My dad always said that horseradish, due to its volatility, lost potency over time. If you add vinegar to your ground HR, adding immediately after grinding results in a milder version. Waiting 10 minutes will produce a more potent version. He also said that the way to get the most potent horseradish was to leave the vegetable in the ground until after the first frost.
Dig the root up early when it is small about the size of your finger, when grinding it up mix some vodka or gin with it.