I started this sweet corn here in MN about 3 weeks ago, it came up nicely about a week ago. The tallest are about 3 inches. When do you recommend me planting into the garden? Less than 30% chance of freeze this far into Spring in MN. The risk of leaving indoors too long would be roots too long. thank you

You're right about the risk of the roots getting away on you I believe they're ready to go out now.

You might pick up a bit of thermal protection by putting milk jugs/water bottles next to your plants. When I put my corn transplants (tomatoes, peppers, melons, etc. too) out in the hoophouse, I snuggle a 1-gallon milk jug (filled with water) up next to the plants and put a 5-gallon Culligan water bottle (with the bottom cut off and the lid removed) over top of the transplants and the milk jug. In my case, I'm planting in groups of three or four plants per hill (a family tradition).

Good luck!