I have not found any growfruit in the wild, though I now have three trees, thanks to Jasmine. I have thoroughly searched the neighborhoods, because I was looking for Basil. One thing I have noticed, is that Jasmine's presence has caused many of the plants in the neighborhood to grow, and evolve. The ones in the Garden Boxes are all mostly mature now. Many wild plants around my house are much bigger than they were. Still, there are still trees that are in their sapling phase.
A while back, when starting a new game, I noticed that some plants didn't exist on the new lot, but the plants that did exist had the same locations, as the other game. So, I am wondering if there are set spawn points in the neighborhood, and un-sprouted locations have a chance of coming up as something new, like a growfruit, but if they've already sprouted, they stay as what they are? That's just one explanation I thought of.
Also, I was wondering, if talking to Jasmine makes any difference? If you are aware of the challenge, but you don't get your growfruit seeds, does that make it more likely that the game will change some plants into growfruit trees, or spawn new ones?