I have to respectfully disagree with some of the advice in this thread concerning transplanting and the size changes.
According to what I have witnessed and remember reading from experiments on upsizing containers, the plants that had the best growth and root systems were upgraded in the size container from solo cup to 1 gallon then to a 3-5 gallon container after the root systems filled the first 2 containers in which they were in. Using test plants that went from solo cup to 5 gallon container size did not expand or root as well as the others that made the multiple container transplants. Another thing to consider was the space that said plants occupied under indoor conditions. And lastly the ability to manage smaller plants in the smaller containers. Especially for pest management.
Actually some people actually cut the root ball with a knife to open up the roots. Some even trim the roots with sissors before transplanting them.
I have found transplanting from 1 gallon to larger container sizes if specific plants were over watered, they would sometimes begin to develop anarobic smells and areas in the bottom of the root mass killing growth or at least slowing it down. If I removed that bad area of soil and transplanted the plant rebounded with normal growth.
Sold advice with results that I have witnessed was the geo pot type container for better root oxygen compared to sold wall containers. The biggest difference was from the one or two gallon pot to the 5 gallon or larger geo pot. With one month of veg in the geo type container, look out because they will explode with growth in flower.
And to transition to the larger pot with little or no transplant shock, use a small amount of seed starter soil (like light warrior) as a layer between higher quality(hot) soil and feed with just plane water and they rebound in no time.
Hope that helps.
