Explore Lulukoko, Tsukuyusa, and Westown in this multi-cultural farming game for Nintendo 3DS. [ Game Guide ] JP release = Jun 2016. NA release = Feb 2017. EU release = Oct 2017.
You have to keep harvesting them. The rank will go up at X-number harvested. I don't remember exactly how many harvests it takes to get each rank increase, though. I think it might be 300 harvested gets you to full rank, but...yeah, it's been a while, lol.