Peat pots do it why can't a easily made card board container be set in a hole directly in the garden! Just like DDog has the gutter with plants in itthis should work also! I was thinking of using i for almost anything! Make any size box by bending cardboard and punch a few small holes in the bottom and staple it together simple an cheap!!
For example a 3 to 4 inch deep box about 6 inches wide and 12 inches or more long. Plant your seeds as if you just planted them in the garden inside the greenhouse early! that set them out when they are ready for the outside with no disturbance to the roots.
The cardboard will decompose over the summer and the roots will grow through the bottom holes! I would set these boxes on a board or any type of flat holder support to be moved later! Worms love cardboard and would be near the roots in the garden! You could even add worms to the box! I think I will be making some this week! Even when cardboard gets wet it will still hold soil for while ! What does everyone think?