It is very important to harvest peachiis after 11 days because if it is harvested too early the fruits will have a bad bitter taste that will lower happiness by 5 and HP by 10. Peachiis are very sweet and they can be given to your pokemon and it will raise happiness by 10, HP by 50, and speed by 1. After they are consumed you will have some pits(seed) leftover and you can plant them during spring and summer on rich loamy soil. They can be used to make sweet Kailuau bars that raises cuteness by 10. Also, if you manage to have 4 peachiis, 2 Amaingos, and 2 Koconuks you can take it to a chef near the Ko cooking school to make a "Power boost smoothie", that will heal 100HP, raise happiness by 15, attack by 1, sp.attack by 1, and speed by 2 when used on a pokemon during battle.