Wasabi is not a root. Please reference the photo below for a visual of the wasabi plant's roots. Horseradish - though a close botanical cousin - grows underground. Wasabi grows above the ground. As wasabi is a brassica, it's related to broccoli. Wasabi is a rhizome, which is essentially the stalk or trunk of the plant like the broccoli stem. Though the growth patterns are different, it's this stalk that gets grated to become the paste. The roots are below ground and the green bits are all above ground.
This plant shows the roots, offshoots (the side shoots at the base) and the central rhizome (behind and slightly obscured by stems - sorry for the difficulty). The offshoots are removed and the rhizome trimmed. We trim all our rhizomes by hand, with hand tools.
The above photo shows Jennifer harvesting the above-shown plant. The plants are surprisingly large. This full plant weighs approximately 4 pounds.
Below is the fully-trimmed rhizome. Note: the stems were removed from this rhizome for weighing purposes. It’s 130g 4.5oz which should be sufficient for dozens of customers’ full sushi dinners.