Wasabi rhizomes can grow in the garden. Absolutely they can. Ours did. No secret sauce added. We planted these in September 2014. Several months later, a small rhizome with enough paste for a sushi dinner for a few diners. We used a couple handfuls of 14-14-14 fertilizer three times during the growing duration (note, the offshoots were planted in autumn and grew throughout the winter). These were just like the small, trimmed offshoots we sell. They grew happily in full shade (I can't stress this enough). They were planted in a wine barrel which had amended soil. That's it. The wasabi paste was spicy and tangy and would definitely accompany fish or do well in a sauce. The reddish colored paste is from the root area and is because it's a mazuma variety. The green colored paste is from the top of the rhizome and is a bit more watery, but definitely flavorful. Note that in photo two, there are little purple offshoots that could be broken off and replanted to propagate more wasabi in the garden. As wasabi suffers very little transplant shock, one could dig up the plant, remove the offshoots and then expand the wasabi patch.
Well, now we know it works outside the greenhouse and without the benefit of the secret sauce. (Though, we knew that already. Check out the very last picture which are wasabi plants that have been growing under our blackberry hedgerow for several years. They're now wild field wasabi.)
Next experiment: grow the rhizome in the garden, cut off the apical meristem and re-plant to see if it regrows (it does in the greenhouses at the farm). Also remember, the leaves and stems can be eaten at any time.
The first photo is at ground height - it's a bit disorienting. The photos are in sequence from harvesting to grating.