Ah OK. Mine is in dirt. Every time I go past it, I am reminded how mine got started. I bought two roots at the grocery store and only used one. After a couple weeks the fridge, I was going to throw it away. I decided to plant it, what the heck, It would either grow or not.
I tilled up[ a place and planted it. tamped the soil and went about the rest of my yard work. As always, nothing in my house is done without the constant supervision of my tow labs Bear & Liam. After a while, I noticed something in Bear's mouth. He has a huge head and can hide two tennis balls in there. I looked and he had the horseradish root. I took it away from, thanked him for being, after all, a retriever, and replanted the root after putting him inside.
The rest is history. It is very prolific and spreads like crazy. I actually dug some a couple months ago and processed it. I love it on a steak. It is time to dig a bit more. I might just try carefully using a trowel and see if I can get a long root. Once it takes hold, you are never without.
BTW - Got a sausage recipe that uses horseradish?