Like horse radish wasabi comes from a root in the Brassicaceae family, which includes horseradish. Wasabi is often referred to as Japanese horseradish. All parts of the plant are edible, stems, leaves, flowers and of course the roots.
Growing Wasabi from starts or seeds
Wasabi can be grown from seeds or starts. Starts are bare rhizome segments, so long as there is an tip, the plant can grow. Plant starts do not need to have roots, stems or leaves to develop into a full wasabi plant.
Prior to planting, it is advisable to soak them in cool water and remove any damaged leaves. Plant wasabi in spring once outdoor temps are about 50-60 F, space plants about a foot apart.
Plant starts or seed can be planted in either spring or fall, they grow year round, even in some pretty cold conditions. Wasabi planted in the spring will need to over winter in order to establish a good root system. Before planting, remove any damaged leaves and soak the start or seed in cool water for an hour or so. Distilled water is best but not absolutely essential. Planting in the summer will only work if you are diligent in watering and plant it in a shaded location. Summer planting starts will be slow to take, so keep watering. Temperatures below 80 F are necessary for any hope of robust growth, 45-60 F is optimal. If temps are over 80 F, the plants will simply stop growing and either die back or go into a sort of dormancy awaiting more ideal conditions such as in Autumn.
Planting in Early Spring [or even late winter] is best, Autumn will work as well for a harvest the following spring, it is best however to allow wasabi to mature another season for best results. Water wasabi persistently and comprehensively. Mulch around the plant bases will help retain moisture.
Make sure the soil is not clay and drains well. You can check water drainage by soaking the soil and observing how long the water takes to soak in. If you end up with standing puddles, your soil is not well draining.
Wasabi thrives in loose, organically enhanced moist soil, not swampy just moist. Soil pH between 6 and 7 is ideal. A mixture of compost and organic, sulfur-rich fertilizer is a good idea. Till the soil to roughly a foot deep and work in organic compost or manure to create an optimal environment. High sulfur fertilizers will enhance wasabi's pungent flavor.
Harvest wasabi roots in either spring or autumn. Leaves can be harvested periodically as needed, but in moderation if you plan on maintaining a robust plant. Roots should not be harvested the first year, although they are still viable and edible they are not prime quality or size. Mature rhizomes should be 4 to 6 inches long minimum, 7 to 8 inches long is best, size does matter in wasabi. Attaining a good size healthy rhizome / root takes about 2 years.
When harvesting wasabi roots, pull up the entire plant, discard any side shoots.
It's a good idea to leave a few plants in the ground when you harvest, it's a perennial and will last as well as reseed and multiply all by itself, you shan't have to replant it season after season.