I live in the US in Wisconsin and this is how I harvest my horseradish root for preparing both my horseradish sauce and also replenishing my plant stock for next year.
Dig up your horseradish root using a pitchfork. You will want to go around the entire plant in a circle pattern to lift the root ball of the plant.
Shake off the soil. Cut the greens off the top and rinse and dry the root.
If you live in a warmer zone where the ground temperature stays above 40ºF all year, grow horseradish as an annual. Store the roots that are to be replanted next season in the fridge (40ºF to 32ºF) until spring – they need a cool rest period to simulate winter. Horseradish can be stored in the refrigerator for 6-8 months. Store your horseradish in a loosely wrapped plastic bag to prevent the root from drying out.