How to grow sweet potatoes in containers: slips, heat, harvest and curing

Sweet potatoes grow well in a large pot if you plant rooted slips once the soil reaches 65°F, give them 6 or more hours of sun, and wait 90 to 120 days. Penn State Extension says sweet potatoes can be grown in large pots and names Porto Rico and Vardaman as bush varieties that do well in containers. The plant likes what a balcony has in summer: heat, light and a pot that drains.

A sweet potato is not a potato. University of Maryland Extension calls the part you eat a storage root, not a tuber, and the plant belongs to the morning glory family. That is why the advice for potatoes in grow bags, with its hilling and soil-adding, does not carry over. Sweet potatoes are planted once, left to vine, and dug at the end.

Pick a bush type if space is tight

Vine length decides how easy the plant is to live with. UF/IFAS Extension says the vines can extend up to 15 feet depending on the variety, and Penn State Extension says they can reach 20 feet. Penn State describes the two bush varieties it names, Porto Rico and Vardaman, as easier to manage and spreading about 2 feet per plant. UF/IFAS also points gardeners with less space to Vardaman, and Clemson describes it as a compact, bush-type plant that takes about 100 days to harvest.

Clemson gives days to harvest for other varieties too: Beauregard at 90 to 100 days, which it calls the quickest to produce harvestable roots, and Jewel at 120 to 135. If your season is short, pick by the days figure first.

Vining types still work in a pot, but plan for them. Penn State Extension says you can grow sweet potatoes vertically on a sturdy trellis, weaving the vines through, which saves space and makes the edible leaves easier to pick. A renter can set one up with the methods in the no-drill trellis guide.

Start slips from a root, or buy them

Sweet potatoes are not grown from seed. Clemson says slips are sprouts grown from roots of the previous season’s crop, and that most gardeners prefer to buy them. UF/IFAS Extension recommends certified disease-free slips from a reputable supplier, because sweet potato weevils can be a serious problem. Clemson also advises against using transplants with black rot spots on the lower stem.

If you start your own, choose the root carefully. Penn State Extension says fresh sweet potatoes, or ones treated with sprout inhibitors, are harder to sprout. Clemson says roots from last year’s home harvest, or organically grown ones, are more apt to sprout than most grocery-store roots, which were chemically treated against sprouting. University of Maryland Extension says to wash any purchased root to remove anti-sprouting chemicals.

Two methods are on offer from the extension sources.

• In a tray of damp mix (Penn State, Maryland). Penn State starts 6 to 8 weeks before planting with a small or medium root about 1.5 inches across. Put 2 to 3 inches of moist potting soil or sand in a container, lay the roots on their sides, and bury them halfway. Keep the lid on in a warm (75°F), sunny place. Maryland slices the root in half lengthwise, lays it cut side down, covers it with 2 inches of mix, and keeps it at 75 to 85°F under plastic until shoots appear, then moves it under grow lights for 14 to 16 hours a day. Maryland counts six weeks from bedding to planting. Penn State gets three to five slips from each root.
• In a jar of water (Clemson, UGA). Clemson pushes three or four toothpicks into a medium root and suspends it so the bottom half sits in water, in a warm spot with indirect sun. Change the water weekly. Sprouts appear in about 2 weeks. Once a sprout is six or more inches long, twist it off and stand it in fresh water, and it roots in about 10 days. Penn State warns that growing slips in water carries more risk of a bad outcome, because sweet potatoes are prone to rot.

A warm spot matters more than a bright one at this stage. If your kitchen is cool, a seedling heat mat with a thermostat holds the 75°F that Penn State describes.

Count backwards from planting day. Penn State says to plant three to four weeks after the frost-free date, and to start roots 6 to 8 weeks before planting, which puts the bedding of the root roughly 9 to 12 weeks before your last frost. To find that date, see the frost date guide for balconies.

Pot, mix and planting

Penn State Extension’s page says to use large pots without giving a volume. What the extension sources do give is the shape of the root zone and the room each plant gets in the ground:

• Penn State says the roots will be in the top 8 to 10 inches of soil at harvest.
• Spacing between plants in the ground is 8 to 12 inches at Clemson, 12 inches at Maryland and Penn State, and 12 to 14 inches at UF/IFAS.
• Penn State plants slips 6 inches deep with at least two nodes underground. UF/IFAS also says two nodes under the surface, and Clemson says 3 to 4 inches deep. UGA’s master gardener guide digs a hole 4 to 6 inches deep.

So the pot needs depth for the roots, with the slip planted at least 3 inches down, and about a foot of width for every plant. Fewer plants in a bigger pot is the safer bet than several in a small one; the plants per container guide has the general logic. Whatever the material, make sure the water leaves. Penn State says sweet potatoes in compacted and waterlogged soil will rot, so check that the holes are open. Pot material changes how fast the mix dries and how hot the root zone gets; the trade-off is laid out in fabric pots versus plastic. A pot this size is heavy when wet, so run the numbers in the balcony weight limit guide before you carry it up.

Use a loose, well-drained potting mix. Penn State says to add 1 to 2 inches of compost to loosen the soil, and University of Maryland Extension says to work in compost before planting. Our DIY potting mix for container vegetables covers a mix that drains without drying out in an hour.

On planting day:

1. Wait until the soil is 65°F. Maryland, Clemson and Penn State give the same number.
2. Harden slips off first. Penn State says to do this when slips reach about 10 inches, putting the box outside a little longer each day for a week or two, starting in shade.
3. Plant in the evening or on a cloudy day to reduce stress (Penn State).
4. Penn State says slips do not need roots to be planted. Maryland says they are rootless when pulled from the original root, so keep them well watered.
5. Water daily for the first two weeks (Penn State). UGA says daily in the first week, every other day in the second.

Heat, even watering and a light hand with nitrogen

University of Maryland Extension says full sun means at least 6 hours of direct light and sweet potatoes prefer 8 to 10. Penn State says they thrive in hot, dry weather. A shaded balcony will not give them that.

Watering is where pots differ from beds. Maryland says sweet potato is sensitive to drought for the first 50 to 60 days after planting and should be watered deeply and regularly without keeping the soil saturated, since excess water can cause root rot and splitting of the storage roots. Clemson adds that rain or irrigation after a long dry period may crack the roots, and UF/IFAS says roots can split in excessively dry conditions. So the aim is an even rhythm. The weekly amounts in those guides are for open ground; a pot dries far faster. Use the finger test in the container vegetable watering guide, and on very hot days follow the heatwave guide for container plants.

Feed lightly. Penn State says soil too rich in nitrogen produces many leaves but few potatoes, and UF/IFAS says too much fertilizer can prevent roots from forming. Maryland rates sweet potato a medium feeder, with compost worked in before planting. For timing and products, see the container vegetable fertilizer guide, and keep to the lighter end of it.

When to dig

University of Maryland Extension says sweet potatoes mature 85 to 120 days after planting and to check root size after 80 to 85 days, because they keep growing and can split when overgrown. Penn State and Clemson say 90 to 120 days. UF/IFAS, writing for Florida, says at least 120 days and up to 150. Clemson digs when 30 percent of the roots are larger than 3.5 inches across, a rule written for rows, so in a pot, judge by a gentle probe at the edge.

Harvest before frost. Penn State says to dig when the leaves begin to turn yellow, ahead of a light frost; if frost burns the leaves, do not wait, and if harvest has to wait, cut the vines so decay does not reach the roots. Maryland adds that soil temperatures below 50°F can cause chilling injury. Clemson says cool soil reduces quality and storage capacity.

To lift a pot’s worth:

• Cut the vines off first (Maryland and Clemson both say this makes digging easier). The leaves and young shoots are edible, according to Maryland and Penn State, so there is no need to bin them.
• Tip the pot over onto a tarp, or loosen the mix with a fork around the edge. Penn State warns that the roots bruise easily and keep less well if broken.
• Do not rub the skin or wash the roots before storage (Maryland).
• Let them dry for a couple of hours (Penn State).

Curing and storing

Curing heals wounds and turns some starch to sugar, and the extension sources disagree on the setup.

Source	Curing conditions	Time
Penn State Extension	80 to 85°F, 85-90% humidity, roots in loosely closed plastic bags with slits	2 weeks
Clemson	85°F and 90 percent humidity; or the warmest room in the house	1 week; 14 days in a house
University of Maryland Extension	85°F and 80 to 90 percent humidity in commercial rooms; at home, a covered porch or garage	About a week
UF/IFAS Extension	Roughly 95°F and 90 percent humidity	About 10 days

Clemson adds that no curing happens below 70°F. Maryland says the roots usually store successfully even without the week of curing, so a cool kitchen is not a reason to skip the harvest.

After curing, keep the roots cool but not cold. Penn State says 50 to 60°F in the dark, and Maryland says 55 to 60°F at 85 to 90 percent humidity. Clemson says to keep them out of temperatures below 50°F and out of the refrigerator, and that well-kept roots last more than six months. UF/IFAS says below 50°F gives an off flavor, or rot.

Problems in pots

Penn State Extension says voles, mice and deer can attack sweet potatoes, and that raised beds or large pots keep rodents out. Scurf, a soil-borne fungal disease that marks the skin, leaves roots edible but can spread in storage (Penn State); Penn State suggests a four-year rotation in the garden and a soil pH of 5.8 to 6. Clemson suggests a two-year rotation between sweet potato crops. In a pot, the easier fix is fresh mix each season, and the crop rotation guide for containers explains what counts as enough.

Sources

• University of Maryland Extension, Growing Sweet Potatoes in a Home Garden
• Penn State Extension, Sweet Potatoes, a Winning Vine for Your Garden
• Clemson Home & Garden Information Center, Sweetpotato (HGIC 1322)
• UF/IFAS Gardening Solutions, Sweet Potatoes
• UGA Extension (Glynn and McIntosh), Growing Guide: Sweet Potatoes