Lining the holes with 80 gallon extra-thick garbage bags and then filling them with growing medium is a simple and very effective method for eliminating any issues with roots. This also allows for enough medium to be used that it can breathe. It also prevents checmicals from entering the cannabis' soil.

Holes poked with a pen near the bottom 2 inches of the bags (maybe 30 holes) allows for drainage. The plastic barrier also reduces water loss to surrounding soil when watering by hand.