Wasabi is a resilient perennial herb thriving in the damp, shaded environments of Japan. Recognized for its cluster of small, white flowers on slender stalks, wasabi has glossy, deeply veined leaves that form at the base. The plant’s spicy-flavored leaves are a testament to its survival in the wild, deterring herbivores with their potent taste.
I. Appearance and Characteristics
Wasabi (Japanese: ワサビ, わさび, or 山葵, pronounced ) or Japanese horseradish (Eutrema japonicum syn. Wasabia japonica) is a plant of the family Brassicaceae, which also includes horseradish and mustard in other genera. The plant is native to Japan and the Russian Far East including Sakhalin, as well as the Korean Peninsula. It grows naturally along stream beds in mountain river valleys in Japan.
Siebold named Cochlearia wasabi in 1830, noting its use pro condimento or “as a condiment”; however, this is a nomen nudum, and the synonym Eutrema wasabi, published by Maximovich in 1873, is thus an illegitimate name. The wasabi plant was first described by Miquel in 1866, as Lunaria (?) japonica, from the type collected by Siebold in Japan, though the precise type locality was not recorded.
In 1899 Matsumura erected the genus Wasabia, recognising within it the species Wasabia pungens and Wasabia hederaefolia; these are now regarded as synonyms of Eutrema japonicum. In 1912 Matsumura recognised the species Wasabia japonica, treating his earlier Wasabia pungens as a synonym. In 1930, Koidzumi transferred the wasabi plant to the genus Eutrema, the correct name and author citation being Eutrema japonicum (Miq.) Koidz.
It has large leaves produced from long, thin stalks. They are simple and large, 3–6 inches (7.6–15.2 cm) long and 3–6 inches (7.6–15.2 cm) wide with palmate veins.
Wasabi flowers appear in clusters from long stems that bloom from late winter to early spring.
Wasabi favors growing conditions that restrict its wide cultivation – among other things, it is quite intolerant of direct sunlight, requires an air temperature between 8 and 20 °C (46 and 68 °F), and prefers high humidity in summer. This makes fully satisfying commercial demand impossible for growers, which makes wasabi quite expensive. Therefore, outside Japan, finding real wasabi plants is rare.
II. Wasabi Varieties
There are a number of popular cultivars of wasabi, including:
- ‘Daruma’ is more tolerant of high temperatures than most varieties.
- ‘Fuji Daruma’ is a fast-growing variety, ready to harvest in the same year it is planted.
- ‘Green Thumb’ is a good choice for edible leaves.
- ‘Sanpoo’ was developed to grow well in poor soils.
- ‘Mazuma’ is a slow-growing variety that takes up to three years for the roots to mature.
III. How to Grow and Care
Sunlight
Wasabi thrives best in conditions of partial sunlight, where light is dappled or filtered through canopy cover. Consistent exposure to gentle morning or late afternoon sun complemented with shade during the peak intensity hours promotes healthy growth. While wasabi can adapt to full sun conditions, excessive light may lead to stress, negatively impacting its health by causing leaf scorch or faded foliage. Outdoors, wasabi should be planted in spots that provide natural shade or alongside taller plants that can offer protection. The subtle light adjustments and potential leaf color variation are indicative of wasabi’s response to its light environment, signaling the need for optimal light balance.
Temperature and Humidity
One important consideration for growing wasabi is that it tolerates only a narrow temperature range, preferring constant temperatures of 45°F to 75°F.
Temperatures below freezing or above 80°F can kill the plant, so these extremes should be avoided. Some coastal locations, such as the Pacific Northwest, provide the suitably cool, temperate weather for growing wasabi. Planting in containers means you can move the plants if the thermometer suddenly gets too high or too low.
If the leaves get droopy or wilted, move the plants to shade and mist them a bit.
Watering
Water your wasabi well at planting and then regularly after this. Misting the plants will also help keep them cool. Take care not to overwater as these plants don’t tolerate waterlogged conditions.
Soil
Wasabi plants in the garden require rich, consistently moist soil with a slightly acidic to neutral pH. If growing in containers, use a one or two-gallon pot with a 10-inch planting depth. Mix up some rich soil with compost; slightly heavier than potting mix medium, so it will hold moisture. Plant your nursery starts so that they stand upright, leaving part of the rhizome exposed.
Fertilizing
Your wasabi plant doesn’t need much fertilizer. They’re slow-growing and as a result, can’t take too much fertilizer. However, it’s best to give them a balanced fertilizer (meaning the NPK numbers are all the same) like a 12-12-12 at the time of transplanting. Additionally, many farmers apply a foliar spray to the leaves about 1-3 months before harvest to improve flavor.
Pruning
Removing any wilted leaves that don’t perk up after misting. This can help to avoid any spread of disease or powdery mildew.
Propagation
While difficult to grow from seed, it is possible. Known for their low germination rates, if you do get some of the wasabi seeds or seed shells, try overplanting and expect under germination. Additionally, if they haven’t already been stratified, you’ll need to put the wasabi seeds in the fridge for 2 months to simulate a cold period for the wasabi seeds to go dormant. You can expect a germination time of 3-4 weeks.
More likely, you’ll be able to source or find pups, plantlets, or starts (all the same thing). These are the baby plantlets that grow along the stem or crown of a more mature wasabi plant. With these starts, you can place them directly into your growing medium, ensuring that the root has sufficient room to grow downwards. This is best done in the fall with several months of ideal growing conditions on the horizon.
Also grown through tissue culture, this option is open to many large-scale farmers who want to grow sterilized products at a faster rate without spreading some of the many diseases common in this crop. This is a relatively new way to propagate plants and not something replicable outside a laboratory. In your search for wasabi, try searching for wasabi tissue culture plants if you can’t find the more traditional starts.
Pests and Diseases
- Growing Problems
One of the most common problems with growing wasabi is finding rot. In order to fight this problem, try searching for rot-resistant starters. There are several varieties that have been bred to be resistant to rots. Improve drainage in your growing area and remove wilted leaves that can harbor disease once dead and decaying.
- Common Pests
Wasabi doesn’t suffer from too many pest problems. They’re much more susceptible to fungal problems. However, aphids are big fans of the wasabi leaf. Neem oil or insecticidal soap will handle the aphids easily.
- Common Diseases
Wasabi suffers from a plethora of fungal diseases including leaf spot, root rot, rhizome rot & petiole blight. It may be hard to differentiate between the diseases as many of them manifest with the same symptoms of wilted leaves, browning and blackening stalks and roots, and a grayish tinge to their leaves.
Prevention is the best path to take as few plants survive the disease, especially given how long it is to harvest. Use a copper fungicide or copper spray as a preventative for leaf spots.
IV. Uses and Benefits
Wasabi is generally sold either as a rhizome or stem, which must be grated very fine before use, as a dried powder, or as a ready-to-use paste in tubes similar to toothpaste tubes.
The part used for wasabi paste is variously characterised as a rhizome, a stem, or the “rhizome plus the base part of the stem”.
In some high-end restaurants the paste is prepared when the customer orders and is made using a grater to grate the stem; once the paste is prepared, it loses flavour in 15 minutes if left uncovered. In sushi preparation chefs usually put the wasabi between the fish and the rice because covering wasabi until served preserves its flavour.
Fresh wasabi leaves can be eaten raw, having the spicy flavor of wasabi stems, but a common side effect is diarrhea.
Legumes (peanuts, soybeans, or peas) may be roasted or fried and then coated with wasabi powder mixed with sugar, salt, or oil and eaten as a crunchy snack. In Japan, it is called wasabi-mame (わさび豆, “wasabi bean”).
V. Harvesting and Storage
- Harvesting
Wasabi is actually edible at almost every stage of its life. However, it is usually harvested when the stalks are protruding above the ground by about 4-5 inches and are about 1 1/2” thick. Simply pull the stalk gently out of the ground, trying not to break it in two. Remove the leaves and any rot at the base of the root.
- Storing
It’s best used fresh and eaten raw after being grated. It usually only lasts about two months fresh in the fridge. It can alternatively be dried and powdered, but this causes the plant to lose some of its precious nutrients. There are no unused portions of wasabi, as the entire plant is edible. Try adding its leaves and stems to a salad, or pickling them to make azuke.