The mystery has been solved! This plant has been cropping up in my garden for several years now, I assumed a reseeding annual. It grows very quickly, reaching eight feet in one season here in my Kanata, Ontario garden. My sister has a purple version growing in her yard in Corpus Christi, Texas. It makes a great privacy screen and has pretty flowers, but apparently it is an “invasive alien”, spreads easily by seed and can take over gardens and yards. I just pulled some seedlings from my lawn to transplant into my garden in strategic bare spots. I might have to rethink that idea!