Gardening, can't graft bonsai shrub?

Actually I don't think the UFO plant grows on Sixam. I think you can only obtain it while travelling in space via the rocket and getting the right combination on a chance card, finding it at a festival or it's also apparently available in the Rare seed packets you can purchase once you hit a high enough level.

Never having been to Sixam yet I can verify that this is 100% true. There are three plants that only grow on Sixam though they are the Fang Flower/Tentacle Tree, Glow Orb & Quill Fruit.

I'll back up what @3KNPen said about the plants, all true, been there quite a few times.

I don't have get to work but i'll keep the romance / spice festival in mind as I've heard there's a low chance there. Prior to all of my space explorations I have bought over 10,000 in rare seed packets and no dice.

Posting for the greater good. I checked the romance festival and got nothing. Eventually after completing the space rock collection, I got a mission with a kid alien who's mom gave me $200 simoleans for bringing her son home safe. Then my sim arrived home and completed the garden collection and then got the pop up for the UFO plant. I still need to complete the Perfect garden but I have one UFO plant and one perfect bonsai buds or bonsai shrub away.

Thanks all who helped, I'm going to take 5 roses and 5 grapes and experiment with grafting one onto another and see if anything results. Apparently with new grafting it's either grapes, OR roses, OR bonsai buds. We'll see.

Signing off.