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.
After Grandma's recipe - which is only a list of ingredients lacking measures or instructions - is found, some of the ingredients laid out on the counter in the following scene do not appear to match those on the list.