Native to southeastern Europe but quickly and widely naturalising as a wild flower, horseradish (Armoracia rusticana) is a deep-rooted member of the mustard family, Brassicaceae. Also called red cole, common horseradish should not be confused a green-rooted cousin native to Asia and used to make wasabi. Gardeners in U.S. Department of Agriculture hardiness zones 4 through 8 know horseradish is an easy-to-grow perennial vegetable, if not a weed. In other USDA zones, grow it as an annual.
Horseradish grows into an upright to rounded clump of leaves maturing to be 24 to 30 inches tall, with a spread of 30 to 36 inches. When in flower, the plant measures 12 to 18 inches taller. Healthy, vigorous, older clumps may reach up to 48 inches tall. Underground, the long, sometimes branching taproot grows downward 18 to 30 inches. The brittle roots taper like those of a carrot but are creamy white. Plants spread via the roots. Over three or four years, one horseradish plant can become several clumping plants in a substantial mass.
During the subfreezing winter months, horseradish leaves die back and rot atop the soil surface. New sprouts occur in spring, often having to puncture upward through the dead, tan leaf layer. The horseradish leaf is elongated, up to 24 inches long and about 4 to 6 inches wide. The blade is leathery but glossy smooth, is an attractive deep emerald to dark green and has small teeth on the leaf edges. The leaves typically grow upright but can display curves or waviness within the blade.
Because horseradish is a cousin to cabbage, radish and mustard, it produces similar four-petaled, crosslike flowers. From late spring to early summer, flower stems rise above the leaves to display a branched, loose cluster of white flowers, each blossom measuring 1/4 to 3/8 inch in diameter. A horseradish in flower is quite attractive, because the ruffled green leaves contrast the upright, cloudlike clusters of white flowers that lure insects for pollination. Seed rarely forms, according to the Missouri Botanical Garden.
If you break off a leaf from the horseradish plant, you may get only a faint smell of pungency, not unlike the smell from cabbage juice or freshly grated coleslaw. Also, a dug-up horseradish root also doesn't emit that strong horseradish scent you may expect. The scent only occurs when the juices in the root are exposed to oxygen in the air. The bitter, hot flavour occurs when the horseradish juice mixes with oxygen and your saliva. Some people may get a rash if any leaf or root juices get on their skin.