The seedlings have been transferred to 2" pots by now and have been moved outside. Sun, wind and fluctuating temperatures make the plants stronger. To harden out the plants, it is best at first to move the flats outside during the day and inside at night. After a 3 or 4 days of this, they can stay outside all the time in a protected spot unless the night temperatures are going to be well below freezing for most of the night, then I move them back in. But since they are hardy perennials, they can take night time lows around 30 degrees or so.
The plants stay outside in the pots until about June 1, when I start transplanting them into the ground. I have better success with larger plants in a natural setting when a standard garden bed has not been prepared, so I wait for the seedlings to grow to 2 to 3 inches tall. Planting is done on cool days and definitely not all at once.