There are many different types of periwinkle. The purple ones like mine, there are 2 types that look exactly alike and one is poisonous and one is not. I know it's not just me being allergic because my husband, my brother, and yard workers have all gotten a rash from it. Maybe I'm a bit more sensitive to it though. Maybe you don't have the bad kind. It's so invasive it's impossible to kill off. My husband is going to attempt to smother it out by putting cardboard over the top. I hate poison and it'd be too expensive to poison it all off because there is so much.