Freshly grated wasabi, less than an hour from harvest to plate, has to be one of the most highly prized culinary experiences. The effect of wasabi on the palette is not subtle or mild; quality wasabi can literally blast the nasal passages with a distinctive heat and flavor which has, in recent years, become much more widely appreciated.
What is exciting about wasabi is that being a semi-aquatic vegetable, it is well suited to hydroponic production, and quality, yields and growth rates can be boosted using soilless methods if the right conditions are provided. This, coupled with the increasing availability of young wasabi plants and offsets sold by herb producers and commercial growers, means indoor gardeners can now grow this exotic, expensive and exciting plant.
Environmental Requirements for Growing Wasabi
While well suited to hydroponics, wasabi japonica does have specific requirements for optimum growth that have given it the reputation of being difficult to grow. Much of this reputation stems from the fact that, unlike many other crops and herbs we grow hydroponically, wasabi is a cool-season plant with little heat tolerance. For this reason, growing indoors, where light, air and solution temperatures can be highly controlled, give optimal results.
In its native environment-the stony riverbanks of cold mountain streams in Japan-the plant is often under shade with its roots semi-immersed in flowing, naturally oxygenated water. Commercial wasabi operations in Japan and other countries often aim to replicate this environment by creating raised beds of gravel through which cool, high-quality water flows. Many smaller growers have had success growing wasabi in pots in well-ventilated shade houses similar to those used for ferns.
In an indoor garden, wasabi needs not only a cool environment with maximum airflow, but also careful consideration of the lighting provided. Hot, direct HID lighting may burn wasabi foliage and result in too much heat loading; LEDs similar in intensity to those used for microgreens and lettuce are ideal.
Since wasabi has similar temperature optimums to other cool-season crops such as lettuce and many salad greens (50 to 72°F, 10 to 22°C), this herb can be easily incorporated in an indoor growing area set up specifically for these crops.
While wasabi plants can withstand warmer conditions, these will result in leaf wilting during the day, reduced growth rates and plant decline, all of which are symptoms of temperature stress. Damping down the plants during warm weather can assist with temperature reduction in summer as can fog or mist systems inside the growing area.
Hydroponic Systems for Small Scale Wasabi Production
Wasabi plants can reach a substantial size at maturity and this needs to be taken into account when planting the small seedlings or offsets. An 18-month-old, healthy plant (cultivar dependant) can be 2 feet tall and more than 2 feet wide as it spreads out during development.
A plant of this size should have multiple harvestable stems as well as small offsets or shoots that can be taken to propagate more planting stock and a dense canopy of rounded, brittle leaves on long, thin stems.
Root systems also become large and dense as most plants are grown for a period of 18 months to two years or longer. For this reason wasabi plants are better suited to a large media bed system rather than nutrient film technique, where channels can become blocked with the extensive root mat.
It is essential that the growing medium be free draining to help prevent stem and root rot. Despite being considered a semi-aquatic plant, wasabi doesn't seem to thrive in a continually submerged system or one where the nutrient stagnates in the media, as this excludes oxygen, which the root system has a high requirement for.
Clean, sterilized river gravel is a good medium for a small wasabi system and has been used successfully for hydroponic crops. Similar substrates such as coarse perlite, different types of grow rocks sold in the hydroponic industry, scoria and chunky bark or coconut fiber chip (similar to that used for many species of orchids) are also suitable, provided all are free draining and highly aerated. Ideally, grow beds need to be at least 1 foot deep with nutrient solution applied via an intermittent drip irrigation system to the top of the substrate using a cycle that does not flood the plants at any time.
The nutrient should be applied so that the surface of the substrate remains relatively dry most of the time while the medium underneath is cool and moist. Wasabi, like many crops, is prone to developing fungal diseases and stem rots around the base of the plant if conditions become overly wet due to excess irrigation.
This is one of the reasons why commercial, outdoor wasabi systems often use a raised row or mounded system to keep the surface of the growing media dry directly under the plants, so that flooding around the delicate stems and leaf bases does not occur.
Read More: Dialing in Hydroponic Drip Irrigation
Nutrient Management for Wasabi
Wasabi plants, like many members of the crucifer or cabbage family, seem to benefit from regular nutrient solution changes. This may be due to plants producing exudates that accumulate in recirculating nutrient systems and begin to slow growth and mineral uptake.
Dumping and replacing the nutrient solution with a fresh batch each week for larger, mature plants is advised, while smaller plants can be run for two to three weeks on the same solution. If plant growth appears to slow or plants are not removing nutrients from the solution (for example, EC levels don't fall over time), then changing the nutrient solution will often cause a boost in growth and nutrient uptake.
Wasabi appears to have similar nutrient requirements to other members of the crucifer family and the addition of extra sulfur during the stem development and elongation phase may assist the development of volatile compounds within the plant tissue.
In Japan, sulfur is sometimes sprayed onto the crop to enhance flavor and this can also be carried out in hydroponically-grown wasabi. Research into hydroponic wasabi production has shown that young plants are best maintained at an EC of 1.2 to 1.8, increasing up to 2.4 as plants mature. EC can be lowered under warmer temperatures to assist plants
in taking up water and transpiring to cool themselves, and increased under lower temperatures to assist with flavor development. As for pH, levels are optimal at 5.8 as this assists with iron uptake, particularly under cooler growing conditions.
Wasabi Planting and Propagation
Wasabi can be propagated from seeds or vegetatively through either offsets or small, rooted shoots that form around the outside of mature plants. Herb suppliers often sell young plantlets that can be shipped in damp sawdust.
Hydroponic growers are advised to start their plants from offsets, as the seed has dormancy requirements that can make germination slow and unpredictable. While there are a few different wasabi cultivars, there is not an extensive selection grown commercially. The most commonly grown varieties in hydroponics are midori and daruma, which vary slightly in growth form, flavor and heat.
Harvesting and Preparation of Homegrown Wasabi
The harvestable portion of the wasabi plant is the stem that forms above ground and elongates as the plant matures. Wasabi stems suitable for harvest develop over a 12- to 18-month period and as the older, lower leaves age, they die and fall from the stem, leaving a leaf scar. It is the increasing length of leafless stem that is harvested. Stem thickness is typically up to an inch in diameter with first-grade stems being at least 6 to 7 inches in length.
The unique flavor and heat of wasabi is derived from a range of volatile compounds that are released from the plant tissue and dissipate rapidly. Fresh wasabi stems have a limited shelf life and once cut from the plant should be refrigerated, but ideally used immediately after harvest and within 20 minutes of preparation.
The cleaned and scrubbed stems are grated--traditionally in Japan, this is done with a sharkskin grater-to gently shred the fresh stem tissue into a smooth paste, a process that creates the unique flavor of pure wasabi. Apart from the thick, first-grade stems sold for fresh consumption, the smaller stems and leaves of the wasabi plant are used for paste and other products as they have a similar but less intense flavor.
There is no comparison between high-quality, freshly grated wasabi stem and the inferior green paste sold in tubes (often not made from wasabi at all). Growing your own wasabi is a culinary pleasure that is highly rewarding and, given the price of first-grade fresh wasabi stems, could also be profitable. With a little attention to environmental control and careful nurturing, wasabi will thrive in a hydroponic system and make a valuable addition to any indoor garden.