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Yacon spends the first half of its long season building a plant and the second half filling tubers. From planting until midsummer you see leaf and stem and very little underground; the tubers are present but small and thin. Then, through the second half of the season, the plant moves sugars downward steadily, and the largest share of the eventual crop is laid down in the final six to eight weeks before frost.
That back-loading is the most useful thing to know about caring for this crop. Anything that shortens or damages the last two months, drought, a plant blown over, a check from being knocked about, costs you far more than the same problem in early summer would. Conversely there is no point fussing over a slow-looking plant in late spring; yacon is simply slow to start.
Watering: steady is the whole point
Yacon tubers are around eighty per cent water and they swell only while moisture is available. The plant tells you nothing useful in the meantime, because the big leaves wilt dramatically in afternoon heat and then recover fully overnight whether or not the soil is dry. Judge by the soil, not the leaves: push a finger in at the edge of the mound and water when the top two inches (5 cm) are dry.
Through the main season give the equivalent of an inch (25 mm) a week in dry weather, applied as one deep soak rather than daily sprinklings, which only wet the mound and encourage shallow rooting. Through the final filling weeks, when the days are shortening, keep that up: a dry spell in the last month is the single most common reason a plant that looked magnificent yields four small tubers.
Do not overcorrect. Yacon on waterlogged ground rots at the crown, and a soggy crown means no planting stock next spring even if you got a crop. Water deeply and let the surface dry between soakings. In a container, where the whole root zone dries in a day in hot weather, expect to water daily and consider a saucer only during a heatwave, never permanently.
| Period | Water | Signs you have it wrong |
|---|---|---|
| Planting to 12 in (30 cm) tall | Just enough to keep soil barely moist | Rotted crown piece, no shoot after four weeks |
| Early to mid season | 1 in (25 mm) weekly in dry weather | Stalled growth, small hard leaves |
| Final 6 to 8 weeks before frost | 1 in (25 mm) weekly without fail | Few, small, cracked or crooked tubers |
| After the tops are frosted | None | Rot at the crown in storage |
Feeding: keep the nitrogen down
Yacon will grow an impressive plant on very ordinary soil, and it will grow an even more impressive plant on rich soil while producing almost nothing worth digging. Nitrogen goes into those big leaves. A bucket of well-rotted compost worked into the planting station in spring is the whole base dressing on decent garden soil, and nothing further is needed.
If the plants look genuinely pale and stunted by midsummer, on thin or sandy ground, give one balanced liquid feed and stop. From midsummer onward, if you feed at all, use something potassium-leaning rather than nitrogen-rich, which supports tuber filling instead of prolonging leafy growth into a season that frost is about to end.
Fresh manure is a mistake here for a second reason as well as the nitrogen: it encourages the forked, split, misshapen tubers that make cleaning and slicing a chore, in a crop where the tubers are already awkwardly brittle.
Weeds only matter in the first two months. Once the canopy closes over, the leaves shade out almost everything underneath and the bed stays clean without further work. Hoe shallowly and well away from the stems, or hand pull, because the tubers run further out than you expect.
Handling brittle stems and heavy leaves
Yacon stems are hollow, sappy and snap cleanly rather than bending. A stem broken in midsummer is gone; the plant does not regrow that stem, though it usually has two or three others. Keep this in mind whenever you go into the row: hill up, stake and tie early in the season, before the plants close over, rather than pushing through them in late summer.
The plant does not need pruning, pinching or thinning at any point. Each crown throws up several stems and they all contribute. Remove a stem only if it is already broken and hanging, and cut it cleanly at the base rather than tearing it, which pulls at the crown.
Some plants flower in the last weeks of the season, with small dull yellow daisy-like flowers. This is late enough that it costs nothing and you can leave them; seed is rarely viable and is not how this crop is propagated. Do not treat flowering as a harvest signal, because it is not one. Frost is.
Keeping notes that pay off next year
Record the date the plants reached about a foot (30 cm), the dates you hilled, the dates and amounts of any watering during dry spells, and the date the first frost blackened the tops. That last date is the fixed end of your season, and having three years of it tells you exactly how early you must start crowns indoors to get a full-length season on your site.
Note anything that checked a plant as well: a snapped stem, a week of drought, a plant blown sideways. When you lift in autumn and one plant gives three pounds (1.4 kg) while its neighbor gives twelve, the explanation is almost always something you saw in summer and did not write down.