Magical Beans!!!

1. The stump turns into a tree with a magic portal when you put all the seeds in. You may find a Forbidden Fruit if you go through the portal, which is used to turn your Sim into a PlantSim temporarily.
2. Keep asking Patchy for seeds.
3. Yes.

@PenguinFoop I believe you can get them from buying the rare plant seed pack, obviously you are not guaranteed to get them all in one go though and might have to purchase it several times.

You can also easily find them in the debug menu if you don't mind cheating :)

Does anyone know if you can get them from fishing as well? I know that some collectable and rare items can be discovered that way

@PenguinFoop, if I'm not mistaken, I got my forbidden fruit seeds on the packs that the gnomes left behind.

And I must add that the gnome glitch had contributed to me having access to those seeds, because they're quite rare.
Specially because of it, I recommend you planting you first forbidden fruit instead of eating, so you can get more of those latter. The plant resultant also has a different glow. It's very nice to see.

I've found a few forbidden fruits while dumpster diving.

Some visuals to go along with this post

Here is the tree that will sprout from the stump, and then you can explore the portal

https://i.imgur.com/YngAGRy.png

If your sim comes back with the forbidden fruit of the plantsim then eats it, they will become a plant sim too, but it is not a permanent condition

https://i.imgur.com/ojdc1RJ.png

https://i.imgur.com/fvz32iD.png

@PenguinFoop The buff says 5 days, but I think it can end sooner if you don't maintain it - like if you keep them inside for a while I think their needs will lower and then they will transform back into their usual self. I haven't really played with plantsims extensively, so I can't remember if it lasts longer than 5 days if you take really good care of them in this form or if it will just end after 5 days no matter what you do

@PenguinFoop Yep, fortunately they do not die from plantsim related needs not being met! They will simply revert back into being a normal sim (I'm guessing a plant sim can still die from regular sim deaths, but again, I have not tested this)

It's a buried hold-over from a special thing they did ages ago. If memory serves duration was 3-5 days. Would be nice if hey brought them back.

I'm guessing around 2016/17. They'd do them around a holiday, Day of the Dead at Halloween was another. EA made a big deal of them & they were fun and a break from the routine. A sim named Jasmine would come by, tell your sims about it & how to participate. Strictly voluntary & fun.