if the pot is only half the length of a dollar bill the tap root might need more room to grow. I think this is the problem with my current grow. I started plants in pots about that height but a bit wider and i couldn't figure out why they were going so slow. After two months they were about 6' tall! I started two more plants off straight into 8" pots about 3 weeks ago and they are already twice as tall as the ones i left in the little pot forever. I would say the moral of that ramble is start in bigger pots. This is just based on a small amount of experience though. I have noticed weed needs longer pots than most plants ive grown. I think that tap root shoots down quickly and stunting it will damage the plant (or make cool little bonsai buds)