It’s not a square versus round issue to begin with

I’ve never seen this before, but some pretty good explanation’s above.

The round pots don't fit tightly together like the square pots do. That allows the round pots to cool down more quickly at night. That means the soil temperature is consistently warmer in the square pots, because most of the heat comes from directly absorbing sunlight, not heat from the air. Soil temperature controls sprouting and root formation. That's why many people, myself included use heat mats to germinate seeds, to callous cuttings and to more quickly root cuttings.

Keith

I filled these containers with dirt (soil if you want to be a nerd about it), planted the seeds and placed all the containers in the black tote they are pictured in. They have never been taken out. The tote gets rotated 90 degrees each day to get them all equal amount of sun time. When I water them, they stay inside the tote so in theory, they all get equal water, heat and sunlight.

I like the square for efficiency in the use of space and the waterproof nature of plastic has its plus, once you get 6000 plants in a small greenhouse the air inside tends to be moist all the time creating its own biosphere

Les, I hoped you would chime in. I never put up my greenhouse this year. You are a heck of an accomplished gardener so can you offer any insight as to why the square black ones sprouted first?

Not really,except that they are all in a group ( same temp), and if one gets active… they all do if they are equal, has to do with pheromones, but I can’t explain it. I do everything in batches all the same. And I quit doing things that don’t work well… like peat pots.

No peat pots? Ok, I appreciate the tip! Thank you.

They do sort of resemble baby pumpkins.

I stuck a rubbing post in the field already. If I plant pumpkins too, I may need a rifle that is belt fed or something.

LOL. Now I am curious what they actually are?

All of the above, black draws more heat, less cooler air around the pots and also the square pots hold less soil and then can warm faster. White reflects sunlight, black draws it in.

Bryce