How to Grow Sweet Potatoes aka Kūmara

March 10, 2022 by Karen 211 Comments

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A step by step guide on how to grow sweet potatoes & how to start slips. Growing sweet potatoes used to be a closely guarded secret among farmers. A mystical, mysterious process - like how to perfectly apply liquid eyeliner. Not anymore!

A pile of dirt covered, freshly dug sweet potatoes from a home garden. — Newly dug Beauregard sweet poatoes.

I've successfully taught thousands of you how to grow luffa in a cold climate (zone 6) and sweet potatoes (known as Kūmara where it's hugely popular in New Zealand) are no different.

It can be done and you can get a HUGE harvest even in a short season - you just need to follow the steps.

In 2010, when I first started growing sweet potatoes (Ipomoea batatas), there was almost no information on the Internet about it. At that time, growing sweet potato slips was a closely guarded secret in the farming community.

Not anymore.

Table of Contents

Two ways to grow sweet potato plants

🌱 It starts with growing the slips. 🌱

In order to grow sweet potatoes, you have to first grow sweet potato slips. These are the sprouts that come off of the sweet potato.

Every slip will grow into a sweet potato plant that will produce around 2 pounds of sweet potatoes

1. IN WATER

2. IN SOIL

Sweet potatoes growing slips in glass of water. — WATER METHOD

Sweet potatoes sprout after they break dormancy just like the perennial plants in your garden do. These sprouts are called slips and they're what you use to grow sweet potato plants.

You can encourage your sweet potato to break dormancy by putting it in a warm room in either a glass of water or some soil.

Water Method

With this method you're generally safe to start your slips 6 weeks prior to when you want to plant them out.

STEP 1. Place sweet potatoes in a glass jar of water with half the sweet potato under water and the rest not. The part under water will grow roots and the part above water will grow slips.

STEP 2. Put the jar somewhere WARM - over 80℉ is ideal. Now you wait about a month for it to root and sprout.

STEP 3. Once the slips are a few inches long they can be pinched off of the sweet potato and rooted in water or planted in 4" pots.

STEP 4. Slips can be planted outside once the soil temperature is 65ºF (or 18ºC).

Soil Method

The soil method produces slips more quickly. With this method you're generally safe to start your slips 4 weeks prior to when you want to plant them out.

STEP 1. Place whole sweet potato(es) lengthwise in a pan of soil so the soil comes halfway up the side of potato.

STEP 2. Place the pan on a seedling heating mat.

STEP 3. Make sure the soil stays moist and wait for it to produce roots / slips in 2 weeks or less.

STEP 4. Once the slips are a few inches long and you can either put them in a glass of water to root, or plant them directly in soil to root. Either way is fine. Rooting in a glass jar takes up a lot less space than putting each slip in a 4" pot with soil.

STEP 5. Slips can be planted outside once the soil temperature is 65ºF (or 18ºC).

TIP! It's the warmth of the heat pad that speeds up the sprouting process.

Sweet potato slips rooting in jar of water.

How to plant

Sweet potatoes will be one of the last things you plant in your garden. They must go in later than peppers, tomatoes and other heat loving plants because sweet potatoes need more than just warm weather. They need warm soil as well as warm air.

STEP 1. Apply a couple of inches of compost to the top of your soil. You can also use a slow release fertilizer; I use Gaia Green's 4-4-4 fertilizer in my garden.

STEP 2. Lay black thermal mulch (plastic) on the planting area 2 weeks before setting out. Sweet potatoes need full sun so make sure your area has that.

STEP 3. On planting day cut a circle in the plastic and push one slip in. Make sure the slip has contact with soil all around. Repeat for all your slips.

STEP 4. Proper spacing for planting is 1 sweet potato slip per square foot. HOWEVER, I find spacing of 16" between sweet potato plants increases your yield & the size of your sweet potatoes.

STEP 5. Keep the plants well watered throughout the summer. Using the plastic eliminates the need to weed and helps retain moisture.

STEP 6. Harvest before the first frost. Once the weather cools down they won’t grow anyway.

* Speed up your soil warming by laying a layer of black thermal plastic in your garden bed. I use biodegradable plastic made of cornstarch that just decomposes on the soil by the end of the season. It will heat the soil up by as much as 10 degrees which means you can plant the slips 1-2 weeks sooner than if you don't use thermal plastic.

Growing in beds

Growing in containers

Sweet potato slips planted through hardware cloth. — Sweet Potato Plants

If you grow sweet potatoes in the ground you may find voles & mice get to them before you do. Hardware cloth can help with this. If you can't find it locally, Amazon carries hardware cloth.

Cover your sweet potato bed with ¼" hardware cloth. Grow your sweet potatoes in a raised bed with wood sides. After laying your plastic down, staple hardware cloth around the edges of your bed.
Plant the slips you have to punch a hole into the plastic with a pencil and push the slip through the hardware cloth, plastic and into the soil. THIS IS A PAIN. But it eliminates 100% of rodent damage.

I grow all of my sweet potato plants in containers.

Plant 1-2 sweet potato slips in a 60 litre pot that measures 60 cm across.
Remember to keep the pots watered as they'll dry out more quickly than a garden bed.

Once your plants are well established you can also harvest and eat the leaves.

Use them in: salads or cook them like you would spinach or chard.

This video shows my sweet potato harvest in 2016 after I tried using the hardware cloth the first time in a raised bed in my 40' x 40' community garden plot. In this video I'm using regular thermal plastic, not the biodegradable plastic.

How & When to Harvest

Near the end of their growing life sweet potato vines will start to yellow and croak. This is a GOOD sign! They're ready to harvest.

Cut the tangle of vines away, leaving only a few stubs to let you know where the plants are.
Using a shovel or digging fork, dig em up! Honestly, the most fun crops to grow are the ones that grow underground because you have NO idea what you have until the day you dig them up.
Be careful when you're digging them and pulling them out. They bruise and break easily.
Once they're all dug let them sit in the sun for a few hours to dry and begin the curing process.

If the vines get touched by frost and start to turn black the sweet potatoes can rot quickly so dig them up right away!

How to Cure & Store

Curing

Sweet potatoes need to be cured for 10 days in an area that is 85ºF with 85% humidity. Getting those conditions at home probably seems difficult but just get as close to those ideal conditions as you can.

Why do you have to cure sweet potatoes? Curing toughens the skin so they keep longer and it develops their distinct sweet flavour. A sweet potato dug straight out of the ground won't taste sweet at all! Try it.

Here's how:

Put your sweet potatoes in a rubber bin with the lid offset so it isn't completely sealed off. Store this near a heat register, wood stove or sunny spot. This will create conditions as close to perfect as you can get in most houses. DO THIS FOR 10 DAYS.
After the initial 10 day curing period move your sweet potatoes to an area that is between 55-60ºF for one month. This develops their flavour. After 1 month they will have developed their sweet potato flavour which will get even stronger as time goes by.

Storing

Store sweet potatoes in an area that doesn't get below 50 degrees in a container that breathes like a slatted wood box or a burlap sack.

How many sweet potatoes do you get per plant?

2 lbs or 4 sweet potatoes per sweet potato plant.

Sweet potato graphic showing how 1 potato can easily produce 30 pounds of sweet potatos.

1 sweet potato plant will produce about 4 large sweet potatoes, or 2 lbs of sweet potatoes. Some varieties will produce 6 or more per plant.

The plant usually creates 1 very large sweet potato, along with a few smaller ones.

A single sprouting sweet potato can provide you with at least 15 slips (that's a low estimate). Those 15 slips will create 15 plants, which will give you around 30 lbs or 60 individual sweet potatoes.

Where to buy slips

If you don't want to grow your own you can buy potted sweet potato plants at many garden centres now and you can order live slips online.

Growing from store bought sweet potatoes

To grow your own slips all you need is a sweet potato that hasn’t been treated to stop sprouting which you can get at the grocery store.

How do you know if it’s been treated? You don’t. You go to the store, buy your sweet potato and hope for the best. Organic is your best bet for an untreated sweet potato, but both organic and "regular" store bought sweet potatoes have produced slips for me.

Tips on picking a sweet potato from the store to grow

Check for cold damage. If the sweet potato has been exposed to below 55 degree temperatures it will probably rot rather than sprout. Cold damage presents with dark marks and lesions.
Bigger isn't necessarily better. Small sweet potatoes, in my experience, have produced more slips than larger ones.
Ask if they were grown locally. Locally grown means it will grow well in your region.

A chill damaged sweet potato showing signs of dark spots and blemishes from being too cold. — Cold damage on sweet potato.

Sweet Potato with cold damage

Once you've established your very OWN crop of sweet potatoes you can use those for producing slips year after year.

Are ornamental sweet potatoes edible?

You may have noticed that your ornamental sweet potatoes also produce tubers. These tubers are edible but not delicious.

The good news is you can propagate ornamental sweet potato vine the same way as regular sweet potatoes! Just dig up the decorative sweet potato tuber in the fall, store it in a cool room, and then encourage it to grow slips in the spring. These slips can be planted directly outside or rooted and potted up for later planting.

Varieties of Sweet Potatoes

The most popular sweet potato variety by far is Beauregard and it'll be the easiest for you to find. But there are a lot more varieties than that.

Beauregard* (best all around sweet potato variety)
Georgia Jet (short season variety)
Jewel (longer season but still doable in colder climates)
Garnet (a purple variety with purple skin and flesh)
Stokes (bright purple variety that retains its colour after cooking)
Covington (a standard variety that grows well in cooler cliimates)

*this is the sweet potato I most often grow.

Note: I have successfully grown all of the above (with the exception of "Stokes") in my Canadian garden. I just haven't tried Stokes, but I'm sure it would be fine.

Sweet Potato VS Regular Potato

Seed potatoes lined up in an egg carton for sprouting. — "Chitting" regular potatoes

To clear up any confusion, sweet potatoes don't grow like regular potatoes. A regular potato is a tuber, a sweet potato is a root.

Regular potatoes are grown by planting whole "seed" potatoes into the ground. (here's my guide on how to grow regular potatoes)

Sweet potatoes are grown by planting only the sprouts aka slips that grow from the sweet potato.

The Start to Finish Guide to Growing Sweet Potatoes.

Yield: A whack of sweet potatoes

Additional Time: 5 months 2 days 22 hours

Total Time: 5 months 2 days 22 hours

Difficulty: Easy(ish)

How to successfully grow sweet potatoes whether your garden is big or small.

Materials

Glass of water
Foil pan with soil
A sweet potato

Tools

Heat mat

Instructions

Start sweet potato slips 6 weeks prior to planting out.
Rest a whole, undamaged sweet potato in soil and set on a heating mat. Slips will start to grow in around 2 weeks. When around 5", break slips off of sweet potato and plant out or root in water.
Rooted AND unrooted slips can be planted directly in the soil.
Speed up how quickly you can plant your slips outside by laying down thermal plastic
To prevent vole/mole/mouse damage either grow sweet potatoes in very large pots or grow in a raised bed with wood sides and ¼" hardware cloth across the top.
Dig up sweet potatoes when the weather cools in fall.
Cure sweet potatoes at 85F and 85% humidity for 10 days.
Cure another month at 55-60F allowing potatoes to develop sugars.
Store long term in vented crates or burlap bags at no colder than 50F

How long does it take for sweet potatoes to grow?

Sweet potatoes can be harvested 4 to 5 months after planting.

How many sweet potatoes do you get from one plant?

You get around 4 sweet potatoes per plant. Usually one very large one and a few smaller but still substantial ones. Some varieties under the best conditions will produce even more.

What soil should I use for growing sweet potatoes in pots?

Any potting soil will work well. It has the nutrients you need. If you are reusing potting soil you'll need to amend by adding fertilizer. Adding a 4-4-4 fertilizer or a few inches of compost to the top of the depleted potting soil will revive the soil. I also use native garden soil in my sweet potato containers.

Can I grow sweet potatoes from a sweet potato?

Yes, that's exactly how you grow them but you don't plant the entire sweet potato. You let the sweet potato sprout in a warm place, pull the sprouts off when they're a few inches long and then root or plant those in soil.

What month is best to plant sweet potatoes?

May or June are the best months to plant sweet potatoes outside when the soil at planting depth has warmed up to 65ºF (or 18ºC).

Can sweet potatoes be planted in a raised bed, bucket or pot?

Yes! Sweet potatoes do really well in beds, buckets or pots. Buckets and pots are especially good for growing sweet potatoes because they keep the soil warm and prevent moles and mice from eating the growing tubers.

Once you have a whack of sweet potatoes that you've grown yourself, if stored in good conditions, they'll last you into April or even May.

You can turn them into my personal favourite guaranteed crispy Sweet Potato fries with a Sriracha/mayo dip, Sweet Potato soup or sweet potato casserole.

Now go forth and grow.

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