There are two categories of crops. The first kind is the crops that can be harvested only once and the other kind is renewable (meaning, they can be harvested numerous times). It is advised to plant crops at the start of the season so they can be harvested more and yield larger profit. Flowers can also be grown on your farm, except they are not shippable (except Red Magic Grasses). That does not mean they are worthless; players can plant them to use as gifts for townsfolk and/or decorations for their farms.