For soil, use a rich, but well draining soil. A good mix is 2 parts potting soil to 1 part perlite or pumice rock. Repot your plant to a pot about 3 gallons (12 liters) in size and at least 8 inches (20 cm) wide, with drainage holes. Leave 4 inches (10 cm) of space below the top of the pot, because you will be "mounding up" soil over the plant later.
Add water until evenly moist (but soggy). For the first 2 weeks after repotting, cool/mild temperatures are best (55-74°F / 12-24°C), with some air ventilation and bright shade. After the 2 week adaptation period, start giving it sun, but protect it from strong afternoon sun. In warmer climates, keep the pot shaded, or at least mulch well.
Feed about every 4-6 weeks using an all-purpose fertilizer. If your potting soil contains fertilizer, you shouldn't need to feed the first 3 weeks after repotting.
Forming tubers -- Like potatoes, the edible tubers form on underground stolons. After a few months, start mounding dirt up around the base of the stems to encourage stolon formation. They begin developing tubers in late summer as daylight hours shorten to 12 hours per day. The tubers normally take another few months to mature and are usually ready in late winter. They should be protected from frost during this time. The foliage above ground usually dies back when the roots are ready, but this doesn't always happen. Try digging in the soil and pulling one or two out to see if they're large enough.
You may wish to leave the roots in the sun for a few days. This does two things: it increases the sugar content, plus it reduces the oxalic acid naturally found in Oca. Oxalic acid may cause problems for some people who are prone to kidney stones. Cooking or par-boiling the tubers also reduces the oxalic acid content.
The skin is edible by the way, just give it a good brushing under running water.
You can store your Ocas in a refrigerator for many weeks, or sometimes months. Just check on them periodically, and if they show signs of deterioration, plant them right away.
Have fun growing them!
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