I can not remember exactly when I started growing Verbena bonariensis but I have always missed it when I was without. I had little luck starting them indoors from seed but I suspect the seeds could be scattered about any time of the year with success. My favorite method of introducing them is to find a plant in a pot and set it out, by next year there will be a nice stand. I have been able to transplant the seedlings in spring to other spots.
Once these are established they come back reliably in my zone 6 garden but are never a nuisance.