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Also known as Japanese horseradish, Eutrema japonicum is cold-hardy perennial member of the Cabbage family (Brassicaceae), that grows to about 45cm high.
It is a native to Japan, Korea, parts of Northern China and Eastern Russia (specifically Sakhalin, a large Russian island in the North Pacific Ocean), where it is usually found in the wet banks and gravel beds of cool mountain streams and springs – an environment for which it is highly adapted.
It figures heavily in the traditional cuisine of Japan, where it has been cultivated both for food and medicinal use for over 1000 years. Outside of its native range, it has also been cultivated in Taiwan, the highlands of Vietnam, New Zealand and parts of Australia, and more recently, in the US, the UK and parts of Europe.
Its culinary properties are similar to those of some other member of the Cabbage family (Brassicaceae), notably mustard and horseradish. In fact most commercially available “wasabi” in Europe and the US – usually sold in paste form – is actually a mix of horseradish, mustard, starch, and food colouring. If wasabi is listed as an ingredient, it is often only a very small, nominal amount of the leaves.
For those who haven’t tried real wasabi, it has a delicious, sharp, hot flavour. Although it is similar to horseradish, the flavour is much more subtle and complex, at once fresh, sweet, while vaguely herbal. The heat doesn’t overwhelm your palette, and the pungency quickly dissipates, leaving an extremely pleasant, milder aftertaste a bit like mustard greens.
In traditional Japanese cuisine wasabi is prepared by grating the fresh stem (sometimes referred to a rhizome) – after stripping off the small side roots and washing. It is often served as an accompaniment, on the side of a dish, often mixed with soy sauce or rice wine, or used as a herb/spice (e.g., spread on the fish used in sushi or grated onto tofu). In New Zealand, a wasabi mayonnaise is apparently popular.
Although the root is still the most commonly used part, the whole wasabi plant is edible. Fresh, young leaves have a mild, peppery but distinctively wasabi-like flavour, but with much less pungency than the roots. The flowers, which usually appear in late January, are also edible and have a sweet, mild flavour. Both are excellent as a garnish or added to salads.
Larger leaves have more heat, and can be used to wrap food, for example, as an alternative to the nori traditionally used in maki sushi. Picked with the petiole (leaf stem) still attached, they are traditionally pickled in sake brine or soy sauce, and served as an accompaniment to white rice. They are also delicious fried in tempura batter, or added to stir-fries. The leaves can also be dried and have, in the past, been used to flavour a distinctive cheese. There are also, apparently wasabi wines and liqueurs, but these are probably more of a novelty than anything else.
Commercially, it tends to be cultivated in a gravel bed which is periodically flooded, as this produces larger roots which command a much higher price. However, the easiest way to grow wasabi at home or on the allotment is in open grown or a planter/raised bed. This tends to encourage plants to divide, so you end up with smaller, but more numerous roots. These are just as flavoursome as the larger ones, and as we only use a small amount at any one time, this suits us perfectly, but it is worth keeping in mind if you have your heart set on huge, showy roots.
As it grows at altitude in its native range, Wasabi is a hardy plant, although not as hardy as some that we offer. Generally speaking it will survive sustained lows around -5, and shorter lived ones down to about -10. However, it can be a good idea to give plants a good mulch, or provide some other protection, as winter approaches.
It grows best in rich, moist soil, and likes some shade, particularly during the hottest parts of Summer, which can be provided by planting near taller annuals, or herbaceous perennials. Removing any wilted leaves will help to prevent disease, although so far we haven’t experienced any issues on this front. We have noticed aphids on the petioles, but they rarely become a problem. And slugs and snails will also take a nibble, but again, we’ve found that Wasabi is troubled much less than some Brassica family plants.