Cochlearia armoracia) is a perennial plant of the family Brassicaceae (which also includes mustard, wasabi, broccoli, cabbage, and radish).

Horseradish grows up to 1.5 meters (5 feet) tall, with hairless bright green unlobed leaves up to 1 m (3 ft 3 in) long that may be mistaken for docks (Rumex).

When cut or grated, enzymes from within the plant cells digest sinigrin (a glucosinolate) to produce allyl isothiocyanate (mustard oil), which irritates the mucous membranes of the sinuses and eyes.

Once exposed to air or heat, horseradish loses its pungency, darkens in color, and develops a bitter flavor.

Horseradish is probably the plant mentioned by Pliny the Elder in his Natural History under the name of Amoracia, and recommended by him for its medicinal qualities, and possibly the wild radish, or raphanos agrios of the Greeks.

It was introduced to North America during European colonization; both George Washington and Thomas Jefferson mention horseradish in garden accounts.

In The Herball, or Generall Historie of Plantes (1597), John Gerard describes it under the name of raphanus rusticanus, stating that it occurs wild in several parts of England.

After referring to its medicinal uses, he says: [T]he Horse Radish stamped with a little vinegar put thereto, is commonly used among the Germans for sauce to eat fish with and such like meats as we do mustard.

[19] In Slavic languages, the word for mustard derives from a root meaning fire or burning, often used metaphorically to refer to spicy or bitter foods.

[22] In the United States, horseradish is grown in several areas, such as Eau Claire, Wisconsin, and Tule Lake, California.

Upon crushing the flesh of horseradish, the enzyme myrosinase is released and acts on the glucosinolates sinigrin and gluconasturtiin, which are precursors to the allyl isothiocyanate.

[citation needed] The leaves of the plant are edible, either cooked or raw when young,[28] with a flavor similar but weaker than the roots.

A variation of horseradish sauce, which in some cases may replace the vinegar with other products like lemon juice or citric acid, is known in Germany as Tafelmeerrettich.

In Russia, horseradish root is usually mixed with grated garlic and a small amount of tomatoes for color (Khrenovina sauce).

In Denmark, it is mixed with whipping cream and as such used on top of traditional Danish open sandwiches with beef (boiled or steaked) slices.

In the Christian tradition, horseradish is eaten during Eastertide (Paschaltide) as "is a reminder of the bitterness of Jesus' suffering" on Good Friday.

[25] The enzyme horseradish peroxidase (HRP), found in the plant, is used extensively in molecular biology and biochemistry primarily for its ability to amplify a weak signal and increase the detectability of a target molecule.