I try to come up with different backstories each time, but a general theme is that the motherplant came to earth as a seed hitched on an asteroid (semi-sentient plant) or as a very young survivor of an UFO crash (fully sentient, but childlike). Whether Earth's scientists wanted to help or or exploit her, she never gets fully socialized. Things worsen when she hits puberty (goes "in season") and as a neophyte her spores completely dominate the ecosystem. There's nothing nefarious about the process, just what always happens when an exo-organism enters an ecosystem that isn't prepared for it.
My favorite, however, is the version from my Star Wars save, where "Godmother" led her crime empire from her base in the crater, The Planter Pot. The spores basically allowed her to rule the planet, except for the north and south polar region, where they cannot survive. My sims were space privateers in her employ. Lt. Agnon at nearby Batuu was pretty concerned about Godmother, but couldn't do much, since he has the resistance to deal with.
Blog link with the intro: https://enkisstories.tumblr.com/tagged/sims4%20dbh%20star%20wars/chrono
