Japan
The plant is known as itadori (イタドリ, 虎杖?). The kanji expression is from the Chinese meaning "tiger staff", but as to the Japanese appellation, one straightforward interpretation is that it comes from "remove pain" (alluding to its painkilling use), though there are other etymological explanations offered.
It grows widely throughout Japan and is foraged as a wild vegetable (sansai), though not a major crop shipped to market in appreciable amounts as to given in statistics. They are called by such regional names as: tonkiba (Yamagata), itazuiko (Nagano, Mie), itazura (Gifu, Toyama, Nara, Wakayama, Kagawa), gonpachi (Shizuoka, Nara, Mie, Wakayama), sashi (Akita, Yamagata), jajappo (Shimane, Tottori, Okayama), sukanpo (many areas).
The young shoots and young leaves are used. The shoots look like asparagus but are extremely sour; the fibrous outer skin must be peeled, soaked in water for half a day raw or after parboiling, before being cooked.
Places in Shikoku such as central parts of Kagawa Prefecture pickle the peeled young shoots in salt admixed with 10% nigari, which are then weighted down. Kochi also rub these cleaned shoots with coarse salt-nigari blend, and it is alleged (though no authority is cited) that the magnesium of the nigari binds with the oxalic acid thus mitigating its hazard.
A novel use for a related species known as oh-titadori (Polygonum sachalinense), done in Hokkaido is feeding it to larvae of sea urchins in aquaculture.
Read more about this topic: Japanese Knotweed
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