Gadenin日本語Get the appWater deeply, and stop about a month before you dig
Sweet potatoes are genuinely drought tolerant. Their roots go down three feet or more, and a plant that looks limp at midday in high summer will be standing up again by dawn without any help from you. Water only when a two-week dry spell has left the soil dry two inches (5 cm) down, and then water heavily rather than lightly, so the moisture reaches the deep roots instead of encouraging shallow ones.
The one period that really matters is the six weeks when the roots are bulking, roughly the second half of the season. Erratic water then, a long drought broken by a downpour, makes the roots crack lengthwise. Then reverse the policy: from about three or four weeks before you plan to dig, keep the bed on the dry side. Roots taken from saturated soil bruise easily, cure badly and rot in storage.
| Stage | Watering | Why |
|---|---|---|
| First two weeks after planting | Every second day, lightly | Buried nodes must stay damp to root |
| Establishment to midsummer | Only in a two-week dry spell | Encourages deep rooting |
| Root bulking, late season | One inch (25 mm) a week if no rain | Steady moisture prevents cracking |
| Last three to four weeks | Withhold water | Dry roots cure and store far better |
Do not feed once the vines are running
The temptation with a big leafy crop is to feed it. Do not. Nitrogen applied after the vines start to run goes straight into more vine, and it does so at exactly the point when the plant should be shifting sugars downward into the roots. Gardeners who feed a struggling-looking sweet potato in midsummer reliably dig strings in autumn.
If the leaves are genuinely pale, yellowing from the oldest upward rather than just looking tired in the heat, one weak liquid feed is defensible. Choose something low in nitrogen and higher in potassium, apply it at half strength once, and then leave the plant alone. In a bed prepared as described in the soil guide, this should never be necessary.
A sweet potato covered in lush dark green leaves in late summer is not a healthy plant, it is an overfed one. Pale, slightly tired-looking foliage in autumn is what a good crop looks like above ground.
Lift the vines so they do not root along the row
Sweet potato vines root wherever a node touches damp soil. Each of those secondary rooting points draws energy away from the main crown and forms its own crop of small, stringy roots, so you end up with a wide scatter of useless little roots and disappointing ones under the plant itself. Lifting the vines is the single most useful job of the growing season.
Go along the row every two or three weeks from the point where the vines start running. Slide a hand or a cane under the vines, lift them clear of the soil and lay them back down in a slightly different position. It takes a couple of minutes per row and it does not damage the plant. Never prune the vines to control their spread: the leaves are the factory filling your roots, and cutting them back directly cuts your harvest.
- Start when runners reach two feet Once the vines are running out across the path, begin the lifting rounds. Before that there is nothing to do.
- Lift, do not cut Run a cane or your forearm under the vines and lift them clear so any new roots snap. Set them back down turned slightly. Do not shorten them.
- Repeat every two to three weeks New rooting points form continuously through summer. Set a fixed day and combine it with your pest check.
- Keep the crown area clear While you have the vines up, pull any weed growing within a foot of the crown. After midsummer the canopy handles weeding for you.
Mulch, weeds and the shade the crop makes for itself
For the first six weeks the plants are small and slow and the weeds are not. That window is when weeds do their damage, and it is worth hand-weeding the ridge once a week. From midsummer onwards the vine canopy is so dense that almost nothing germinates under it, and the crop effectively weeds itself for the rest of the season.
Black plastic laid before planting handles the whole problem and warms the soil as a bonus. If you prefer an organic mulch, straw works, but wait until the soil is thoroughly warm before laying it, because straw put down early insulates cold soil and keeps it cold. Straw also gives voles cover, so in a garden with a vole problem, plastic is the better choice.
| Mulch | Advantages | Drawbacks |
|---|---|---|
| Black plastic | Warms soil, total weed control | Needs removing at harvest, no organic matter |
| Straw | Cheap, keeps roots cool in heat | Shelters voles and slugs, keeps soil cold if laid early |
| None | Nothing to buy, easy digging | Six weeks of hand weeding on the ridge |
Reading the plant through the season
Above ground the crop gives you very few signals, which is why people find it hard to judge. The few signals it does give are worth knowing, because most of them are answered by doing less rather than more.
- Leaves wilt at midday, recover by morning Completely normal in heat, even in moist soil. Do not water on the strength of a midday wilt. Judge at dawn instead.
- Enormous vines, no sign of slowing in late summer Too much nitrogen. There is no cure this season. Stop all feeding, keep lifting the vines, and prepare a poorer bed next year.
- Older leaves yellow from the bottom up Genuine hunger, or waterlogging. Check drainage first. Only if the bed drains freely should you give one half-strength low-nitrogen feed.
- Purple or bronze leaf color in autumn A normal response to cool nights as the season ends. It often appears just as the roots are finishing and is not a problem to fix.