I live in northwestern PA and I have this cactus that I grow year round, outside in summer, inside in winter. It does very well. I also always put in impatiens. Had one of my cacti sitting near my impatiens 3 yrs ago, brought it inside in the fall. I thought weeds were growing in the cacti, but I let them go to be sure. Well, sure enough there were impatiens growing in it. I have impatiens year round and I do nothing special for them than I do for my other plants. Is this uncommon for impatiens and should I be doing anything special for them? I have to keep cutting them down, have started other plants, and they are getting ahead of me.
I had the same thing happen many years ago. This is fairly common. Impatiens are a type plant that "throws" it's seed when they are mature. As a 'survival of the fittest' trait, the further a plant throws it's seed, the better it's chances for more plants. I've had some of the prettiest blooming ferns you've ever seen! While impatiens and cactus reequire different culture in nature, it is amazing how well they can live in the same container for a "brief" time. Enjoy your roommates that prove opposites attract.