Horseradish (Armoracia rusticana).

http://saijiv.notlong.com/ 9 May 2008 Horseradish (Armoracia rusticana)

Horseradish (Armoracia rusticana, syn. Cochlearia armoracia) is a perennial but I treat it as an annual, and propagate the following year's plant from a piece of the current root. It is a member of the Brassicaceae family, which includes mustard, wasabi, and cabbages. I put it in a convenient place for digging in the late Fall, since the roots are a real hassle to remove from the ground, and the plant gets about two feet in diameter and about four feet tall. When mature in the Fall, horseradish condiment is made from the roots.

http://ieraf.notlong.com/ 15 May 2008 Horseradish (Armoracia rusticana) -producing seed.
Apparently seldom does horseradish produce seed in NA. This particular plant was growing a short distance from the main plant in 2007. I chose it for the 2008 planting since it seemed to be well established with leaves. The other plant was grown simply from a root cutting. It has no seed head. I will determine if the seed is viable later in the season.