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You need to fertilize the female To prevent self fertilization, the pumpkin has a "Male" phase followed by a "Female" phase. If you only have one plant you won't get any pumpkins as you won't fertilize the female. I had this problem so went with an ear bud to an allotment nearby and got some pollen from somebody else's male plant to fertilize my female flowers. I now have two excellent large pumpkins growing.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?