If you’re a regular patron of Japanese restaurants, fiery green wasabi paste is a familiar and welcome sight. So, it might come as a surprise to learn that you’ve probably never tasted real wasabi. Most restaurants use a substitute made from Western-style horseradish and other ingredients, including vinegar and mustard powder.
That’s because real wasabi, Wasabia japonica, is rare and expensive even in its Japanese homeland. It’s native to the cold upland streams of that mountainous country, and is notoriously difficult to grow. It flourishes only in a narrow range of conditions, and most cultivars are specific to one small region of Japan. However, commercial growers in Japan, Taiwan and North America have slowly learned to coax this uncooperative plant into producing reliable crops.
Wasabi is part of the larger brassica family, which also includes the cabbages and mustards as well as Western horseradish and other root plants including turnips, daikon and the familiar garden radish. Like horseradish and the mustards, its fiery nostril-clearing character is a chemical defence against predators. When the cell walls are crushed or grated, enzymes in the root quickly convert stored sulphur compounds into an irritating chemical called sinigrin. It’s highly volatile and aerosolizes quickly, which is why you feel the effect primarily in your sinuses.
Growing wasabi can prove a fascinating project for experienced gardeners in search of a challenge. The plant requires lots of shade, ideally 75 % coverage or better. It requires high humidity and cool air temperatures, ranging from 8-20°C (46-68°F), and flourishes best at 12-15°C (53-59°F). In warmer, sunnier or drier climates the shade-type greenhouse can be the best solution, providing shelter and a microclimate that can be kept humid and temperature-controlled. A fertilizer containing sulphur will help increase the root’s potency.
In nature, wasabi grows in a semi-aquatic environment along stony riverbeds. If you have running water on your property, or have an existing hydroponic system, you can use that water to cultivate wasabi. Prepare a deep bed or large box-shaped planter, approximately 75-125 cm (30-50″) in depth. Fill the bed with stones approximately 6 to 8 cm (2.5-3″) in diameter, leaving 10 cm (4″) at the top. Cover the stones with a layer of rounded gravel the size of large peas, approximately 6 to 8cm. Finally, fill the rest of the bed with fine sand.
The plants will flourish best when the water flows gently down a slight slope, usually 2 to 4 degrees. Water should be distinctly cool, with an ideal temperature of 12-15°C (53-59°F). Water flow should be gentle, no more than 10 cm (4″) per second, to produce straight and healthy rhizomes. If the water flow is too strong, the roots will grow in a curved shape. The flavor will still be fine, but if you’re growing them for sale curved roots don’t fetch as high a price.
Wasabi can also be cultivated on dry land, in a light, well-drained soil with lots of sand and organic material. Prepare your beds in an area where there’s a lot of natural shade, or where you can easily shade the plants with a cover. The plants require constant moisture even on land, so plan to irrigate with soaker hoses or some other form of low-flow irrigation. Monitor your plants closely. Provide more water if they wilt, and less if you begin to find stem rot.
Wasabi can be propagated from seed, slips or the small plantlets that grow around the crown of a mature plant. Plantlets 3 cm (1.25″) long, with four or five leaves of their own, can usually be planted directly into your aquatic or dry-land beds. Place each plantlet in its own hole, about 30 cm (12″) apart, with 1 cm (.4″)of the root left above the surface once you’ve patted the sand or soil back into place. Plants grown from slips or seeds must be grown to a height of approximately 5 cm (2″) before transplanting into their beds.
The fiery roots will usually be large enough to harvest in 16 to 24 months, depending on your growing conditions. Remove plantlets from the main stem for replanting, and then wash the root thoroughly. Trim away any small roots then peel the stem with a paring knife or peeler. As with chilli peppers, it’s best to wear gloves while working with fresh wasabi. The juices can be decidedly unpleasant if they find their way to a sensitive spot.
Real wasabi is best when grated as needed. Japanese chefs use a special grater made from shark skin, but a very fine conventional grater will also work. The flavor will reach its peak within 3 to 5 minutes, and will diminish within 15 to 20 minutes if it’s left uncovered. Fresh wasabi has the familiar, fiery effect in the diner’s sinuses, but mellows quickly to a sweet and complex flavor quite unlike the artificial variety. Grate only as much as you need for one meal, then store the remainder of the root tightly wrapped in your refrigerator. It will keep for several weeks. To preserve your wasabi for the longer term, slice and dehydrate the roots. Grind the dried roots into powder in your spice grinder, and store it in airtight packaging in a cool, dark place.
[alert type=white ]This article was written by Fred Decker. It was originally published in Garden Culture Magazine, Issue 1 under the title, “Grow Your Own: Fresh Wasabi”.[/alert]