Being inspired to give zinnia breeding a try by maineman and jackier_gardener, I am working on the system.
One of my hopes was to grow the plants to first bloom stage before setting them to the garden. I proposed to use 6 inch sections of 4" lightweight PVC pipe as open bottom pots. The pipe was to be cut lengthwise to allow the soil to be released and the pot pulled up and over the plant, resulting in a minimum of root disturbance.
The original concept
I bought a 10 ft joint of the pipe yesterday for a little under $5. The lengthwise cut was made in some of the pipe and then chopped into 6" sections. I had thought that in use I would need to use duct tape to pull the cut edges together, but after cutting them I found that the pipe tries to spring together on its own. If the edges are allowed to offset the pipe will bend to overlap about a half inch as shown below.
So it doesn't look like I'll need the tape after all.
I have made a design addition of two half inch by #10 sheet metal screws. The pipe was drilled and the screws set leaving a gap between the pipe and screw head as shown.
Screws are a little above halfway up the pipe from the bottom.
Next a special planting tool was made from a shelf bracket as show below.
Shown below is a simulated use of the Pipe Pot and planting tool. The pot is set in the hole and the bottom point of the tool is placed between the two screws.
The tool handle is then pressed down which spreads the pipe open to release the soil block and the screws lock into the handle notches.
One hand is then used to pull soil around the plant as the other uses the handle to pull the pot up and over the plant.
Next I will need to make up a tool to lift the bottomless pots from the flat surface used for growing and transferring them to the garden hole.