Most Wasabi Served Outside Japan Is Not Real Wasabi

Real wasabi (Wasabia japonica) is extremely difficult to grow, requiring cold running water and specific soil conditions — making it very expensive. Most "wasabi" served in sushi restaurants outside Japan (and much inside Japan) is actually a paste made from horseradish, mustard, and green food colouring. Real wasabi has a more complex, less harsh flavour that fades within minutes of grating.

Source: Food and Agriculture Organization of the United Nations; various culinary sources