Growing Horseradish

Horseradish, known as Kren in Bavaria and Austria, is a hardy perennial from the cabbage family, which makes it a relative of cabbage, radish and mustard. It forms large, coarse leaves and a thick white taproot whose heat only comes out once you grate it. The plant grows to about a metre tall and is fully hardy. It hardly ever sets seed, so you propagate it from pieces of root, known as sets or thongs. Horseradish is one of the oldest crops in our gardens and escapes easily into the wild. Along paths and stream banks it often still marks the spot where a garden stood decades ago.

You almost never grow horseradish from seed. Instead you use root cuttings, also called sets or thongs. These are the strong side roots that branch off the main root and come up at harvest time. Good cuttings are about 30 cm long and at least as thick as a pencil. Cut them straight across at the top and at a slant at the bottom so you don't mix up which way they grow. Plant them in autumn or in spring from early April. Lay the cuttings at an angle, always the same way up as they grew, in well-loosened soil enriched with compost, with the top end sticking out about 3 cm. Leave 70 to 80 cm between rows and 40 to 50 cm within the row.

If you plant in autumn, the harvest only starts in autumn of the following year and runs until spring of the year after that. Grove only plans this as far as December of the following year.

You can also give horseradish a permanent spot in the garden. If you leave small pieces of root in the ground when you dig it up, each one sprouts into a new plant the next spring. That way the horseradish stays in its corner and comes back by itself every year. This saves work, but it spreads over time and is then hard to get rid of. So plant it at the edge of the garden or contain the roots with a root barrier.

Horseradish grows in almost any soil, but it only forms good, sturdy roots in deep, humus-rich soil that stays evenly moist. On light sandy soil it loses flavour, and on heavy clay it quickly turns woody. It likes a pH between 6 and 7.5 and can't cope with waterlogging. Full sun is ideal, though it puts up with partial shade. Horseradish doesn't do well in a pot on the balcony.

There are hardly any true horseradish varieties. Gardeners grow regional strains instead, such as Bavarian, Hamburg, Spreewald, Baden or Styrian horseradish. They differ in root size, heat and how firm the flesh is. Styrian grows vigorously and is hot and firm, Bavarian and Baden horseradish taste milder, and Spreewald is very hot. Many strains are prone to white rust and viruses, so buy healthy plants or root cuttings from a healthy stock.

Horseradish needs steady moisture to form good roots. Water especially after planting and in August and September, when the roots are thickening. In good garden soil a layer of well-rotted compost each year is enough, about 3 litres per square metre. Don't use fresh manure, as it makes horseradish prone to disease. For straight, smooth roots, uncover the top of the root in June and again in July, snap off all the side shoots except the strongest one and cover it back up with soil, ideally on an overcast day.

In a small garden horseradish usually stays healthy. Problems mainly come up when it sits in the same spot for years or when you keep propagating from diseased plants.

The most serious disease is verticillium wilt, which German growers call horseradish blackening. The fungus lives in the soil, gets in through the roots and turns the vascular tissue brown. When you cut through the root, you see dark dots or a ring. The only defence is prevention: use healthy root cuttings, keep it away from potatoes, tomatoes, strawberries and cabbage, and leave no root remains of diseased plants in the bed. Viruses also find their way in through root cuttings and aphids. Turnip mosaic virus shows up as pale ring-shaped spots, and cucumber mosaic virus makes the leaves turn yellow in patches.

In cool, damp weather white pustules appear on the leaves. That's white rust. Brown leaf spots usually come from Alternaria or Cercospora fungi, which overwinter on old leaves. Remove affected leaves and don't propagate from diseased plants.

Flea beetles, including the yellow-striped horseradish flea beetle, and the shiny metallic horseradish leaf beetles chew holes in the leaves, especially in dry weather. Cabbage white caterpillars rarely do much damage to horseradish. Slugs and snails love horseradish, but it copes well with the damage, and you can easily pick them off from under its large leaves in the morning and evening.