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I would like to grow Wasabi. My climate seems, according an old Times World Atlas, very similar to the (central) Japanese one. But on documentaries I've seen on TV, it seems that Wasabi grows in wet to submerged conditions. This is my biggest problem, as in my garden I have no natural rivulet.
Is watering enough, and how should I do it?
Also, which soil does wasabi prefer? And should I look for seeds or roots?
Contrary to what you often see, Wasabi can be grown in moist, but not submerged, soil. You'll do best starting with fresh, viable seeds. Here's a few points on preferred conditions in soil growing (this does not all apply to hydroponic growing):
Soil should be free draining, highly organic soft soil at least 10 inches deep. The soil should not compact easily, and while staying moist, it should drain well, to allow oxygen into the root zone. Aim for a pH of 6-6.5 (acidic)
Fertilizer is very useful. High nitrogen is good. I like 10-10-10, applied bimonthly in the waterings.
Light is where many people go wrong. These do not do well in bright lighting, If you have an area that gets no direct sun, great. Otherwise you have to create your own microenvironment (I used laths, spaced out over a frame, with a liht row cover stapled over the top, about 11' above the ground surface).
Temperature should not go below 45 degrees Fahrenheit, or above 75 for prolonged periods. Again, this limits where the plant can be grown, because of the 24 month maturity. In my area, this is only feasible in a heated greenhouse.
Water is time consuming unless you have an automated system. The soil, while it should not be waterlogged and muddy, can not begin to dry out, or the plants will stop growing and decline. The foliage needs to be misted regularly. You can set up a mist sprinkler system, to run twice daily (or however often you find necessary).
Now, about growing the plant itself. Remember these are extremely picky, don't feel like a failure if you take multiple attempts to reach your goal.
Make sure your growing environment is ready to go before you plant. Anything like a hoophouse, structure, or sprinkler system should be installed before you get the beds ready.
Get the soil ready. It should be fluffed up 10" down or so. If you want, you can mix slow release fertilizer into this worked soil.
Plant the seeds just lightly covered in soil, and pressed, about 18" apart (If you want to plant 2 per spot and thin them later, you may have a fuller bed).
Water them in well. From this point on, the soil should not be allowed to begin to dry. But remember, this is not a hydroponic growing strategy, and the soil should not be allowed to become waterlogged, as that will deprive them of oxygen and increase likelihood of root infections such as phytophthora, which won't be treatable (I learned that the hard way).
Fertilize regularly. I recommend using a soluble high-nitrogen fertilizer suitable for foliar feeding, applied twice monthly with waterings.
Maintain humidity at all times. This is easier in a covered hoophouse. The moist soil will help, but it would be useful if the plants could be misted at least twice during the day. If you can set up your waterers to do this, that would help.
Control weeds early. It's good to familiarize yourself with wasabi seedlings, and how they look, so that you don't destroy them while weeding. Weeds generally germinate much faster, and grow faster than wasabi, and need to be eliminated. This is a good opportunity for a careful soil cultivation, to keep the soil loose.
Harvest comes after 24-36 months of healthy growth, when the rhizomes are about 8" long. You may want to leave some plants in, to save seed.
Please feel free to let me know if you want additional details, or expansion on any point.
