I'm a pretty new grower, but I've transplanted a number of plants in plastic pots, cannabis and otherwise, and I'm using smart pots right now.
I transplanted from one gallons into 5 gallons and a couple of 7 gallons. I probably overthought it and over-read it online regarding smart pots, and came across a lot of people saying they just bury the smart pot in the larger smart pot, while others said they just roll down the sides of the smart pot and transplant like normal. I first tried rolling down the smart pot on one of my largest plants, I was very worried by how much I was agitating the soil the whole time, and when I'd transplanted plants before, I'd seen roots all around the containers. the roots weren't very apparent in the smart pots. The scariest part, however, was that the bottom did have roots poking through, more than I thought, I guess, because it put up a real fight, the roots had a deathgrip on the bottom of the smartpot. When I finally got it off it almost sounded like velcro! Scared me to death. I finished up the transplant and decided to go with the burial method for the rest of my plants. I was surprised that there was very little apparent shock on the plant that I actually removed from its pot to transplant-if you check out my journal it's midnight kush in the back right of the tent. absolute beast! I moved her from a 1 gallon to a 7 gallon.
Edit:Just re-read some of the thread and noticed the guy right above me used the velcro analogy as well!
However, I have good faith that the burying method works as well. My starbud is my baby, the main event for my first real grow, carefully trained and vegged, I wasn't gonna rip her out of her pot like I did the first transplant! I buried her 1 gallon in a 7 gallon, and she's huge! there's no denying her roots are going all over the place in that 7-gallon pot. Others on the internet say that the sides of the pot, if buried, allow the roots to expand through, since they won't be air-pruned anymore. I was skeptical of that, but I convinced myself with some of my own logic. this is the bottom of a different plant, but shows my point. If you feel the bottom of a smart pot, it's exactly the same as the side. Same material, same thickness.
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So overall I think they're great and I'm probably gonna keep using them, The walls might slow down root growth a little if you bury them when you transplant them, but you might stress them a little more by removing them from their pots when you transplant them. Might just be a matter of personal preference, and if you've honed some mad smart pot transplant skills maybe you could remove the smaller pots with minimal stress.
EDIT:also, at the end of my grow, I plan on digging up the plants I don't reveg (if I reveg any), to examine the structure of the roots with the smart pot-barrier. I'll definitely take pics if it's remotely interesting.
I'd love to hear someone more experienced than I weigh in heavily on this.
there's one idea I had regarding the burial transplant method which I think could be awesome, but maybe not. If vertical height isn't as much a concern as saving on your square footage, then would it help to bury a pot halfway? more gallons of soil in less square footage stacked vertically, but would it work well if it was stepped like that? or would it confuse the rootball, if you know what I mean?