Every fall, pumpkin growers put record-breaking giant pumpkins on the scales. Some pumpkins weigh more than 2,700 pounds (1,225 kilograms). But the world’s largest apple barely reaches 4 pounds (1.8 kg), and the heaviest blueberry ever grown weighs less than 1 ounce (28 grams). So why does pumpkin grow to such incredible sizes while other fruits (yes, pumpkin is a fruit and even a type of berry) remain relatively small?
Giant pumpkins are specific varieties of Cucurbita maxima that have been bred to grow to giant sizes, most commonly the Mammoth or Atlantic Giant varieties. Vikram Baliga, assistant professor of horticulture practice at Texas Tech University, told Live Science that the fundamental reason plants can grow so large is because they are amorphous. Deterministic plants stop when they reach a certain size, whereas indeterminate plants grow indefinitely.
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Determined plants tend to produce all their fruit at once, which gives them an advantage in harvesting, but limits the size they can grow.
“Some plants have programs that limit the size of certain things they grow when they grow organs, whether it’s leaves, fruits, or flowers,” Jessica Savage, an associate professor in the Swenson School of Science and Engineering at the University of Minnesota Duluth, told Live Science. “Other species don’t have that kind of limit. … For some reason, pumpkins don’t seem to have very strong limits that limit their size, so they can breed to get bigger.”
Since there is nothing to limit its growth potential, the pumpkin plant can continuously produce more leaves and produce energy for the fruit.
“The goal of an indeterminate plant is to produce as much biomass as possible as quickly as possible, so the pumpkin will have leaves and stems and all sorts of other things,” Barriga says. “Then if you need to produce more energy, you just make more leaves. There are no genetic choke points.”
How to grow giant pumpkins
Growers take advantage of this by removing all but one pumpkin from the vine.
“If you have this plant that can store all these resources, and you pull eight pumpkins from that plant and only leave one, it’s like, ‘Great, I’m just going to repurpose everything into this one fruit. This is my only chance to pass on my genes,'” Barriga explained.
Technically, this trick also works with other fruits. For example, removing all but one peach from a tree will yield larger peaches. But this is where the laws of physics come into play.
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First, pumpkins grow on the ground, so they are less affected by gravity. Giant peaches cannot grow as large as pumpkins, because they fall from the tree long before they reach their weight.
Additionally, pumpkins have hard skins, so they grow larger than fruits with softer skins. “If it’s really soft, it’s not going to produce big fruit because it’s going to start to bear weight and break down,” Savage says.
However, if the skin is too hard, the pumpkin will not grow large. “People who grow pumpkins strive to get that sweet spot. You don’t want the pumpkin to become so hard that it can’t expand. … If the skin splits, it becomes uncompetitive. So the skin has to be soft enough, but the skin has to be strong enough to support its own weight.”
Giant pumpkins have soft, thin skin when young, which allows them to grow quickly. As they grow, their skin becomes tougher, Savage said. Growers cover their pumpkins with tarpaulins to protect them from the sun and keep them in the soft, rapid growth stage for as long as possible.
At peak growth, the giant pumpkin can weigh up to 44 pounds (20 kg) per day, and all of that mass has to move through the fruit’s vascular system, which Savage says makes it “super-strong.” Savage and her team found that compared to other pumpkin varieties, giant pumpkins have more phloem, the part of the vascular system that transports sugars.
“I often think of it like a highway,” she said. “You can move the same amount on a small highway, but there’s a limit to how fast you can do it. If you want to move more resources faster, you need more roads.”
Giant pumpkins also take a long time to grow. “Pumpkins stay on the vine for months…it’s like you can harvest them for five, six months, sometimes 180 days,” Barriga said. “Apples, peaches, pears and many blueberries tend to go from flowering to harvest much more quickly.”
But an equally important reason why giant pumpkins are so big is that we made them that way. “These have been selectively bred for a very long time just for size, which is different from a lot of other foods that are also selected for taste,” Savage said.
Pumpkins are a symbol of fall and central to Halloween and Thanksgiving traditions, so they hold a more important place in our culture than other fruits with similar growing potential, such as cucumbers.
Savage believes the pumpkins will continue to grow. “There will probably be a ceiling eventually, but I think we will continue to find ways to push it,” she said.
Fruit and Vegetable Quiz: Do you know where pumpkin, blueberries, and broccoli come from?
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