It is easy to provide those conditions with the double-cup method. Using 16-oz cups the soil volume is large enough to grow a substantial root mass, retain moisture in the soil to grow in a non-humid area with less frequent waterings.
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The double-cup method is using two plastic cups stacked inside one another.
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The cups are tapered from small at the bottom to larger at the top so the transplant will easily slide out of the cup when you transplant it.
Just like a mini-greenhouse.
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The bags can be taken off as the seedlings grow and then put back on during the night, effectively hardening off the seedlings to acclimate to room conditions.
The inside cup has a hole punched in the bottom or clip off 3 small slivers of plastic around the base to allow water to contact the seed starting mix you place in the top cup.
From what I've seen the idea is to get roots to grow out of the first cup into the liquid in the second cup. That isn't what a tray does. I don't have roots growing into my trays. I don't think it's a good idea to have the plant sitting in a pool of water, so if there is water in the tray some time after watering I tip it away. So roots wouldn't grow into my trays.
I watched the video: he is giving one plant nutrients and the other one just gets water. So I would say the difference in growth is because one plant is getting nutrients and the other isn't!
I don't see how using two cups eliminates watering worries.
Plants that fit those cups don't need more feeding than can be found in good potting compost.
A have more than one beef, possibly a small herd:
Plastic cups are too floppy. A plant pot needs a certain amount of resilience so you can handle it without the root ball being disturbed.
You can apply nutrients just as easily by watering the plant from above or below, standing the pot in a tray. Has he done a video like that?