|The following is based on Fallout Tactics and is not canon.|
Through careful nurturing you've gotten a small fruit tree to grow out of your head. Fruit will appear in your inventory from time to time.
This perk is based on Harold's tree from Fallout 2. While Harold looks like a ghoul, he isn't one - he's a unique creature created by a unique, limited exposure to the Forced Evolutionary Virus, just like his tree he is a completely new species. Because of that, this perk probably should not apply to "normal" ghouls.
Behind the scenes
Bonsai is a Japanese art form using miniature trees grown in containers.
Testing suggests this perk is broken; when wandering the wastes for weeks on end, no fruit will appear in inventory. Likewise, standing around in a bunker for hours on end will result in no fruit. It seems likely this perk does nothing. [verification overdue]