Dig up the horseradish. Rinse the roots and cut just below the base of the stems. Set the stems aside to replant as they will grow new roots. Scrub the roots clean and cut into 2" pieces.
Pour the cider vinegar and water into the blender. Add all the cut roots. Add water to the full line and cover.
Blend for several minutes until the roots are finely ground. Strain the away the liquid. Store in a well-sealed plastic container. Freezes well.
Food processor instructions: Pour the cider vinegar, ½ cup water, and the cut roots into the processor bowl and cover. Do not add more water. Process until the horseradish is finely ground and strain as above.