Gardeners tend to grow horseradish for culinary use, rather than ornamental value, so harvesting the roots is an integral part of cultivating these plants. In the United States, the plant we call horseradish is Armoracia rusticana; Wasabi japonica, dubbed the Japanese horseradish, is much more difficult to grow. Digging Armoracia rusticana roots at the proper time is not only important for the palatability of horseradish preparations, but for propagating the plants, as well.
It is best to harvest horseradish roots in late fall or early spring. This timing maximises growth, assures the optimal storage conditions, as well as the best flavour. Wait until a hard frost kills the tops before digging. Timing the harvest after that depends on when you planted the roots, your intentions for the harvest and the weather.
Timing for Peak Flavor
Horseradish recipes, often made with freshly grated roots, may call for cooked or pickled preparations, instead. Regardless of the culinary treatment, harvested roots deteriorate, so waiting to harvest your roots until you intend them may be best. One long-standing use of horseradish is in the springtime observance of the Jewish Passover Seder. If you intend to use your roots then, an early spring harvest makes more sense than storing the roots for months.
If you planted or replanted your horseradish the previous spring, you may want to wait until the next spring to harvest. New plantings may take a year or longer to produce sufficient growth to justify digging them. Oregon State University advises commercial growers that harvested horseradish roots are ideally at least 3/4 inch in diameter and 8 inches long. This is a useful guideline for home growers for ease of storage and processing. Replant side shoots and smaller-diameter roots if you want to propagate more plants. Very large roots, which are older, may be too woody, so dig the roots every few years, even if you don't plan to use them.
Weather may thwart your harvest, so plan your fall digging early enough to avoid unearthing horseradish roots from frozen ground. Don't wait until the top growth begins sprouting in spring, as the roots may become bitter. If the harvest is primarily for dividing your roots to share or rejuvenate your planting, replanting as soon as possible is better than storing the roots. Fall harvest and replanting assure greater growth before the next year's harvest season.