Zone 4b here at 5000 ft elevation. Mine were started indoors under growlights near the woodstove for warmth. Maybe the darker green of the square pots allowed more solar gain from the nearby window. It was not entirely consistent, but plants started in the thin green plastic pots "outperformed' same seedling varieties planted in styro.
Another thing that may matter, I started directly into these larger pots instead of sprouting in 4 packs, then potting up.
I also started some in 4 packs, which kept up till the point where they were getting crowded. Then, the ones in the larger pots outpaced the 4-packs.
Thinking further on this, the styro cup plants stayed wet longer between waterings. Up here, that made for wet feet. In your locale, that would probably be a good thing!
I shouldn't make such drastic changes in how I start seeds year to year. I'm attempting to find the easiest way to grow up a few flats of heirloom toms for sale at the local nursery. Potting up is a major effort and the larger pots take more space, so I hoped to simplify and start them in their final pot size right away. Styro apparently isn't my best choice, but I can now see why it would work better for you!
Hope you have a great season!
-Ed
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