I've planted lemon verbena in my herb garden almost every year. It only rarely makes it through the winter, so I assume it is an iffy one for my zone (ya think). The tag on this year's model states "happiest when allowed at least semi-winter dormancy in the NW." What do you suppose that means? Has anyone had any luck growing this as a perennial, or should I continue to think of it as an annual? Any tips or tricks much appreciated. Aloysia triphylla or Lippia citriodora.