- Written also: Horseradish
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Perennial herb. Rootstock cylindrical.
- Height: 50–150 cm (20–60 in.). Stem upper part branching, glabrous.
- Flower: Corolla regular (actinomorphic), white, approx. 1–1.5 cm (0.4–0.6 in.) wide; petals four, 6–8 mm (0.24–0.32 in.) long. Sepals 4. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage. Flowers with pleasant fragrance.
- Leaves: In basal rosette and alternate on stem. Basal leaves long-stalked, large, stem leaves almost stalkless. Basal leaf-blades elliptic-ovate, with cordate or round base, with toothed and wavy margins, bluish green, lowermost stem leaf-blades commonly pinnately lobed, upper usually lobeless, large-toothed.
- Fruit: Egg-shaped–spherical, unveined, 4–6 mm (0.16–0.24 in.) long, commonly with weakly-developed silicula. Stalk ascending oblique, over 10 mm (0.4 in.) long.
- Habitat: Yards, gardens, roadsides, banks, waste ground, rubbish tips, waterside meadows. Formerly cultivated, left-over and escape from old gardens.
- Flowering time: June–July.
Horseradish is native to southern Russia. In Finland it is mainly cultivated but it commonly grows wild. Horseradish was originally valued as a medicinal herb and in the Middle Ages it had a reputation as a general tonic, and like many other large-rooted plants it was invested with magical properties. Its culinary use only began in the 16th century, but if it is finely grated it has an affinity with many different kinds of food.
Horseradish is not cultivated much in Finland nowadays, certainly not enough to supply demand. In the Middle Ages it was probably grown a lot more, especially when its cultivation was promoted during the Age of Utilitarianism in the 18th century, and there are many plants still growing all over Finland as leftovers and escapes from cultivation as a reminder of that time. In the wild it usually grows in ones or twos, usually at the edge of gardens, in young meadows, on banks and in waste ground. It is rarer to find it in places that resemble its natural habitat such as coastal places and even in water. Horseradish can survive for decades in the same place: it can be commonly found on the site of old vegetable gardens around manor houses, villas and small houses, even if there is nothing to be seen of the original buildings. The species has spread to new places when earth is shifted, and sometimes under its own steam, for instance floating in streams.
Horseradish in Finland usually spreads vegetatively via its rootstock. Seeds develop very rarely even though the plant blooms abundantly and is visited by flowerflies, honeybees and other insects. It is assumed that horseradish is cross-pollinated: the seeds only form when fertilization occurs between two separate plants, but most horseradish spreads from pieces of rootstock and new plants seem to be clones, i.e. genetic copies of the mother plant. Cross-pollination can only occur between plants that are not clones of the same original mother plant.