Edible Tubers

Site and planting

Plant wasabi in partial to full shade — direct sun above 60°F will cause leaf burn and stress. A north-facing site, under a shading canopy, or on the shaded side of a structure is ideal. The crown (the point where leaves meet roots) should be planted approximately ½ inch above the soil surface — burying the crown promotes rot.

Amend planting soil with generous finished compost worked one foot deep to provide high organic matter content and excellent drainage. Wasabi requires consistent moisture but absolutely cannot tolerate waterlogged conditions — the combination of moisture and warmth rapidly causes crown rot. In cold-winter climates (below Zone 7), grow wasabi in 2–3 gallon containers using a well-drained vegetable garden mix; move containers indoors or to a frost-free space in winter.

Care and harvest

Keep plants consistently moist but not saturated. On warm days, misting foliage with cool water helps maintain the temperature range. Do not allow plants to dry out — stress accelerates disease susceptibility. Top-dress with compost annually.

Wasabi rhizomes (the pungent, grated green portion) develop slowly — typically 18–24 months from planting before the rhizome is large enough to harvest. Harvest by lifting the entire plant; use the rhizome immediately or store refrigerated for up to a few weeks. Wasabi's heat compounds are volatile — grate fresh immediately before serving for maximum flavor impact.

Yacon is a member of the sunflower family native to the Andes, producing large, crisp, sweet storage roots that taste somewhat like a cross between a water chestnut and a mild apple. The flavor comes from fructooligosaccharides (FOS) — a type of carbohydrate that passes through the human digestive system largely unabsorbed, giving yacon a low glycemic index despite its sweetness. It can be eaten raw, cooked, or pressed into a syrup with a flavor resembling molasses.

Yacon is a relatively forgiving crop within its season requirements. It tolerates both heat and cool temperatures reasonably well — the key limitation is season length: the storage roots need at least 200 frost-free days to develop fully, and a long, frost-free autumn produces substantially larger tubers. The top of the plant will be killed by the first frost; the tubers in the ground are hardy to about 10°F and will regrow from the ground the following spring in suitable climates (Cultivariable.com).

Planting and care

Plant yacon tubers in spring after all danger of frost has passed, in rich, well-drained soil with a pH of 6.5–7.2 amended with compost. Plant in a sunny location; yacon is not shade-tolerant. In Zone 5–6 locations with growing seasons shorter than 200 days, start tubers in containers 4–6 weeks before the last frost date to extend the season; transplant outdoors when frost risk has passed.

Keep soil evenly moist throughout the growing season — consistent moisture is important for root development. Mulch with compost to encourage vigorous growth and retain moisture. Plants grow 4–6 feet tall in a full season; they do not need staking but appreciate some shelter from wind, which can shred their large leaves. If growing in a mounded or raised bed, drainage is improved and yields are often higher.

Harvest and curing

Harvest after the top of the plant has been killed by frost — the frost signals maturity and slightly sweetens the roots. Dig tubers carefully with a fork; they are near the surface and fragile. The roots will taste bland at first, similar to a water chestnut; cure at room temperature for several days to 2 weeks until the skin darkens to a maroon color and sweetness develops markedly. Store cured roots cool and dry for up to several months.

Oca is a Peruvian and Bolivian food staple — the second most widely cultivated root crop in the Andes after the potato, yet largely unknown in North America. The small, waxy tubers come in a remarkable range of colors: white, yellow, orange, pink, red, and purple-streaked. Their flavor is mildly tart (from oxalic acid) and slightly nutty; when exposed to sunlight after harvest for a few days, the oxalic acid diminishes and the tubers become sweeter and more palatable. They can be eaten raw (thin-sliced in salads), roasted, or boiled.

Oca tubers are hardy to 0°F, but the above-ground plant will be killed by frost — the tubers in the ground will overwinter and regrow if the soil does not freeze solid. However, leaving tubers in the ground over winter is not ideal for yield — they will all sprout and crowd each other, producing smaller tubers in subsequent years. The best approach is to treat oca as an annual: plant each spring, harvest fully in fall, and select the best tubers to replant the following season (Cultivariable.com).

Planting and care

Start tubers in a container using a well-drained vegetable mix 4–6 weeks before the last frost date; transplant outdoors when frost risk has passed. Or plant directly in well-drained garden soil after last frost. Oca prefers full sun and well-drained soil with a pH of 6.0–7.2; yields improve with the addition of compost.

Keep soil evenly moist during the growing season. Hill oca plants once or twice during the growing season, the same way you would hill potatoes — mounding soil up around the stem base encourages additional tuber formation along the buried stem. Tubers form as day length decreases in late summer and fall, so avoid harvesting prematurely.

Harvest and storage

Harvest after the first frost kills the tops, or in fall before hard frost to prevent tuber damage. Dig carefully — tubers are near the surface. Expose tubers to indirect sunlight for a few days after harvest to reduce oxalic acid and increase sweetness (the skin darkens and sweetness improves noticeably). Then store cool and dry. Do not refrigerate immediately after harvest — allow the curing period first.

Mashua is the least familiar of Raintree's tuber offerings and, for PNW gardeners in particular, one of the most worth trying. A close relative of the nasturtium (Tropaeolum majus), it produces vigorous climbing vines with attractive orange-yellow flowers, and large, peppery, earthy tubers beneath. The flavor has been variously described as a cross between hot radish and cooked turnip — bold and distinctive, either raw (thin-sliced) or roasted. It is a significant food crop in the high Andes of Peru, Bolivia, and Ecuador, where it has been cultivated for at least 3,000 years.

Mashua's soil pH tolerance (5.3–7.5) is among the widest of any edible tuber, making it adaptable to a range of garden conditions. It is well-suited to cool, moist climates and is explicitly noted by Raintree as being particularly well-suited to Pacific Northwest cultivation — the cool summers and reliable moisture of western WA and OR suit its growing requirements closely.

Planting and care

Plant tubers in a sunny location in spring after all risk of frost has passed. In short-season climates, start tubers indoors in pots 4–6 weeks early to get a head start. Mashua tolerates most soil types as long as they are moist and moderately well-drained; it will grow in both sandy and heavier soils. Amend with compost for best yields.

Provide a trellis, fence, or other sturdy support structure — vines grow vigorously and will sprawl untidily without support. Mashua vines can reach 6–10 feet in a full PNW season; provide adequate support from the beginning. Hilling after first emergence and again at first flower may increase tuber yield by providing additional rooting zone along the buried stems.

Keep soil consistently moist during the growing season. Mashua is generally pest- and disease-resistant — one reason it has been grown for millennia in the Andes with minimal crop protection inputs.

Harvest and storage

Tubers are ready for harvest in 6–8 months, or following the first frost that kills the tops. Tubers form near the surface and are harvested like potatoes. Harvest promptly after the first frost kills the top — do not leave in the ground through hard freezes that would damage the tubers.

Store harvested mashua tubers for up to 6 months in a cool, well-ventilated location protected from strong light. Light causes greening (as with potatoes). Tubers left in the ground will sprout and regrow the following spring in frost-free or mild-winter climates.