The wasabi plant (Wasabia japonica) is a close relative of the mustards and is native to Japan, Korea, China and New Zealand. It has large, heart-shaped leaves growing from a small underground rhizome and can be 60 cm (2 feet) tall. Wasabi plants need cool, shady conditions and running water to thrive, and they are challenging to grow in the garden. Wasabi rhizomes are grated in Japan to create a much-prized hot dip. Most wasabi paste sold in the West is an imitation product based on horseradish and Chinese mustard.
Grow wasabi plants in an area of the garden that receives 75 per cent shade. A spot next to a shaded garden pond or flowing stream with water temperatures of 12.2 to 15 degrees Celsius (54 to 59 degrees Fahrenheit) is ideal. Wasabi plants require slightly acidic soil that is loose, wet and rich in organic matter. Work the soil to a depth of 20 to 25 cm (8 to 10 inches), digging in 30 cm (12 inches) of compost.
Plant your wasabi plant in the spring once temperatures are above -1.11 degrees Celsius (30 degrees Fahrenheit). Leave 30 cm (12 inches) between plants.
Provide additional shade or cover your wasabi plant with straw during the summer. Aim to keep the air temperature below 20 degrees Celsius (68 degrees Fahrenheit). Take care not to damage the leaves as this will slow down growth.
Water your wasabi plant regularly and thoroughly, and never let it dry out. If possible, set up a flowing water system or water feature that continually splashes the leaves with cool water. Fertilise with a slow-release, balanced fertiliser enriched with micronutrients. Apply fertiliser every three or four months at the manufacturer's recommended strength.
Harvest your wasabi plants after three years of growth, when the rhizome is 15 to 20 cm (6 to 8 inches) long and 2.5 cm (1 inch) in diameter. Wasabi leaves and stalks can be used as a garnish or in salads.
Grow wasabi plants in containers in areas with summers hotter than 21.1 degrees Celsius (70 degrees Fahrenheit), or winters colder than -1.1 degrees Celsius (30 degrees Fahrenheit). Plant your wasabi in a 15 cm (6 inch) pot with rich, organic compost on top of a layer of sand or grit for drainage. Transplant into a 30 cm (12 inch) pot after a year. Water container-grown wasabi plants thoroughly before the soil surface starts to dry out and let all excess water to drain away.