Perennials take a little while to mature, but then provide Suns for the rest of the game. When you plant a perennial, place it with the Plant / 1 value along the edge facing upwards. Then, each time you enter the marked season, rotate the card clockwise 90 degrees. Once the Mature edge side is facing upwards, you may harvest and flip the card over.
Unlike regular plants though, if you spend the Suns on a perennials card, it is not discarded. When you spend the Suns on a perennial, flip it back over to the face side. You may harvest it again in the next appropriate season.
For example, this lemon tree takes three years to mature. It grows every summer. Then, when harvested, it will provide two Suns.