widely grown vegetables on Earth. It is among the five most important vegetables of commerce in many agricultural societies. This is remarkable when we consider that this humble fruit was only grown in limited areas of what is now southern Mexico and Guatemala in the early 1500s when invading Spanish conquistadores found it growing in Aztec villages. This fruit was called tomatl, or “swelling fruit,” in the Nahuatl language of the Aztecs, which became tomate for the Spanish. The origin of this cultivated form is mired in mystery, as all of its wild relatives are native to mountainous regions of western South America, from Ecuador and Peru to northern Chile, including two species that are endemic to the Galapagos Islands. For many years the weedy cherry tomato of Mexico, Solanum lycopersicum var. cerasiforme, was thought to be the progenitor of the cultivated type, but recent genetic profiling at Cornell University indicates that this weedy form is in fact a feral mixture between wild and cultivated types.
Tomatoes were brought back to Spain by Hernán Cortés and were grown as a botanical curiosity. Within very few years they made their way to Italy. In 1544 Pietro Andrea Matthioli, a Tuscan physician who studied the medicinal value of plants, described them in an herbal text he was writing and suggested that tomatoes might be edible. In the second edition of this text 10 years later Matthioli first used the term pomo d’oro, golden apple. While many have speculated since that time that the first Italian tomatoes must have been yellow-fleshed, it seems that pomo d’oro was a generic phrase used for all soft tree fruit at the time and wasn’t specific to the color of the fruit. In Italy, the tomato was embraced as food in the poorer southern regions of the peninsula as well as in Sicily before becoming widely grown. There is also some evidence that tomatoes were grown as a vegetable in parts of Spain relatively soon after their introduction, but many parts of Europe only grew the crop as an ornamental for many years, fearing that the fruit was poisonous like many of the wild native members of the Solanaceae. Northern Europeans were late to the party as far as accepting tomatoes as a food plant, with the British and their colonies (and former colonies like the United States) not eating them widely until early in the 19th century.
Tomato seed has historically been produced wherever tomatoes are produced. As tomatoes spread across the globe people easily saved seed from the fruit and adapted the crop to their regional and climatic needs. The specialization of growing tomato seed in ideal environments didn’t start until well into the 20th century. As several seedborne diseases, including fusarium and verticillium wilts, along with bacterial spot and speck, became more prevalent in commercial tomato acreage, it became obvious that growing tomato seed in drier climates and controlling the irrigation after fruit set was definitely desirable for controlling the spread of these diseases via the seed.
The cultivated tomato is a short-lived perennial that is grown as an annual crop under most cultivation systems. The plant has a range of sizes, from extremely small dwarf determinates that can be 10 in (25 cm) tall to some very vigorous indeterminate types that easily reach 72 in (183 cm). Most modern commercial tomato varieties are determinate or vigorous determinates with an inflorescence borne between each leaf and range in size between 18 to 36 in (46 to 91 cm) in height. Determinate types also have terminal flower clusters at the end of each shoot. Indeterminate varieties usually bear an inflorescence every three to four leaves along the length of their shoots, and apical growth continues throughout the season until frost kills the plant.
Each tomato inflorescence usually has between 4 and 12 flowers that are formed and mature sequentially on a raceme. Individual flowers are perfect, with six bright yellow petals that curve outward, away from the flower as the flower matures. The ovary can have anywhere from 2 (especially in cherry types) to 15 or more locules, which contain the ovules. The six stamens have compact fused anthers that form a yellow cone, 0.5 to 0.75 in (1.3 to 2 cm) long, that surrounds the pistil, with its style and stigma that usually terminates within the cone but can occasionally extend slightly beyond the tip of the cone, which has a small opening. The anthers have slit openings on the interior of the cone, and when pollen dehisces it will shower out of these pores with any kind of motion of the flowers, whether from wind or insect visitation.
As the anther cone of the flower usually points downward, the pollen will thoroughly cover the bulbous stigma, it is well within the anther cone as it is with most modern tomatoes, or the cone is exerted out of the tip of the cone as it often is with many heirlooms. The pollen, which is shed over a 2-day period, will usually pollinate its own stigma within the anther cone, supplying the pistil with plenty of pollen to fertilize a full complement of ovules.
However, the stigma is often receptive a day before pollen shed and remains receptive 2 or 3 days after the pollen from its flower has shed. This means that there are opportunities for crossing to occur, especially with the exerted stigma of the older varieties. When the style pushes the stigma out of the end of the anther cone, it is exposed to possible insect activity. While tomato flowers are not visited by a wide number of insect species, they are often visited by several types of bumblebees (Bombus spp.). Bumblebees have a unique way of clinging to the flowers upside down while vibrating their wings rapidly and shaking the pollen out of the cone onto their abdomen. If the stigma is exerted then it is possible that pollen on their abdomen from a previous flower can be transferred to the flower they are currently visiting, producing a cross-pollination. This is obviously much less likely to occur with more modern tomato varieties, which have stigmas that are well encased in the anther cone; other insect pollinators, however, will sometimes pry the flowers open and cause a cross to occur.
Climatic and Geographic Suitability
Tomatoes can have problems setting seed at temperatures that are too high or too low. At temperatures above 90°F (32°C) and below 60°F (16°C) the pollen of many varieties will be affected and fertilization of ovules will be impeded, both resulting in poor seed set. In extensive experiments with tomato pollination in the 1930s, Ora Smith of Cornell University found that the optimum temperature for pollen to germinate on the stigmatic surface is 85°F (29°C); at 100°F (38°C) or 50°F (10°C) pollen germination was virtually stopped. Smith found that even at favorable temperatures pollen tube growth is slow, taking 2 to 3 days to reach the ovules following pollination. This means that, even if temperatures are favorable at the time of pollination, any temperature swings below 60°F (16°C) or above 90°F (32°C) may severely slow or stop the growth of the pollen tube on its journey to the ovules. Therefore, even when the temperature for pollen tube growth is at or near the optimum during the day, if the temperature drops to lows at or near 50°F (10°C) during the night, any of the pollen tubes that started their journey within the last day or two can stop growing. Alternatively, in hot climates the pollen can germinate and start growing during the cooler temperatures of the morning or evening and then be stifled when hot temperatures approach or exceed 100°F (38°C) in the middle of the day. Once the pollen tube stops it usually will not resume growth. If this happens repeatedly over the course of the several days that the flower is receptive then there is a good chance that most of the embryos won’t be fertilized; hence the fruit won’t “set” and will abort.
A potentially worse situation may occur when a tomato seed crop sets a full complement of fruit, but because of less-than-optimum environmental conditions the fruit can have very little seed at harvest. This can happen when the temperatures at or during the pollination and subsequent fertilization of the embryos are either too hot or too cold, as in the previous example, resulting in only a minority of pollen tubes reaching their destination. Some tomato varieties will successfully set fruit with only a fraction of the available embryos becoming fertilized, resulting in a fruit with potentially very few seeds. This often occurs when seed growers attempt to grow the so-called parthenocarpic tomatoes that can be seedless due to their unique ability to set fruit even when temperatures are too cool for most tomato varieties to successfully do so. A number of farmers have been sadly disappointed when growing a parthenocarpic tomato for a seed crop and having a good fruit set, but then finding the crop is barren of seed (or yielding very little) when they crush the ripe fruit to extract it.
Tomato Seed Products Practices
Soil and Fertility Requirements
Tomatoes can be grown on all types of soils, but they seem to benefit greatly from growing in a good agricultural loam, ranging from sandy loams to heavier clay loams that are well drained. These soils, if well drained to avoid root rots, can supply continuous moisture and fertility, which promotes good seed yields in tomato. A soil pH of between 6.0 and 7.0 is desirable, and excess fertility, especially nitrogen, should be avoided; it can promote luxuriant foliar growth to the detriment of fruit growth. Phosphorus should be readily available, which is sometimes problematic under organic fertility regimes, especially under cooler-than-optimum temperatures. In most cases high-quality compost with a good humus fraction is adequate to meet the needs of a tomato seed crop.
Growing the Seed Crop
Planting the Crop: The first step in growing a successful seed crop is to produce healthy, disease-free tomato starts. Seed can be sown into greenhouse hot beds or seedling flats; in warmer climes it can be planted into cold frames. Plants are then frequently pricked out from these initial thick plantings and transplanted into flats or individual pots within a couple of weeks at 6 to 10 in (15 to 25 cm) apart on the greenhouse bench. Seed can be sown as early as 10 to 12 weeks before the projected transplant date. However, many growers prefer a transplant that is stocky and no more than 8 to 10 in (20 to 25 cm) tall when setting them out into the field. Producing a stocky transplant that resists getting leggy can be accomplished by subjecting the plants to good, strong air currents on a regular basis over the duration of their time growing under cover. Seedlings should be grown at a moderately rapid rate and hardened off without exposing them to extremes of cold, below 55°F (13°C), or extremes of direct sun when first being moved outdoors. Lastly, starting with seed that is free from seedborne diseases is very important, and appropriate steps to avoid contaminated seed should be strictly adhered to by all tomato seed growers.
Crop Spacing: When transplanting to the field, tomato plants can be spaced at much the same spacing as when growing the crop as a vegetable. Spacing depends in large part on the harvest methodology and the degree of vigor in the tomato variety being produced. Determinate tomato varieties vary greatly in their size and the extent of the canopy they produce. Some vigorous determinate types are in fact intermediate in their stature between the average determinate types and the indeterminate types. Indeterminate tomato varieties can also vary considerably in their size and stature and will require a range of plant spacings in order to grow the best seed crop.
Upon transplanting into the field, plants of determinate varieties are routinely planted anywhere from 14 to 24 in (36 to 61 cm) apart within the row if staked, and often at increased spacing if no support system is used and plants are allowed to sprawl on the ground. The between-row spacing is anywhere from 36 to 72 in (91 to 183 cm) for determinate varieties, depending on how vigorous the variety is that is being grown. Some growers will also plant two rows on a bed with support to increase the population. Compact determinate varieties that don’t require any staking can be grown at even tighter spacing based on their size and stature.
Indeterminate tomato varieties usually require greater spacing and are usually staked for seed production. Staking tomatoes for seed production is always desirable when possible as it minimizes the soilborne diseases that may possibly infect the crop. The spacing between plants within the row for determinate plants is from 24 to 36 in (61 to 91 cm), and the spacing between rows is similar to the spacing for determinate types, though usually starting at 48 in (122 cm) and going up to 72 in (183 cm), depending on the vigor of the crop and the spacing required for your harvest methods.
Tomato harvest for seed is done in essentially the same way that it is done for the fresh market. The major difference is that fruit harvested for seed is often picked at full dead-ripe maturity, where fruit harvested for the fresh market is often harvested at various stages of immaturity, even when sold locally. Damage to the fruit at harvest should be minimized to allow for roguing out diseased or rotting fruit at the time of processing for seed (this is also very important if you want to extract an additional product such as sauce or juice from the fruit pulp). While the tomato seed may be at its peak maturity when the fruit is dead-ripe or even a little overripe, it is important to not leave the fruit on the vine to the point where it is rotting, as any excessive rotting can either damage or discolor the seed.
Once the tomato fruit is harvested it is also important to extract the seed in as short a period of time as possible, as the inevitable rotting of the fruit from saprophytes (bacterial or fungal organisms that feed on damaged or dead tissue) will grow quickly, especially on any damaged fruit. To harvest the seed the fruit needs to be crushed and mashed until the seed is largely freed from the locules, which are the cavities in which the seed is borne inside the tomato. In many of the machines used for wet-seeded crops, the seed is then forced through a circular wire mesh screen by centrifugal force. This allows the seed, the juice, and small pieces of pulp to pass through and go into a container but catches larger pieces of pulp and most of the tomato skin. This fruit debris then comes out the end of the cylinder, and if the operation is done with properly maintained food-grade equipment, the pulp can be used for salsa or sauce. Historically there were companies that harvested tomato juice in connection with seed production using specialized equipment. If you are processing small batches of fruit it is possible to simply press the tomatoes through a wire mesh screen that allows the seed to easily pass through, while catching much of the pulp.
Seed Extraction Methods: Tomato seed is enclosed in a gelatinous sac that clings tightly to each seed. The traditional method used to break down and eliminate this sac is to ferment the seed, juice, and pulp with the endemic yeast that occurs naturally on the skin of the fruit. The fermentation of tomato seed is an effective way to separate it from the gel and one that is acceptable under organic farming practices and organic certification. In contrast, most of the conventionally grown tomato seed since the 1970s has been extracted using an acid separation technique. It is often used in conjunction with partial fermentation and has become popular, as it saves time and produces a very clean-looking, light tan–colored seed. In this method a controlled amount of hydrochloric acid is added to the seed pulp, mixed thoroughly, and only allowed to interact with the seed for up to half an hour. However, it is a potentially dangerous process, and policymakers agree that it doesn't conform to most organic standards worldwide. At the time of this writing it is still the industry norm, but there is debate over whether it should be acceptable for certified organically grown tomato seed.
Seed Fermentation: The fermentation process is a fairly straightforward process using tubs or tanks to hold the seed that is still suspended in the tomato juice and pulp that has been run through a screen. Some authors encourage adding water to this mash, but this dilution is seen as a hindrance to achieving the full potential of the fermentation process. Some tomato seed growers also believe the presence of water increases the likelihood of germination occurring during this process. This may be based on the fact that the juice of the tomato, which is largely from the locules of the fruit, has sprout inhibitors that are diluted when water is added to the mix.
An important factor in encouraging fermentation is to ensure that temperatures are held at between 72 and 80°F (22 to 27°C) during most of the time that the seed is fermenting. The time it takes to achieve full seed extraction using fermentation is in large part dependent on the temperature that the seed mash is exposed to. The fermentation period will only take 2 to 3 days if temperatures are maintained at the upper end of this scale. Most tomato seed producers agree that it is desirable to have this process take no more than 4 days. If the temperature of the fermenting mash remains much below 70°F (21°C) for any appreciable period of time the gel may not fully separate from the seed, and undesirable fungal or bacterial growth can affect seed quality and lower the germination percentage. If the fermentation temperature goes much above 82°F (28°C) for any appreciable length of time during the process, then the viability of the seed can also be lowered. Therefore, fermenting seed in very hot climates will need to be done within a cooled environment. Conversely, in cooler climates it may be necessary to ferment the seed in a greenhouse or heated building.
From the beginning of the fermentation process, the fibrous pulp and the enclosed seed float to the surface of the fermentation vats. As the seed separates from the gelatinous sacs it will sink to the bottom if it is sound, while the pulp, remnant sacs, and any unviable seed will float to the surface. To encourage this separation regular stirring of this mash should occur at least twice a day and sometimes more often in rapidly fermenting batches. This stirring is also very important to discourage the formation of mold on the pulp at the surface of the mash, which can discolor or damage good seed in this floating material.
Fermentation and Seedborne Diseases: Fermentation of tomato seed is often purported to kill a number of seedborne diseases that may affect tomatoes. Unfortunately, the only pathogen that seems to be affected is the bacterium that causes bacterial canker (Corynebacterium michiganense), and the effectiveness of fermentation on it is dependent on the temperature that is maintained and the duration of time that fermentation occurs.
Washing Seed: When the seed has fully separated and collected on the bottom of the fermentation vessel, it is time to wash out all of the pieces of pulp that remain. There are at least two methods commonly used for commercial quantities of seed. For both methods the first step in washing the seed is to remove as much of the floating mass of pulp as possible. After a final thorough stirring to release good seed that will sink from the floating mass, it is important to either scoop and discard as much of the pulp as possible, or to slowly and gently add water to the vessel till the top few inches of pulp spills over the lip of the vessel. This latter option must be done quite slowly and gingerly with a low-volume stream of water so as to not disturb the good seed that is on the bottom of the vessel.
The first seed-washing method uses a sluiceway or flume, which is a long narrow trough, built at a slight decline of approximately 1 in 50. This is the same technology used by gold miners during the California Gold Rush of 1849. These seed-cleaning sluices are usually made of either stainless steel or wood, with a trough 12 to 18 in (30 to 46 cm) wide and anywhere from 10 to 25 ft long (3 to 7.6 m). On the bottom of the trough are a series of riffles or crosspieces that are 2 to 3 in (5 to 8 cm) high and run diagonally every 12 to 24 in (30 to 61 cm) along the length of the flume.
As the contents of the fermentation vessel are mixed with water and gently poured into the top of the trough, the heavier, viable seed is caught behind the riffles and the lighter pulp is easily carried down the length of the trough until it is eliminated over the spillway at the end of the sluice. After the riffles have accumulated a good amount of seed, only water is run until all of the pulp and debris has cleared. Then the riffles are taken out and the seed is washed down onto a clean fine-mesh screen at the spillway. Controlling the flow and speed of the water to properly clean tomato seed with a sluice requires practice and experience, and the trough should always be equipped with a fine-mesh screen at the spillway during the cleaning to capture any good seed that may be washed through the system due to error.
The second seed-washing method is useful for smaller batches that are frequently done in varied-sized pails, from 5-gallon (19-liter) buckets to 55-gallon (208-liter) drums. Be sure to be conscious of the previous contents of these vessels. For organic seed producers it is important to remember that any recycled receptacles used for seed cleaning have to meet organic certification standards.
At the end of the fermentation process the floating pulp that has accumulated at the top of these smaller vessels is easily eliminated by tipping and scooping off as much of the pulp as possible before the actual washing of the seed begins. Always make sure you have stirred this pulp one final time, and give the good seed that has been freed a minute to settle to the bottom of the vessel.
After scooping off the bulk of the pulp and debris, it is time to rinse and decant the seed mass in repeated cycles with cool, clear water. This is done by first filling the vessel with water, stirring, allowing the good seed to settle, and then gently decanting off the pulp that is suspended in the water without disturbing the good seed sitting on the bottom of the vessel. At first the liquid will be quite cloudy with debris, but upon repeated cycles the water will get clearer each time. The idea is to stir up the debris every time you add water, then wait long enough to allow the seed to settle, and then pour off the liquid while the pieces of pulp, placenta, and non-viable seed are suspended in the liquid. This process usually needs to be repeated at least 8 to 10 times to eliminate most of the debris before the water runs clear and the seed is clean. The debris at the end of the process is the hardest to decant off as it is the heaviest and will sink almost as fast as the good seed; it takes a deft hand to get these pieces out of the vessel without losing any good seed.
Drying Seed: Wet-seed extraction requires that the seed be dried as soon as possible after cleaning. When processing large quantities of tomato seed it is actually desirable to wring or squeeze the excess moisture out of large seed masses, which can be done after the seed is placed in strong cloth sacks. In fact, tomato seed in these cloth sacks can be run through unheated spin-dryers, then placed on drying screens. Seed racks need to be elevated to allow airflow both below and above the seed, which should be spread out evenly and fairly thinly on the racks. Tomato seed can be dried in direct sunlight as long as the heat at the surface doesn't exceed 90°F (32°C); higher temperatures can damage the seed. The important factor in drying all seed crops is to always have good airflow. Never hesitate to use fans when good airflow is lacking, even sometimes if you’re drying seed outdoors. As the seed dries on racks, stir it at least twice a day.
In humid climates or during cool, wet weather, tomato seed is often dried with the help of controlled, supplemental heat. In large seed-processing facilities there are often cabinets where seed racks are placed on the top of long wooden or metal open-topped cabinets in rows, then warm, dry air is forced up through the racks. This warmed air must be blown through the cabinet with enough force to reach all of the seed racks along the length of the structure.
Tomatoes are largely a self-pollinated species, so they do not usually exhibit as much genetic variation as many cross-pollinated crops. Because they are a fruiting crop, most of the evaluation selection of the characteristics that distinguish each particular variety is done after the crop has matured fruit. However, a number of traits can be checked before the plant sets fruit.
It is usually possible to determine differences in leaf shape or leaf color when the plants are still in the pots in the greenhouse with only the first few sets of true leaves. When the plants are forming their first flower clusters it is usually quite easy to see the proportion of flowers to leaf nodes. Determinate types usually have one flower cluster per node, while indeterminate types often bear only one flower cluster every third node. Some tomato breeders evaluate tomato fruit shape shortly after flowering has started, by examining the very small, newly formed fruit shortly after fertilization has occurred. The shape of these small immature fruit is essentially the same as what they will become when they mature. Seed growers have used this method to their advantage by growing tomato plants to the point of flowering while still in pots, then selecting for shape before transplanting to the field.
As the fruit attains full maturity it is possible to judge if each plant’s fruit is true to type. Important traits to consider when roguing to type include fruit shape, color, and relative size. Quality traits such as flavor, texture, and juiciness can also be evaluated when the fruit ripens. The fruit of any plant should always be judged collectively and as an average of all of the fruit on that plant, as any one fruit may differ from the others due to the environmental conditions during its period of initial growth.
Determining adequate minimum isolation distances for tomato seed production has become a lively topic, with few people involved in organic seed production wanting to pin their reputations on specific recommendations. The published isolation distances that have been used for years by the conventional seed industry have been discredited with the advent of unacceptable levels of crossing between adjacent tomato crops. There are many factors that contribute to increased cross-pollination in all crops. The accepted theory to explain this is that two fundamental differences in organic production methodology increase the likelihood of cross-pollination in this largely self-pollinated species.
The first factor that contributes to higher rates of cross-pollination is that there is often more biological diversity among plant species within their farms. Many organic farmers strive to have more crop diversity in their fields with both a more diverse number of crop species on their acreage at any given time and more active rotations across years. Both of these factors will usually lead to a richer, more diverse cohort of insect species. This is almost always true when a number of these crops are flowering crops that attract insect pollinators. Many organic growers also try to have some percentage of their diverse crop mix flowering at all times to attract beneficial insects.
The second factor is that organic farmers also often use less insecticide than their conventional counterparts. Many insecticides are broad-spectrum compounds that have the effect of killing a wide range of insect species, including the beneficial pollinators. Also, when organic farmers choose to use an insecticide that is certified by organic standards, it is likely that these products will degrade quickly and have less of an overall impact on the populations of beneficial insects on their farms. The potential increase in beneficial pollinators on organic farms from these two factors must certainly increase the number of cross-pollination events in any given season.
Tomato flowers are of two somewhat distinct types:
1. Modern tomato varieties, usually those bred and released after about 1920, have styles that are shorter than their wild ancestors did and that are usually well encased inside the anther cone. This greatly reduces the chances of the stigma coming in contact with any foreign pollen, even if an insect visits the flower.
2. Heirloom types or varieties with wild ancestry often have styles that are long enough to extend past the end of the anther cone, with their stigma clearly exposed to contact with a visiting insect. Many heirloom tomatoes, especially the potato-leaved and older beefsteak types, have this phenotype that can often be crossed, especially by diligent bumblebees that grab the cone from the bottom and push it right up against their abdomens. The tomatoes with wild ancestry—both the currant tomato (S. pimpinellifolium) and most cherry tomatoes, which are derived from the feral cherry of Mexico and Guatemala (S. lycopersicum var. cerasiforme)—also usually have the exerted styles.
The recommended minimum isolation distances between two different tomato varieties for the modern types should be 75 ft (23 m) if the crops are planted out in the open with no barriers and can be reduced to 50 ft (15 m) between them, when the two crops are separated by a significant barrier of the landscape. While these distances are much increased over the isolation distances published in many older seed guides, they are currently being used by a number of the larger conventional seed production companies in Europe.
The isolation distance will need to be doubled for the heirloom class of varieties, which includes many of the potato-leaved types and beefsteak types. These varieties have long been known to cross-pollinate at higher rates than the modern types that are more commonly grown for commercial production. In a thorough study of cross-pollination in tomatoes from the 1920s that was based on differences in style length, J. W. Lesley found that varieties with exerted styles could cross-pollinate up to 5% per generation. For this reason it is important to separate any heirloom varieties from other heirlooms or any other tomato varieties by at least 150 ft (46 m) in open terrain, or by 75 ft (23 m) when natural barriers are present.
Jeff McCormack, a pollination biologist who has thoroughly examined the research into cross-pollination in the Solanaceae, believes that the minimum isolation distance used for the currant types and most cherry tomatoes should be at least as great as the minimum distance used between the heirloom types.
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Reprinted with permission from The Organic Seed Grower by John Navazio, published by Chelsea Green Publishing, 2012. Buy this book from our store: The Organic Seed Grower.