Remember the famous stand at Thermopylae? The Greeks trying to keep the Persians out? They failed, and alfalfa arrived in Europe. Yes, really. That was the invasion of alfalfa, and it was only the beginning. The earliest record of cultivation of this Asian was Iran before the Greek wars. Now it is grown and has escaped throughout the temperate world. It also grows wild throughout, so why aren't we fighting this invader? Alfalfa is not, in itself, a threat to native plants. These flower heads are more or less a half inch, and are occasionally white, yellow or varicolored. Alfalfa now occurs in every state and province. Lenawee Co MI, 5/23/12.