If you were born before the 1990’s, you remember a time before seedless watermelons. Today, seedless watermelon is immensely popular. I think half the fun of eating watermelons is spitting the seeds, but then I’m no lady. Regardless, the burning question is, “Where do seedless watermelons come from if they have no seeds?” And, of course, the related query, “How do you grow seedless watermelons with no seeds?”
First off, seedless watermelons are not completely seed-free. There are some small, almost transparent, seeds to be found in the melon; they are unremarkable and edible. Occasionally, you will find a “true” seed in a seedless variety. Seedless varieties are hybrids and are derived from a fairly complex process.
Hybrids, if you remember, do not breed true from seed. You may end up with a mutt of a plant with a mix of traits. In the case of seedless watermelon, the seeds are actually sterile. The best analogy is that of a mule. Mules are a cross between a horse and a donkey, but mules are sterile, so you can’t breed mules together to get more mules. This is exactly the case with seedless watermelons. You have to breed two parent plants to produce the hybrid.
All interesting seedless watermelon info, but it’s still not answering the question of how to grow seedless watermelons with no seeds. So, let’s move on to that.
Seedless melons are referred to as triploid melons while ordinary seeded watermelons are called diploid melons, meaning, that a typical watermelon has 22 chromosomes (diploid) while a seedless watermelon has 33 chromosomes (triploid).
To produce a seedless watermelon, a chemical process is used to double the number of chromosomes. So, 22 chromosomes are doubled to 44, called a tetraploid. Then, the pollen from a diploid is placed on the female flower of the plant with 44 chromosomes. The resulting seed has 33 chromosomes, a triploid or seedless watermelon. The seedless watermelon is sterile. The plant will bear fruit with translucent, nonviable seeds or “eggs.”
Seedless watermelon growing is much the same as growing seeded varieties with a few differences.
First of all, seedless watermelon seeds have a much more difficult time germinating than their counterparts. Direct sowing of seedless melons must occur when the soil is at a minimum of 70 degrees F. (21 C.). Ideally, the seedless watermelon seeds should be planted in a greenhouse or the like with temps between 75-80 degrees F. (23-26 C.). Direct seeding in commercial enterprises is very difficult. Overseeding and then thinning is a costly solution, as seeds run from 20-30 cents per seed. This accounts for why seedless watermelon is more expensive than regular watermelons.
Secondly, a pollinizer (a diploid) must be planted in the field with the seedless or triploid melons. A row of pollinizers should be alternated with every two rows of the seedless variety. In commercial fields, between 66-75 percent of the plants are triploid; the rest are the pollinating (diploid) plants.
In order to grow your own seedless watermelons, either begin with purchased transplants or start the seeds in a warm (75-80 degrees F. or 23-26 degrees C.) environment in sterile soil mix. When the runners are 6-8 inches (15-20 cm.) long, the plant can be transferred to the garden if soil temps are at least 70 degrees F. or 21 degrees C. Remember, you need to grow both seedless and seeded watermelons.
Dig holes in the ground for the transplants. Place one seeded watermelon in the first row and transplant seedless watermelons into the next two holes. Continue to stagger your plantings, with one seeded variety to every two seedless. Water the transplants in and wait, about 85-100 days, for the fruit to mature.
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Fruit development normally begins when one or more egg cells in the ovular compartment of the flower are fertilized by sperm nuclei from pollen. In some plants, however, fruit develops without fertilization, a phenomenon known as parthenocarpy. Parthenocarpic fruit has advantages over seeded fruit: longer shelf life and greater consumer appeal.
The most frequent reasons for lack of seed development are pollination failure, or nonfunctional eggs or sperm. In many plants, self-incompatibility genes limit successful fertilization to cross-pollination between genetically different male and female parents. This property is exploited by citrus farmers who grow seedless fruits, such as navel oranges and clementines. Because these cultivars are self-incompatible, they fail to set seed when they are planted in orchards of identical plants (clones). These plants have a high frequency of parthenocarpy, however, so they still produce fruit. Such trees do not require seed for propagation. In fact, propagation by seed would be disadvantageous because the progeny would differ from the parent. Instead nurserymen frequently propagate fruit trees asexually, usually by grafting.
Another frequent reason for lack of successful fertilization is chromosomal imbalance. For example, the common banana is triploid. In other words, it has three sets of chromosomes. Instead of having one set of chromosomes from each parent, it has two sets from one parent and one set from the other parent. Triploids seldom produce eggs or sperm that have a balanced set of chromosomes and so successful seed set is very rare. Bananas, too, are parthenocarpic and produce fruit in the absence of successful fertilization. These bananas are asexually propagated. After the stalk has flowered and borne fruit, it dies. But there are side shoots or suckers at the base of the main stalk, which can be removed and replanted to continue the cultivar. Growers also propagate bananas by tissue culture.
Seedless watermelons are particularly interesting because they must be propagated by seed, and yet growers can still exploit parthenocarpy. One way to make seedless watermelons is to produce triploid seed. As in the case of bananas, triploid watermelons cannot produce functional seed, but they still develop good fruit through parthenocarpy. Plant breeders produce triploid seed by crossing a normal diploid parent with a tetraploid parent, which itself is made by genetically manipulating diploids to double their chromosome number. In the case of watermelons, this manipulation has to be performed each generation, so it is a somewhat expensive proposition but still worthwhile.
Plant biologists have learned that if the plant hormone auxin is produced early in ovule development, parthenocarpic fruit can grow on plants that do not usually exhibit this property. Thus, genetic engineering will most likely give consumers parthenocarpic fruit in many other species in the near future.
Watermelon is a very flavorsome and nutritious food. It is juicy, refreshing and filled with a lot of nutrients. It can be enjoyed on its own and also as juice, salad and dessert. The seeds in watermelons are quite a nuisance especially if you like to drive into the juicy slice and take a large bite. The solution is simple get seedless watermelons. Yes! It is possible! Growing your own watermelons may seem as a very difficult task to do. However, contrary to the notion, watermelon is a very undemanding fruit and is very easy to grow. All it asks for is a lot of water and sunshine. The seedless varieties are a bit harder to grow than the seeded ones and are therefore usually pricier. Seedless varieties can be grown by crossing them with seeded watermelons.
Watermelons demand warm temperatures to grow. The seeds need a temperature of 75 degrees Fahrenheit. So it is better to start the seeds indoors and then transfer the watermelon vines outside in the ground when they are 6-8 inches long. You can also buy transplants from the nursery if you do not want to grow right from the seed.
Get watermelon seeds from your local seed shop or nursery. You have to get both diploid (seeded) and triploid (seedless) seeds because the seedless watermelons are sterile and cannot pollinate. They need a seeded watermelon to pollinate and grow a fruit.
Dig holes on the ground where you want to grow the watermelons. Now sow one seeded watermelon in the first hole. Sow seedless watermelons in the next two holes. Follow this pattern for all the seeds one seeded and then two seedless. Cover the seeds with 3-4 inches of soil so that they can germinate well. Press the soil down a bit so that the seeds are in contact with it. You should remember the sowing pattern so that you can distinguish your seedless watermelons.
Water the seeds regularly as the melons need a lot of it for the juice. Make sure that you follow a particular schedule for watering and taking care of the watermelons. You will be able to enjoy the fruits within a couple of months. Do remember that you will also get regular watermelons in this way, though their quantity will be half of the seedless ones.
In more temperate parts of the country, direct sow watermelon seeds in ground after all danger of frost is past. Make sure they have full sun, which is at least six hours per day— though more is better! Add a balanced 10-10-10 fertilizer to the area according to package directions. Poke a hole in the soil, place them about ½ to 1 inch deep, lightly cover with soil and pat down. Put 4 to 5 seeds per hole because birds and other critters sometimes go after them. Keep watered until germination you’ll see sprouts in about 5 to 7 days. When the plants get their first three leaves, thin them out so that 2 to 3 hardy-looking seedlings remain. Use mulch or straw around the plants to keep down weeds and retain moisture.
To nature, sterility is an atrocity – a heinous crime. Nature encourages the survival of its species (precisely, its genes) at all costs. Such a crime can only be committed by tinkering with a plant’s cellular structure either, naturally (very unfortunate) or artificially, through deliberate cross-breeding, for example.
A watermelon embedded with seeds possesses 22 chromosomes — thread-like entities that carry the genes. Such a species is called a diploid. However, the chromosomes can be doubled by treating the plant with the chemical colchicine. The new species, a tetraploid, now possesses 44 chromosomes. Now, when the tetraploid is pollinated by a diploid, or when the diploid’s male reproductive cell fuses with the tetraploid’s female reproductive cell, a seed possessing 33 chromosomes is formed, a triploid. This seed grows into a watermelon plant that bears seedless watermelons.
The seedless watermelon plant is often called the “mule” of the plant kingdom, for a mule is the sterile offspring of a male donkey and a female horse. (Photo Credit: Pixabay)
It is imperative to know that by “seedless”, we mean that the watermelon isn’t replete with the seeds that we normally refer to as seeds — the brown, minuscule pebbles — but rather, it possesses the white, translucent and softer ones that are less hated. These are immature seeds whose coatings haven’t completely developed. The first problem is therefore solved we discovered how to create one, but how do we ensure that it procreates?
In a vegetation of seedless watermelon plants, only two-thirds are triploids, while the rest are diploids. A watermelon plant sprouts both male and female flowers, but its sterility renders its male flower impotent. To ensure the continuation of its seedless species, the female flower must be pollinated. The pollens are provided by the diploids planted around it. This is primarily achieved by agents, such as a honey bee, which acts as a wing-man and carries pollen from the neighboring diploids and drops them onto the triploid’s females. The flower matures and transmutes into a delightful fruit, except that it is seedless.
Seedless fruits aren’t necessarily grown by cross-breeding. Vines that bear seedless grapes are grown by cloning. They are descendants of the very first vine that bore seedless grapes. For more details, click here.
Seedless watermelon has advantages and disadvantages, but everybody wants to know, “How do you grow watermelon if it doesn’t have seeds?”
Commercial production of seedless watermelon began in the 1990s. Since then it has steadily increased to be a major part of today’s watermelon market. Early seedless varieties did not have the sugar and flavor levels of seeded types, but plant breeders have improved these traits and new varieties no longer have these problems.
However, one problem that does continue is seed germination. Initially, seed germination of seedless watermelon was quite low. One solution is to keep seed warm (90°F) until it germinates and emerges from the planting media. Still, this is difficult in cool climates where well water can have temperatures in the 40°F range. Each time the seeds are watered it lowers their temperature.
On a small scale, warm temperatures can be maintained by watering transplant flats, covering them and letting them heat up in the sun in the greenhouse for a day or more. Then, plant the seed and cover them again until seedlings emerge. On a large scale, they can be placed in dark rooms heated to 90°F with 95 percent relative humidity and held until seedlings emerge. Either process will take four to five days. After emergence, seedlings are then finished off in the greenhouse for three weeks and then transplanted to the field late May or early June. Following these steps generally produces a more than 90 percent germination rate. High germination rate is important since seed of seedless types is quite expensive compared to seeded varieties.
The standard number of chromosomes in watermelon is 22. This is called the diploid number (di meaning two, as in dissect – cut in two). With this even number, cell division is highly regular and produces pollen and egg cells with 11 chromosomes that recombine to produce seed with the usual 22 chromosomes. Through a chemical process, the chromosome number can be doubled from 22 to 44 (tetraploid, tetra meaning four). Cell division in plants with 44 chromosomes is, again, highly regular and will produce pollen and egg cells with 22 chromosomes that recombine to produce seed having 44 chromosomes. However, if pollen from a plant with 22 chromosomes is placed on a female flower of a plant with 44 chromosomes, the resulting seed will have 33 chromosomes (triploid – three sets of the base number of 11 chromosomes). This odd number does not produce (or rarely produces) viable pollen and eggs in the resulting seedlings.
Seedless watermelon fruit will have white seed traces, but only occasionally will it have a mature, brown, hard seed. Since the pollen of these plants is not viable, a diploid, seeded watermelon needs to be planted along with the seedless variety. The diploid will provide good pollen for the bees to move around and pollinate the flowers of the seedless variety. Viable pollen is needed to stimulate fruit set and growth, even though the resulting fruit will be seedless. These diploid varieties can be commercial, seeded types or simply be there as a pollen source.
Seed companies maintain diploid and tetraploid parental lines and then perform controlled crosses by hand pollination to produce seed. These additional expenses in seed production are what cause seed for seedless types to be more expensive. More watermelon information can be found at the National Watermelon Promotion Board website.
For more information on commercial vegetable production, contact Ron Goldy at 269-944-1477 ext. 207.
This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).
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