I created my first Plant Sim today with one of the pre-made sims in Freetime, Julien Cooke. He has a mustache and goatee, but wears a green skirt and leotards I guess. See picture below.
To achieve plantsimism, I planted 13 fruit trees and sprayed them whenever they needed it. I began in the middle of the summer and he became a plant sim 
in the middle of autumn. It didn’t seem like he had sprayed that much, but as I understand, Sims become plant sims when they spray for bugs a certain number of times. He earned a bronze Gardening talent badge while tending the trees, and when he became a Plant Sim it must have jumped to a Gold badge! Now he can talk to the trees (which need it since most were sick). Since Julien’s hobby is cooking, I’ll have him cook with all the fruit that he’s grown!