No need to cloud fantasy with the practicalities of reproduction. There are many legends of creatures, gods, beings with clear genders being spawned from rocks, foreheads, and various other weird sources. Perhaps these dwarves did it the old fashioned way and got cursed/blessed into being tubers at a later point.
at a place were magic gathers, for what ever reason, the trees of this place produce fruit that is filled with magic energies . these fruits when eaten cause different effects. much like the potion a wizard makes.