Andy Corbin runs an ancestry website—for gigantic pumpkins.

The IT worker started growing pumpkins 15 years ago. To help figure out which seeds to use, he built a database that now traces the family trees of thousands of gargantuan gourds back generations.

His pumpkin genetics tool is considered a necessity among growers of the world’s heaviest fruit. Because before they compete at weigh-offs, these hobbyists vie at auction for seeds with lineages of largeness.

“It’s like breeding racehorses,” said Ron Wallace, the first grower to break one ton in 2012.

The race to grow ever-larger gourds has unleashed a blitz of one-upsmanship at contests around the globe. The world record for heaviest pumpkin has been broken 21 times since 1993, including a record 2,819-pounder this year.

The prize for winning a weigh-off is typically a couple thousand dollars, but the horticultural bragging rights are endless. Plus, watching a pumpkin balloon from seed to giant in a few months is a prize all its own for these proud pumpkin parents.

All this means that giant pumpkin seeds are more crucial to the hobby—and more in demand—than ever.

Enthusiasts have been known to spend hundreds of dollars on a single seed. The most expensive are “proven” seeds, meaning people wait years to see how they perform with other growers before agreeing to spend a pretty penny.

Breeding two big pumpkins often yields a bigger pumpkin, but not always. “Genetics are funny,” said Wallace, who sells Wallace’s Whoppers . “It’s a roll of the dice.”

A seed from Andy Wolf’s 2,365 pound pumpkin, grown in 2021, sold for over $1,000 last year. The seed had already spawned Travis Gienger’s world record pumpkin in 2023, a 2,749-pounder nicknamed “Michael Jordan,” among others.

Travis Gienger of Anoka, Minn., holds his two-year-old daughter Lily and poses behind his pumpkin called “Michael Jordan” after winning the Safeway 50th annual World Championship Pumpkin Weigh-Off in Half Moon Bay, Calif., Monday, Oct. 9, 2023. Gienger won the event with a pumpkin weighing 2749 pounds. (AP Photo/Eric Risberg)

Spending big doesn’t guarantee success. Some seeds don’t germinate. And even the cheaper seeds favored by beginners have the genetics to produce a 1,000-pounder, said Wallace.

Many established growers simply give their seeds away. Gienger always carries some in his coat pocket to donate.

“If you have the capability to put their seed on the map, then the motivation is, ‘Hey, I would love for you to grow my seed. Here, you can have it,’” said Brandon Dawson.

He received a free seed from Gienger after losing to him by 6 pounds last year in the coveted Safeway World Championship Pumpkin Weigh-Off in Half Moon Bay, Calif.

“I think he felt a little bad,” said Dawson, who won the contest this year.

For longtimers, choosing which seeds to grow can take all winter. Space in the pumpkin patch is limited, given that one plant needs up to 1,000 square feet and 150 gallons of water a day at its peak.

That’s where Corbin’s genealogy tool can help. Pumpkin Fanatic includes more than 50,000 entries, mapping out each pumpkin’s seed and pollinator. Growers can also build their hypothetical dream seed.

Then they get ready to bid. “The growers who know what they’re after in terms of the genetic line they want to grow, they go to the auctions to get some of the most sought-after seeds,” said Ken Desrosiers, who runs bigpumpkins.com .

Each winter, his site hosts 30 to 40 auctions, which can feature hundreds of participants each. Seeds are donated, and the money goes back to local groups for prizes.

The seed is only the start, though. Enthusiasts must carefully manage the breeding process, manually transferring pollen from male flowers over to female ones, and the growth. They install heat cables in the soil to keep their pumpkins warm, build contraptions to cover them from the sun and add grow-lights for cloudy days.

“It’s just like a little pumpkin palace for them,” said Cindy Tobeck, president of the Great Pumpkin Commonwealth, a governing body for the local growers’ clubs.

Dawson, a manufacturing engineer at electric-vehicle startup Rivian, monitored his patch’s temperature and moisture levels from his phone while on business trips in Japan this summer.

The final stretch requires more coddling and, of course, cost.

Gienger spent $1,200 on gas to move his “Michael Jordan” from Minnesota to California in 2023. He added padding so the pumpkin didn’t split when he drove over bumps.

In total, he spends about $15,000 a year growing pumpkins. That’s on the higher end compared with others, but Gienger gets free supplies from sponsorships and has sold pumpkins afterward for thousands of dollars.

For all the nurture and effort, winning may simply come down to a bit of luck.

Chris Hernandez, who judges his local weigh-off and leads the University of New Hampshire’s Cucurbit breeding and genetics program, said the difference between a 2,300-pound pumpkin’s seed genetics and a 2,500-pounders’ is negligible at this point.

The genetic pool has been whittled down, and these pumpkins are “super inbred” now, he said. The current giants are all likely from the same genetic pool. Howard Dill, a Canadian pumpkin breeder, crossed and then certified the Dill’s Atlantic Giant pumpkin variety about 40 years ago.

Hernandez expected the limit to be around 2,000 pounds—but growers have blown past that. They’re now on the brink of a 3,000 pound pumpkin.

That’s going to make judges’ jobs even harder. They’re required to check the underside of the pumpkin for defects, a task Hernandez jokes could land him in the news.

“If this thing falls on me and kills me, it’s gonna be like, ‘Giant-squash squashes squash-professor,’” he said.

Write to Roshan Fernandez at roshan.fernandez@wsj.com