Horseradish (Armoracia rusticana)

 
A perennial plant of the Brassicaceae family (which also includes mustard, wasabi, broccoli, and cabbage). Probably native to southeastern Europe and western Asia. It grows up to 1.5 meters (4.9 feet) tall and produces spikes of white flowers which attract insects. Although is cultivated primarily for its large, white, tapered root, the young leaves make a delicious salad ingredient with a mild wasabi-like taste. Older leaves can be cooked as spinach, before they get too big and tough.
 
Roots can be harvested any time but are best when the leaves have died back in autumn through to spring. Intact horseradish root has hardly any aroma. When cut or grated, enzymes from the broken plant cells break down sinigrin (a glucosinolate) to produce allyl isothiocyanate (mustard oil), which gives horseradish its distinctive hot taste. The grated root should be used immediately or preserved in vinegar for the best flavour as once exposed to air or heat it will begin to lose its pungency.
 
Plant horseradish where it is to stay as even if you try to dig it up completely in future, any small piece of root remaining will sprout into a new plant.  It does well in partial shade and is supposed to be a good companion plant for fruit trees.