I forgot about another variable. Rabbits! In less than a week they devoured every one of the outdoor romaine plants. Yeah I could have put up a fence or planted them in one of my raised beds, but the greenhouse plants are clearly the winners of this contest.
Can't see the lettuce? That's because it's gone!
The indoor plants, on the other hand, have nearly doubled in size this week, and have required very little watering. Humidity levels inside the greenhouse allow me to skip days of watering.