From
Sims2WikiBecoming a Plant SimThe way one becomes a plant sim through gameplay is through gardening. Becoming a plant sim while gardening is fairly rare.
First, adult or elder sims must have sprayed pesticides 10 times on plants on that particular lot. Then, every time you spray after that, there's a 10% chance of your sim being turned into a Plantsim.
Basically, you must be abusing pesticides for this to happen.
It may be easier on premade Maxis lots like the Greenmans as the lot's preexisting "pesticide abuse level" may already be high enough to provide the 10% chance of going Plant.
For a much easier way to become a Plantsim, get
Wintermuteai1's PlantSim Mod , which is a potion which will let your sim instantly become a plant sim.