I've been able to boost plant growth at this stage by watering with a rooting fert, like Clonex solution, or Root 66. Once the roots hit the side of your container though, transplant asap. I just had a plant stop. After about 1 1/2 feet it didn't grow any more for about 2 weeks in the same environment. As soon as I got it in a bigger pot it took off within days. I'm just figuring things out myself, but I also had a sickly plant (seedling stage 1 set of leaves) in a big container, where her sisters took off fine. After a couple weeks with zero growth I pulled the plant out to toss it, and noticed it's root system was basically one carrot shaped root that was small. I misted off the dirt, dipped it in Clonex gel, and replanted it in a plastic cup just to see what would happen. That took off great also. She is a few weeks behind the other seeds I started at the same time, but has a great root system and will be ready for a bigger container in another few days. I really don't know if the smaller container had anything to do with it or not though.