The Pumpkin Harvester is an automatic harvester used in conjunction with the Pumpkin Farm to automate the farming of Pumpkins and Melons. It is added by the Forestry mod, as are other harvesters and farms. It will harvest any mature pumpkins or melons within a range of 21x10x21 (X-Y-Z) Pumpkins and Melons are automatically ejected into Buildcraft pipes connected to the Pumpkin Harvester. Items can also be placed into adjacent Chests. The Pumpkin Harvester harvests Pumpkins as blocks and Melons as Melon Slices, which can later be turned into Melon blocks using a Crafting Table.
This video shows an alternative to pumpkin/melon farming using redstone.
This video demonstrates how to use all forestry farms, including the pumpkin harvester.