Tade (polygonum, called water pepper in English, scientific name: Persicaria hydropiper) is polygonaceous an annual grass. When you simply say tade, it indicates a water pepper. Tade of 'Tade ku mushi' (a part of proverb, 'Tade ku mushi mo suki zuki,' meaning 'Everyone has his own taste') also indicates a water pepper.
It has a unique aroma and an acrid taste, so it is used for spice and garnishing served with raw fish as condiment.
Its varieties are yanagi-tade (water pepperm, also called hon-tade), beni-tade (red polygonum), ao-tade (blue polygonum), and hosoba-tade (lobulet polygonum).
A product called tade-su (tade vinegar) is made by grinding tade leaves and diluting them with vinegar. It is added to a broiled ayu (sweetfish) with salt as garnishing.
The pungent component is polygodial.