Plantasia is played from the point of view of Holly, a fairy-in-training who must complete Faerie Tasks in order to become a full-fledged fairy. Her last task, and the one for which the player is responsible, is granting a wish. She is assigned a wish-recipient, Jack, and finds him an unpleasant young recluse whose only wish is for the gardens on the property to be fixed.
Guided by the oldest plant in the garden, Winkleroot, Holly plants flowers, waters and tends them, and then harvests them. Upon harvest she receives mana, which she can use to cast spells to buy more seeds, fairy powers, tools, or tool upgrades, or to create the statues that give her the mana to fill the whole garden. The first statues in every level counts towards the basic completion level, the last statue is for expert completion. The last statue can be delayed for strategy reasons - once it is completed the level is over, but more mana and points can be gathered even if enough mana is available to purchase the second statue.
In some cases she must prepare the earth by removing stones. Various garden pests and weeds populate the higher levels, and must be removed before they destroy the flowers. The pests are Caterpillars, Snails, Ants, Hercules beetles, Beetles, and Moths. The weeds are Weed, Crabgrass, Rockroot, Clusterweed, Dryleaf, Strangleweed, and Planteater.
Aside from the "Holly's Story Mode" there is the "Garden Challenge Mode". Mana is not used to buy statues, but instead to reach higher levels. Additional flowers for the Greenhouse (a display room reached from the main menu) are earned through this mode.