I never spent very much time playing with Plant Sims in The Sims 2. I never got around to making one myself, although at one point I downloaded one from the Exchange and played with it a little bit before I got annoyed and bored and moved it out.
Even though I bought the Pets expansion pack when it first came out, I have only barely done any breeding of my pets until recently. Sure I bred a horse or two, but nothing like the concerted effort I have been making lately to create my own hideous Frankenstein-like dog breed. (I think ugly things are more interesting than pretty things!)
If you have successfully managed to impregnate your Sim by an alien, the next step is to raise your alien/human hybrid. Unlike the occult states (werewolf, vampire, fairy, etc) aliens are an entire full-fledged race, with their own DNA. The occult states are an on/off thing, where you're either a werewolf or you aren't.