Horseradish makes its greatest growth during late summer and early autumn. For this reason, delay fall harvest until late October or early November, or just before the ground freezes.
Store horseradish roots for fresh grinding in dark plastic wrapping in the refrigerator. Protect the roots from light to prevent their turning green.
The most common way to preserve horseradish is pickled. This pungent sauce is stored in the refrigerator. For instructions see http://nchfp.uga.edu/how/can_06/pickled_horseradish.html
Make a Horseradish-Tomato Relish using this recipe from Oregon State University