How do you think the first americans discovered that they could grow their own plants

That depends upon the species. There are records of tropical pitcher plants (nepenthes) that have grown over sixty feet tall on their vine, however, this is quite rare. The largest and tallest sundew (drosera) was a d. erythrogyne that grew seven feet tall, had over a thousand leaves and seven hundred flowers. Some larger sarracenia (north American pitcher plants) can grow four foot tall traps, which make these the largest plant traps in the world. Examples include the endangered S. Oreophila and the common S. Alata. The discoverers were multiple. I can only give the data recorded.