Wasabi: The Real Rhizome, Its Heat, and Why Most “Wasabi” Is Horseradish

That green dab next to your sushi is probably horseradish. Outside Japan, most of what’s sold as wasabi is horseradish, mustard and green coloring, and one estimate puts the share of North American “wasabi” that isn’t real at about 99%.¹ Real wasabi is the sharp, pale-green paste you get by grating the rhizome of Eutrema japonicum, a perennial in the mustard family native to Japan, Korea and Sakhalin. Its heat climbs into your nose and fades fast, which is why it’s grated just before serving.

At a glance

Measured values

2 figures, sourced

Measure	Value	Scope	Source
Allyl isothiocyanate	86 to 94 % of total isothiocyanates measured	New Zealand-grown wasabi, seven isothiocyanates measured (Lincoln University doctoral study)	researcharchive.lincoln.ac.nz
Heat compound retention in frozen storage	original levels kept after two months at −10°C or colder (down to −80°C)	Wasabi rhizomes, Lincoln University doctoral studyThe roughly 15-minute loss of pungency after grating is a Chowhound account, not a measurement, so it is not listed.	researcharchive.lincoln.ac.nz

Each figure is quoted as its source states it, with the samples or standard it applies to. Where sources differ, both are shown; nothing is averaged. All measured values

What it is

Eutrema japonicum is a perennial of the temperate zone. You’ll still see the name Wasabia japonica in older references and on labels, but Kew now treats it as a synonym. The part you grate is the rhizome. In the New Zealand study cited below, rhizomes yielded the most heat compounds of the four plant parts tested. Stalks (petioles) and leaves yielded less.⁴

Flavor and the chemistry of the heat

Freshly grated wasabi tastes clean, green and a little sweet, with a heat that climbs into your nose, peaks fast and fades. The Embassy of Japan’s magazine notes that it gets rid of fishy smells and brings out the fresh flavor of seafood.⁵

Made on grating. Wasabi releases volatile isothiocyanates when its cells are broken by cutting or grating.⁶ A Lincoln University (New Zealand) doctoral study of seven isothiocyanates in New Zealand-grown wasabi found allyl isothiocyanate the most abundant, at 86 to 94% of the total.⁴ That’s the same compound that gives horseradish its bite.

How it differs from horseradish. The same study found that wasabi’s isothiocyanate mix differs from European horseradish’s. It picked out 2-phenylethyl isothiocyanate as a marker found only in horseradish, where it gives a radish-like flavor.⁴

Why it fades. Fresh wasabi paste loses its pungency in the air, so the rhizome is usually grated just before serving, traditionally on a sharkskin or wooden grater.⁶ Chowhound puts the loss of flavor at about 15 minutes after grating.¹

Water and the burn. People often say wasabi’s heat dissolves in water and washes away with a drink, unlike chili heat. Allyl isothiocyanate is only slightly soluble in water: about 2 grams per liter at 20°C, per the U.S. Hazardous Substances Data Bank as compiled by PubChem.⁷ The burn passes on its own within minutes because the compound is volatile, same as with horseradish.⁸ We found no test of water against wasabi burn.

Origin and history

Native range. Kew gives the native range as southern Korea and southern Sakhalin to Japan, with an introduced population in Taiwan.² Britannica gives Japan, South Korea and eastern Russia.³

The start of cultivation. Shizuoka Prefecture’s own account says wasabi farming is said to have begun in the Keicho era (1596–1615), with wild wasabi growing on Mt. Butsukoku in the Utogi district of what’s now Aoi Ward, Shizuoka City.⁹ The Embassy of Japan’s magazine also dates the first cultivation by Utogi farmers to the early 1600s.

Tokugawa Ieyasu. According to the prefecture, Tokugawa Ieyasu, who spent his later years at Sunpu Castle, banned taking wasabi out of the area, so for many years it stayed in Utogi. A second prefecture article tells it as an anecdote: Ieyasu liked the wasabi he was given so much that he kept it secret. The embassy says the Utogi growers held a monopoly on cultivation until the mid-18th century.

Spread. In the mid-18th century, growing techniques reached the Izu region, where the tatamiishi (layered stone) method was developed in the Nakaizu area. From there it spread through Shizuoka and to other parts of Japan.

Recognition. “Traditional Cultivation of Shizuoka Water Wasabi” is recognized by the FAO as a Globally Important Agricultural Heritage System and by Japan as a Japanese Agricultural Heritage System.⁹ We couldn’t confirm the year of designation.

Several widely repeated details couldn’t be traced to a source, so we’ve left them out: that wasabi appears in a botanical dictionary called the Honzō Wamyō of 918 and in the Engishiki; that its leaves resemble the hollyhock on the Tokugawa crest; that it was eaten in temple vegetarian cooking from about 1000 to 1500; that it was first paired with sushi in the late Edo period; and that powdered wasabi and field cultivation date to the Taishō period. Some may well be true. We just couldn’t confirm any of them.

How it is grown

Water wasabi grows in beds fed by cold, clean spring water. Shizuoka Prefecture describes stepped fields built in the mountains where spring water runs at around 13°C all year, grown with very little fertilizer.⁹ In the tatamiishi method the embassy describes, the bottom layer is reinforced with large rocks and each layer above is filled with smaller stones, so the water filters through the bed. Wasabi’s specific growing needs limit where it can be farmed.

It’s slow, too. The embassy says a rhizome can take up to three years to reach harvest size.⁵ A prefecture article on Izu wasabi gives a year and a half for its growers.¹⁰ They’re describing different growers, so they don’t need to agree.

Shizuoka Prefecture says it had the highest production volume of water wasabi rhizomes in Japan in 2022.¹⁰ We didn’t check a tonnage figure.

Real wasabi vs. “wasabi”

Western restaurant wasabi paste is commonly made from horseradish. Chowhound reports that the vast majority of commercial wasabi is made from European horseradish, often with mustard and green coloring, and cites The Washington Post for an estimate that about 99% of the wasabi sold in North America isn’t real wasabi.¹ The embassy’s magazine says some restaurants in Japan also use a cheaper mix of horseradish, mustard and coloring.⁵

How to tell:

• Read the label. If the first ingredient on a tube or tin is horseradish, it’s horseradish-based, whatever the front says.
• Look at the texture. Freshly grated rhizome is slightly coarse and pale green.
• Notice the heat curve. Freshly grated real wasabi loses its pungency in open air, within about 15 minutes by Chowhound’s account.¹

For more, see wasabi vs. horseradish, fresh wasabi vs. wasabi paste and wasabi powder vs. paste.

How it is used

Wasabi paste is a quintessential Japanese condiment, mainly paired with sushi and sashimi. It’s dabbed between the topping and rice in nigiri, or stirred into soy sauce as a dip. The prefecture also notes that wasabi pickles are said to have first been sold at Shizuoka Station.⁹

You’ll also find wasabi in dressings and mayo, with grilled steak, and as a coating on snack peas and nuts. Since the heat fades in air, it usually goes in at the end or on the side. See cooking with wasabi.

Buying and storage

Fresh rhizome. If you can find one, grate only what you need, just before serving. We found no sourced fridge life for a whole rhizome.

Frozen. The Lincoln University study found that storing rhizomes at −10°C or colder (down to −80°C) for two months kept their original levels of heat compounds.⁴

Tube paste and powder. Check the ingredient list to see what’s actually in it. Mix powder with water and let it sit briefly before using.

Nutrition and health

You eat wasabi in tiny amounts, and we didn’t check USDA FoodData Central figures for it. Lists that call it rich in vitamin C, fiber or minerals couldn’t be checked against a nutrient database here. Claims that its isothiocyanates prevent cancer, strokes or food poisoning rest on research we didn’t review, and we make no claim that wasabi treats or prevents any disease.

Substitutes and comparisons

• Wasabi substitutes, with horseradish the usual stand-in
• Wasabi vs. horseradish and the horseradish profile
• Too much wasabi, if a dish has gone too hot
• Mustard powder, another isothiocyanate heat

Footnotes

1. Elias Nash, “Why Genuine Wasabi Is So Expensive In The US”, Chowhound, 2023. chowhound.com · Publication ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Eutrema japonicum, Plants of the World Online, Royal Botanic Gardens, Kew. powo.science.kew.org · Reference work ↩ ↩²

3. Melissa Petruzzello, “Wasabi”, Encyclopaedia Britannica. britannica.com · Reference work ↩ ↩²

4. Tamanna Sultana, Factors affecting the yield of isothiocyanates in wasabi, PhD thesis, Lincoln University. researcharchive.lincoln.ac.nz · Reference work ↩ ↩² ↩³ ↩⁴ ↩⁵

5. “Full of Zest: Shizuoka’s Wasabi Fields”, JCC E-Magazine, Embassy of Japan in Singapore, 2021. sg.emb-japan.go.jp · Regulator ↩ ↩² ↩³

6. “Isothiocyanate”, Encyclopaedia Britannica. britannica.com · Reference work ↩ ↩²

7. Allyl isothiocyanate (CID 5971), PubChem, National Library of Medicine. pubchem.ncbi.nlm.nih.gov · Reference work ↩

8. Sylvia Tomczak, “Why Horseradish Is So Potent For Such A Short Time”, Tasting Table, 2022. tastingtable.com · Publication ↩

9. “Pickled Wasabi”, Shizuoka Prefecture. fujinokuni.shokunomiyako-shizuoka.pref.shizuoka.jp · Regulator ↩ ↩² ↩³ ↩⁴

10. “Izu Wasabi”, Shizuoka Prefecture. fujinokuni.shokunomiyako-shizuoka.pref.shizuoka.jp · Regulator ↩ ↩²

Also drawn on

• Horseradish (revised by Melissa Petruzzello, updated 18 Sep 2026), Encyclopaedia Britannica. britannica.com