In Japan, Wasabia japonica sometimes grows naturally in the gravel beds of mountain streams and is highly adapted to this environment. For commercial Wasabia japonica growing two types of cultivation methods are used. These are soil grown wasabi (Oka) and water grown wasabi (Sawa). Japanese growers select the method depending on where they live and the particular end use of the plants after harvest. Most wasabi farms are kept in the family and the last wasabi bed was reputed to be built alongside a mountain stream some 200 years ago. Due to the effects of acid rain and the weather in the mountains of Japan, a number of family farms are now being abandoned as younger members of the family immigrate to the cities.
Wasabia japonica plants grown in the soil require large amounts of organic fertiliser added to the soil before planting, and also require ongoing herbicides and pesticides used to maintain the health of the plants. Being a member of the cabbage family it is susceptible to the diseases of that family. The ground also needs to be kept damp at all times as the plant is regarded as semi-aquatic in its growth characteristics. It is regarded as good practice that the growing plot be abandoned after three harvests and not returned to for at least ten years.
If Wasabi japonica is grown in running water then less fertilisation needs to be used, although it is known that some Japanese and Chinese farmers will put sacks of chicken manure or blood and bone upstream from their farms to make the wasabi grow faster. Less pesticides and herbicides are used with this growing method, although the environmental impact is just as great.
All parts of the plant can be used for a number of products. The rhizome is the preferred part of the plant as it has the most active ingredients, but the petiole, roots and leaves are also used.
Most of the plant is used in food products, although now more and more of the rhizome is being used for Nutraceutical purposes. This was not the case in 1993 when the rhizome only was used for food products. The leaf and stem were pickled and only available in Japan.
The general concensus is that water grown Wasabia japonica (Sawa) produces larger rhizomes with more active ingredients, and for that reason is highly sought after and, therefore, command higher prices.
Soil-grown Wasabi japonica
Wasabia japonica requires specific environmental conditions to thrive. Soil-grown wasabi requires an air temperature from 6-20°C with 8-18°C considered optimal. Soils containing well rotted organic material with a pH 6-7 are considered best. It is most often grown on well-drained soil under mulberry or plum trees in Japan, whereas in New Zealand and China soil-grown Wasabia japonica is usually grown in shade houses rather than under trees.
Water grown Wasabia japonica
Water grown Wasabia japonica requires air temperatures ranging from 8-18°C. However, a narrower range of temperatures (12-15°C) is considered ideal. An air temperature of less than 8°C inhibits plant growth and at less than 5°C plant growth ceases. Other environmental factors can have an effect on the growth of Wasabia japonica and need to be considered carefully e.g. light levels, stable water temperature, good nutrient supply, and well aerated, neutral or slightly acidic pH water containing a high dissolved oxygen level and a large supply of water to maintain consistent flow (this particularly depends upon the growing system being used).
Rainfall accumulation is also important, with an even distribution desirable to stabilize the water supply and temperature. Spring water is considered best because of its clarity, stable temperatures and high level of oxygen. At warmer temperatures the dissolved oxygen in the water decreases, which inhibits the growth of plants.
Silty or muddy water is undesirable as it may contain insufficient oxygen, but some silt in the water is considered beneficial as a source of nutrients. In Japan, Wasabia japonica grows on the wet banks of cool mountain streams and springs in specially built growing beds. Overall, construction and establishment of a traditional growing bed is expensive and labour intensive.
Water grown Wasabia japonica is produced in 42 prefectures, and soil grown Wasabia japonica in 21, out of 47 prefectures in Japan, which indicates that flooded cultivation is popular and is considered to produce a high quality product.
The unique environmental requirements and shortage of cultivatable lands limit Wasabia japonica production areas to 880 hectares in Japan and 400 hectares in Taiwan, but demand for Wasabia japonica condiments is spreading from Japanese cuisine to modern western food. The increasing interest in Wasabia japonica and the inability to expand production in Japan has seen prices rise steadily since 1970. High prices have stimulated research into soil production methods and the investigation of production areas outside Japan. The Japanese have invested heavily in soil-grown wasabi farms in China.