While traveling through Appalachia, the player character may receive a prompt to visit the Pumpkin House and subsequently speak to Jack O' Lantern. Jack O' Lantern will task the player character with bringing it ten pumpkins to carve.
Jack will mark several nearby areas on the map where pumpkins can be collected (one is near Fissure site Sigma, which usually has a scorchbeast flying overhead). However, as long as the player character has ten pumpkins in their inventory, the quest can be completed, regardless of where these pumpkins are from.
While Jack marks the nearby Fissure site as a place to collect pumpkins, it is much easier (and safer) to search the back of the Pumpkin House, as there are several pumpkin patches that can be used to quickly complete the quest - 4 patches are around the garage and another 2 are south from the house, next to a cliff.
Additionally, using the Green Thumb perk allows for collecting double the yield from each pumpkin picked.
After handing over the pumpkins, Jack O' Lantern will provide a reward. This reward always includes some pumpkin grenades, the plans for crafting the grenades (if not yet known), and radioactive pumpkin seeds which are a necessary component for crafting the grenades.