I'm not the best source for generalized wasabi growing information, though I could dig up some of the sources I've used, if you want. The experiment above has wrapped up for now and wasabi cultivation has moved out to the greenhouse. I have been putting together some special purpose systems to try growing wasabi outdoors in the heat this year, and will try to remember to post about that if I can get them online.
Some things I learned from the above iteration:
Nighttime lows are more important than daytime highs. They can handle more heat than I expected as long as it cools off at night. Room temperature is too warm for this, and preliminary testing indicates nighttimes shouldn't get much above the 50s (F), though I need more data here.
Plants in coarser media did better initially (1/2" expanded shale), but were eventually overtaken by plants in finer media(1/4"), with more soil-like media (coir and soil-less mixes) being most successful long-term in my aquaponics systems. Other factors may have affected this, so I wouldn't take it as gospel, but I will be using finer media in later experiments. A notable counter example are a couple of wasabi plants that have been in my 3/4" expanded shale flood and drain siphon bed for 2 1/2 years now through all kinds of temperatures, but these have failed to grow very fast despite their tenacity, and I wouldn't rate their growing conditions as favorable.
Wasabi wants even less light than I thought. My lighting systems weren't calibrated for going as low as the plants wanted, and I had some early damage as a result. Like many plants, they do seem to tolerate more light at lower temperatures.
When temperatures are high, drier plants can go fully dormant, but the ones in my aquaponic system struggled and died under similar water/media and air temperatures. Outdoors, dormant plants in Pro-Mix sitting on a 1/2" water table remained dormant through 110F highs in the shade, and then grew vigorously in the winter (still on the water table). Dormancy is not ideal for production of a long-growing crop, and may adversely affect the product, but does open up possibilities for saving plants in the (inevitable) event of things going wrong.
I had a little bit of powdery mildew set in at one point, but a few applications of dilute peroxide cleared it right up. I think it wouldn't have been a problem with more air movement.
