The "wasabi" served with Japanese food here IS horseradish. Real wasabi is very hard to obtain outside of Japan and tastes very different. Almost 100% of the time wasabi in the western world is really dried ground horseradish with some green food coloring.
(I have no idea of the processing required, however, to get from horseradish root to wasabi.)
Wasabi is difficult to grow outside of its natural range due to the growing conditions. It must have certain climate, soil, and water conditions. It is primarily a water plant and can not be grown in just soil. It is being grown in the Pacific Northwest but not yet on any large scale.
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