The pungent, assertive flavor of wasabi (Wasabia japonica) belies its delicacy as a plant. It requires very specific growing conditions and, as a result, is seldom cultivated outside its native Japan. Intrepid gardeners in cooler climates may have success cultivating wasabi if its needs are met. However, it takes approximately two years to produce a crop of rhizomes and is instead best grown as an ornamental for its lush, lotus-like foliage.
Wasabi occurs naturally along shaded waterways high in the mountains of Japan, so it has adapted to cool, moist conditions. Gardeners within U.S. Department of Agriculture plant hardiness zones 8a to 10b have the best luck growing wasabi, especially in coastal areas where temperatures stay steadily cool year-round. However, even within the ideal climate range, the growing location must be carefully selected and prepared to limit stress to the plants.
Shade and moderate temperatures are vital to growing wasabi. The plants perform best under bright, moderately shaded locations with northern exposure where daytime temperatures stay between 50 and 70 degrees Fahrenheit. Damage can occur if temperatures rise above 80 degrees or if they dip below 40 degrees, with permanent frost damage occurring below 27 degrees. In areas with hot, dry summers or cold winters, wasabi should be grown in pots filled with organically rich potting soil so they can be moved indoors during times of inclement weather.
Soil and Spacing
Soil and spacing have a great impact on the longterm health and vitality of wasabi plants. Loose, fast-draining soil with a pH between 6.0 and 7.0 is best, although it will also tolerate slightly sandy soil. Amend heavy soil with a 10-inch-thick layer of compost worked into the top 8 to 10 inches of soil to improve its drainage and nutrient content. Mature wasabi plants reach a height of 8 to 18 inches tall with a 12-inch spread. Space them 9 to 12 inches apart to provide adequate space for root production, and spread a 2-inch layer of mildly acidic mulch between them to keep down weeds and conserve soil moisture.
Due to their semi-aquatic origins, wasabi plants thrive under moist conditions. Constantly flowing water is not necessary, but their soil must be periodically flooded with clean, 45- to 60-degree Fahrenheit water to keep the roots and foliage healthy. Water daily during the summer months, but allow the soil surface to dry slightly before watering again to prevent rot. If signs of stress such as wilted leaves or yellowing occur during hot weather, increase watering to twice daily. Reduce watering by half during the winter months and withhold all water during rainy, cold weather.
Wasabi plants are not heavy feeders and seldom require fertilizer if planted in rich, organic soil. However, older plants or those in sandier soil may benefit from feeding in mid-spring and midsummer to improve their growth. Side-dress the plants with a 3-inch layer of well-rotted green manure to provide a constant source of nutrients, or apply a slow-release, general purpose fertilizer with an N-P-K number of 10-10-10 or 12-12-12. Apply the fertilizer to pre-moistened soil at a rate of 2 to 2 1/2 pounds per 100 square feet.
Pests seldom trouble wasabi plants, but they are susceptible to stress and damage under poor growing conditions and snails and slugs can be problematic. Sunburn is one common problem with wasabi grown in hotter climates. Shade cloth helps prevent sunburn and heat stress by blocking out strong sunlight. Ideally, shade cloth should provide 70 to 75 percent coverage and, to provide maximum coverage, it must be oriented toward the south-facing side of the bed. Control slugs and snails by handpicking them from the plants on a daily basis. Wear gardening gloves and check the plant's foliage after sundown using a flashlight. Place the slugs and snails in a bucket filled with soapy water.