Given where it's growing, I'm guessing the seeds were carried down in a rainstorm and sprouted late. Which suggests there's more of it somewhere up in the hills, which pleases me.

A few years ago some was growing in a different area of the park, and then was pulled out. I was disappointed, but had to agree that they were probably right: you don't want poisonous plants growing next to a street that is blocked off during the day so elementary school kids can play at recess.