I've been growing horseradish in a container for about 7 or 8 years now (it especially loves being in the self-watering container). I've been growing wasabi in a (non-self-watering) container for 2 years. Both do just fine -- surviving thru' the winter, putting out a fantastic amount of leaves every year -- except where one thing is concerned: actually producing roots that are big enough to be used for cooking. On the horseradish i get super skinny roots that don't leave much leftover after peeling, and the wasabi hasn't put out any usable roots at all.
Any suggestions? I'm thinking that the 1st obvious thing to try is changing the dirt, but i'm not sure what sort of change to make.