Brett tells Jenny the Harvest Moon is the fullest moon of the year. All full moons are the same "fullness". The Harvest Moon is just the full Moon that occurs nearest to the fall equinox.
In the greenhouse, Brett has Jenny moving small plants to larger pots & Brett says they are small plants to be used for next year's harvest. Pumpkins are an annual plant growing, fruiting and then dying in a single season. A Pumpkin plant's life cycle is too short for this method. Additionally, commercially grown pumpkins are grown by sowing seed, not transplanting.