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Giant pumpkin project update July 26

A hand holding a green stem, inserting a yellow flower part into a blooming yellow flower. The flower is attached to a larger vining stem and the ground underneath the leaves is covered in black landscape fabric
Hand pollinating the northern pumpkin

Week of July 11

With the giant pumpkins beginning to flower, we began to hand pollinate female flowers. Although the pollinators do a great job, we wanted to make sure that the flowers in the best locations were pollinated. In order to hand pollinate our flowers, we selected 4 or 5 male flowers, removed their petals, and rubbed them directly onto the female flower’s stigma.

We do not have to worry about cross-pollination from our Hubbard squash affecting the size of our pumpkin because we are not harvesting the seed for next year, however, the two plants can cross-pollinate.

Most of the Hubbard squash have been removed to minimize competition for the giant pumpkin and we discovered several squash vine borer larvae within their stems on removal. Hopefully, this discovery means the squash is successful as a trap crop and we won't see any larvae in our giant pumpkin plants. We check the plants regularly for any frass or entry points, although we are slightly nervous about potentially finding any.

As the secondary vines begin to reach the edge of the plot, we have started to use stakes to guide them. By guiding the pumpkin’s vines, we are able to help the plant gain nutrients more efficiently — it also helps us with basic maintenance and removal of female flowers once our primary pumpkin appears.

Week of July 18

A basketball sized white pumpkin sitting on black landscape fabric with a vine in the background and with leaves above providing shade.
Northern pumpkin at one week of growth
An elongated softball sized white pumpkin sitting on black landscape fabric. A thin pumpkin tendril begins to reach across the top of the pumpkin and there is a papery old petal still around the top of the pumpkin
The southern pumpkin started later, but will hopefully catch up soon

The giant pumpkins have finally started to fruit! The hand-pollinated flower on the northern plant has grown to the size of a basketball over the course of one week, and an insect-pollinated flower on the southern plant has grown to the size of a softball.

Initially, we left several female flowers that had been pollinated on the vine, however, once the pumpkins had grown to the size of a softball it was apparent that these two would be the best options. Removing all of the other female fruit and flowers also removed competition for the pumpkins and they have begun to grow exponentially.

Although field pumpkins used for jack o’ lanterns are rotated for a rounder, more symmetrical appearance, giant pumpkins should not be rotated. Giant pumpkins are grown for their weight, not their appearance, and rotating a pumpkin can potentially damage its vine. To minimize potential rot, we will be placing sand and a pallet underneath the pumpkin as it grows larger.

We check daily for potential damage and vine health and have only found minor scratches on the pumpkin’s surface. The scratches are likely from a chipmunk or squirrel, however, they are not deep enough to cause any damage to the pumpkin’s health and we do not foresee this becoming an issue as the pumpkin grows larger and stronger.

Giant Pumpkin team: Julie Weisenhorn, Extension horticulture educator; Annie Klodd, Extension fruit educator; Noah Burley, Extension horticulture intern

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