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Giant pumpkin project update June 25

Two-leafed pumpkin seedlings growing next to larger green  leafy squash plants.
Giant pumpkin seedlings sprouting in the greenhouse

University of Minnesota Extension's horticulture team is growing giant pumpkins at the Arboretum! Track our pumpkins' progress throughout the season in Yard and Garden News.

The giant pumpkin (Cucurbita maxima) has its origins in Argentina thousands of years ago.

Originating from wild squash (Cucurbita andreana), evidence suggests through fossilized megafauna dung, that creatures were eating and then depositing Cucurbita maxima seeds across the landscape. Humans began to domesticate this plant around 4,000 years ago and it was seen in many cultures throughout what is now South America.

Although this species of pumpkin is larger than its relative used for jack o’ lanterns (Cucurbita pepo), it wasn’t until the 18th century that it began to increase to the massive sizes seen in state fairs.

As Cucurbita maxima began to make its way north to European and North American countries in the 1700s through colonization, the change in the environment allowed for an increased growing period. Combined with an improved environment and selective breeding, the “giant pumpkin” began to take shape.

At the end of the 19th century, the first recorded giant pumpkin at 365 lbs. won a contest at the Chicago World's Fair. But it wasn’t until the late 1970s that, through selective breeding, a variety known as Dill’s Atlantic Giant, grown by Howard Dill, allowed the giant pumpkin record to grow from 459 lbs. (1980) to 2,703 lbs. (2021).

Today there are many different varieties of giant pumpkins grown by enthusiasts and plant breeders. The varieties being grown at the Arboretum were sourced from and bred by members of the St. Croix Growers Association. 

About the planting site

The planting site is 1,000 square feet,  gated, and surrounded by a 6-foot high fence. It is the site of a former biochar trial (2012-2015), and pollinator flowers and pepper trials (2016-2019).

On April 21, a soil test sample was submitted to the University of Minnesota Soil Testing Lab on the St. Paul campus, and results were received two weeks later: 

  • Coarse soil texture means the soil is well-drained. 
  • High organic matter (10.4%)
  • Slightly alkaline soil (pH 7.3)
  • Very high phosphorus (P)
  • High potassium (K)
  • Recommended fertilizer analysis (N-P-K): 30-0-20

Starting our seeds

We ordered seeds in the winter from St. Croix Growers who specialized in giant pumpkin seeds. The seeds were kept in sealed plastic bags in the refrigerator till planting.

May 2: In the greenhouse at the Minnesota Landscape Arboretum Head Operations building we started hubbard squash seeds. The squash seeds were planted in a germination mix in 6-inch nursery pots and the flowers in cells and placed in the greenhouse on heat mats.

The hubbard squash (Cucurbita maxima) will serve as a “trap crop” in order to lure striped cucumber beetles away from the giant pumpkins.

You might notice that hubbard squash has the same genus and species as the Giant Pumpkins. Being of the same species, but of different varieties lends itself to the effectiveness of these crops at pest attraction.

The hubbard squash will be removed along with the cucumber later in the season. With only 5 or 6 hubbard squash planted around the 1000-square-foot area, the giant pumpkins should be protected from the cucumber beetles. 

May 12: We started the pumpkin seeds in the greenhouse. Six different varieties were planted, making up eleven plants total. The top six plants will be planted in the plot, based on size. We scraped the seeds on a file to break the seed coat and allow the seed to take up water better. The seeds were planted in a germination mix in 6-inch nursery pots and placed in the greenhouse on heat mats.

 A small plant with three leaves planted in soil, surrounded by black landscape fabric.
The first giant pumpkin has been planted.

May 26: We prepared the planting site by weeding and laying down landscape fabric to reduce future weed pressure. We used a propane torch to burn away areas of landscape fabric, planted the hubbard squash transplants, and direct-seeded and transplanted flowers and herbs to attract pollinators along the perimeter of the fence.

The flowers and herbs included Music Box Mix semi-dwarf sunflowers, nasturtiums, Calendula, Tithonia, Malabar spinach, bachelor buttons, Italian and Thai basil, and golden runner beans and pole beans.

A green plant with four large leaves and young new leaves growing in its center.
Giant pumpkin plant loving the sun

June 2: It was finally time for the pumpkins to be transplanted. The weather was rather windy and sat at 73 degrees. After having waited to plant due to unexpected cold days and storms, this may not have been the ideal time to plant, but with upcoming rains, would work just as well.

Only seven of the 11 plants had grown successfully, so the smallest was chosen to continue its growth in the greenhouse as an emergency replacement.

The six pumpkin varieties chosen to be planted were 854B Anderson, 1540B Anderson, and 1963 O Stevens on the south side. And 1540B Anderson, 854 Anderson, and 1887.5 O Johnson on the north side of the bed. A single giant pumpkin needs at least 500 square feet to grow, so these plants will be allowed to grow and the team will select one giant pumpkin on the north side and one on the south side.

After planting, we watered the pumpkin plants and wrapped the stems near the soil with a 2- to 3-inch-tall piece of aluminum foil to prevent squash vine borer.

Fertilizer was applied as directed by the soil test results to increase the nutrient levels in the soil. Nitrogen fertilizer is added early on in the giant pumpkin's life cycle in order to aid in the production of vines and leaves. 

June 13: As the heat of the summer arrives, we water the pumpkins daily and check for stress in the leaves from lack of water or nutrients. This stress can show itself as wilting or yellowing of the leaves.

The giant pumpkins' foliage has expanded dramatically since their initial planting and some of the old dead foliage was pruned so the plant’s nutrients are focused on new growth.

There is no sign of cucumber beetle or squash vine borer on either the pumpkins or the hubbard squash at this point, the pollinator plants are growing rapidly and the sunflowers have already started to bloom.

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