For engineer David Hu, an assistant professor at the Georgia Institute of Technology in Atlanta who studies how the shells of plants and animals get their shape and size, these fruits are a biological puzzle. Small pumpkins are roughly spherical. But giant pumpkins tend to be irregular, lopsided, and as much as twice as long as they are tall.
"They look like ugly green peppers," said Hu, commenting on the shape, not the color, which remains orange.
explore why giant pumpkins change shape, Hu built a computer model. A giant pumpkin is made of billions upon billions of cells, each pulled and pushed by forces under the weight of its neighbors. To simulate these forces, Hu represented clumps of these cells as points connected by springs.
The simulation suggested that the shape of the pumpkin is not due only to gravity pushing down, but to how the internal forces affect the growth of the pumpkin. The sides of the pumpkin seem to feel the most force which, Hu believes, causes cells in these areas to divide faster and the pumpkin to grow outwards.
However, other scientists cautioned that this model is limited. Because giant pumpkins are difficult to grow and work with, Hu smashed ordinary-sized store-bought pumpkins to test their strength and fed the data into the model. But the strength of these small pumpkins might be different than that of giant pumpkins. The model also lacks pre-stresses, forces caused not due to the weight of the pumpkin, but to anatomical features built into its structure that might resist gravity.
Still, the general conclusion that some cells might be dividing faster than others is "not unexpected" said biomedical engineer Larry Taber, a professor at Washington University in St. Louis, Mo.
"The location of the greatest stresses (near the bottom) is expected, as that part must support more weight than regions nearer the top (like pressure increasing with depth under water)," Taber said.
Hu offered a lesson for any would-be mega-pumpkin growers.
"There cannot be any sudden changes in weight," Hu said. "The pumpkin has to have time to redistribute the weight as it grows."
That's a lesson that Don Young, a prize-winning giant pumpkin grower in Iowa, has learned through years of experimentation. He knows that rain and other factors in the environment can play a huge role in shaping how fast the pumpkin grows -- and that steadiness is the key.
To prepare his pumpkins for a coming storm, which could cause them to swell and burst, he gradually increases how much he waters them, slowly ramping up their growth rate.
"It's like competing in a NASCAR race," said Young. "There's a fine line between running the pumpkins so hard that they blow up and getting them to the finish line in one piece."
During the giant pumpkin growing season, which starts around April and lasts as long as 160 days, or until the last frost, Young also collects soil and pumpkin samples every week and sends them off to a lab for analysis. He wants to know which nutrients the pumpkin uses during different parts of the season.
Young and other growers, who spend as much as 40 hours a week caring for their pumpkins, are also starting to use symbiotic fungus to enrich the uptake of nutrients from the soil.
"We're experimenting a lot with trial and error," said Young. "Sometimes we move forwards, sometimes we move backwards."
The race is on to grow a pumpkin that weighs more than a ton.
"The future is thicker, longer, and heavier," said Young. "We'll see a pumpkin weighing more than 2,000 pounds in the next couple of years."
No one can predict just how big the world's biggest pie-fillers can get. Some growers think that the pumpkin's size is limited by its genetics and others, by the number of days in the growing season.
One thing is clear: to get these gigantic gourds to judging booths across the country, everyone's going to need a bigger truck.