Wasabi

Stratify seed at about 41F for two months first, then sow a half inch deep and two inches apart in a well-draining sand and compost or vermiculite-perlite-peat mix. Sow once outside temperatures sit in the 50F to 57F range, which is February into March in mild climates and later where it still freezes. Keep the mix moist but never saturated; seedlings emerge in about 20 days. Freshly harvested wasabi seed is deeply dormant and simply will not germinate without a cold spell; in the wild it is winter that breaks dormancy and the seed sprouts in February.

Turn leaves over and look for the small shiny spots of white blister rust that swell and turn milky white; it shows up in spring and fall, spreading around 55F to 57F and stalling once it hits 66F. Check for green peach and turnip aphids causing leaf discoloration and distortion, and hand-pick slugs, which are worst in the first two months after planting and can take out young plants entirely. Pull weeds around the planting by hand. Aphids matter less for the chewing than for the viruses they carry, and weeds beside a wasabi bed have been found hosting the very same white blister rust fungus infecting the crop, so they act as a reservoir that reinfects it.

Hold the planting under heavy shade from shade cloth, shade trees such as black alder or poplar, or an evergreen canopy. Up to 70% shade suits established plants and up to 80% suits seedlings; a straw cover also works. Measure air temperature two feet above the plants and add shade if it climbs past 77F. Aim for 54F to 59F, and treat 46F to 64F as the workable band. Above 77F, and especially past 82F, soft rot and blackleg take hold and leaves burn outright. In nature wasabi grows on shaded, wet banks of cold mountain streams, and heat is the fastest way to lose the crop.

In a soil-based planting, spray a foliar sulfur application one to three months ahead of harvest. In New Zealand trials, Daruma given 560 kg sulfur per hectare at 380 days after planting reached 2,486 ppm isothiocyanates in the harvested stem. Sulfur is the raw material for allyl isothiocyanate, the compound that gives wasabi its heat, and loading it late puts the pungency into the stem right before you cut it rather than letting the plant spend it on growth.

Each mother plant throws up to 20 plantlets around the crown. As you lift plants for market, cut the healthy plantlets free and replant them immediately, keeping the crown intact. Use only plantlets at least an inch and a half tall with four or five leaves, dark green and free of leaf spots. Harvest and replant together in fall or spring. After three consecutive vegetative generations, start over from tissue-cultured or seed-grown stock. Offshoots are clones of the mother and carry her diseases with her; blackleg overwinters as mycelium in infected tissue, so each vegetative round concentrates it. Japanese growers cycle back to seed or tissue culture specifically to break that chain and rejuvenate the planting.

Start checking size early in the second year of growth. Harvest when the stem is about 6 in. long and 2 in. across, typically 16 to 20 months in a rock-mat semiaquatic bed. Pull the whole plant up by hand, wash the stem, cut away dead leaves and any diseased tissue, keep the newest center leaves, and trim leaf stems to about a third of their length. Store cold and humid for up to four weeks. Per NC State Extension Gardener Plant Toolbox (https://plants.ces.ncsu.edu/plants/eutrema-japonicum/), the best flavor comes from grating the cleaned stem within thirty minutes of harvest. The stem reaches peak harvestable size early in year two and before flowering, and the plant then diverts itself into bloom. An evenly tapered, medium green stem brings the highest price; uneven taper is the plant telling you growing conditions wobbled.

Move seedlings out when they are about four inches tall with at least four leaves. Set each plant so the crown sits roughly a half inch above the soil surface, and give extra shade until the plants take hold. September through October is the best window. Plants must establish roots before temperatures drop below freezing; at 27F the central stem starts to freeze, and while offshoots usually survive and regrow, an unrooted transplant will not.

Broadcast and incorporate compost from poultry or dairy manure, green manure, or 12-12-12 at a rate supplying about 100 lb plant-available nitrogen per acre, plus 25 to 40 lb of sulfate-form sulfur per acre. Add borax at 18 lb per acre broadcast, or 9 lb banded, only if a soil test shows boron deficiency. Test both soil and water before planting. Wasabi wants an open, friable soil high in organic matter, and the sulfur feeds the isothiocyanate chemistry that makes the stem pungent in the first place. Working it in before planting is the only clean chance you get under a perennial crop that sits two years.