Harvested it well into winter..and it came back the next year!
The seeds germinated very easily in mid May in our zone 6 garden in NY. We ate the small plants in salads as we thinned the row. Heads filled out nicely and were larger than I thought they would be, about 10-12 inches tall. Plants turned nice red color (with the ribs still slightly green) in late September. We harvested by cutting whole heads just above the soil line. I was surprised to find that the plants sent up new heads from the root, and a few plants even came back from the root the following spring. Definitely growing this one again.