I'm with you, Kim! I thought an invasive species would only be defined as a non-native species, usually one that has a negative effect on the environment. I too wonder if it's according to farming experiences; still 'invasive' doesn't seem correct, the term 'Noxious' might be more accurate.
If the pollinators like them, so do I. Thistles are especially loved by bees and butterflies and the seeds enjoyed by many birds. We get lots of those huge Bull thistles (cirsium horridulum) here, and I let them grow as much as possible... even though dh will mow them down.
