When you create a character you must select a plant type, there are 3 plants types (Cactus, Tree, and Flower) to choose from. The bonus you recieve is determined by your level (bonuses start at level 20). Plants gain 1 bonus for every 200 levels each time they train their bonus attribute. Plant type can be changed in the 'Settings' menu for $100,000.
Cactus: Offensive plants, their bonus attribute is potency.
Tree: Defensive plants, their bonus attribute is girth.
Flower: Speedy plants, their bonus attribute is responsiveness.