It worked!
It didn't take long, once I'd got Petunia to give up her job and buy a few more trees to make an orchard (only about 6 trees). I kept mousing over the trees to see when the 'spray' or 'spray many' option came up and made her do it, no matter what she was in the middle of (sleeping, showering, etc). Within two Sim days she changed into a Plant Sim. It helped that it was summer at the time.
I moved her out and into another house in the neighbourhood which I then redesigned. I replaced the roof with glasshouse walls and roof, with a tree growing up through the middle and a mezzanine floor. I may take a pic next time.
After rearranging the inside and out, neighbours visited. She spread her happiness spores over them (so another Malcolm Landgraab and Gilbert Jacquet talked about kissing), then decided to shake her head and produce a baby plantsim called Salvia.
