per·en·ni·al /pəˈrenēəl/ Adjective Lasting or existing for a long or apparently infinite time; enduring. Noun A perennial plant. Synonyms perpetual - everlasting - eternal - permanent
Correct. If they ever do die, they have so many seeds, it wouldn't matter. They reseed.
They breed really well, and someone planted a few in their field and now it's a whole field of them several years later.