Sweet potatoes, Ipomoea batatas, are native to the Americas and unrelated to potatoes, Solanum tuberosum, which are native to South America. Sweet potatoes are the sweet-tasting root of a plant in the morning glory family, while ordinary potatoes are a tuber belonging to the nightshade family.
Even though sweet potatoes are sometimes called yams in grocery stores, it isn’t a yam either. Yams are Dioscorea, a tuber that is native to Africa and Asia.
That’s interesting, but how do you know when your sweet potatoes are ready to harvest? A good rule of thumb is to harvest sweet potatoes when the leaves have turned yellow. In cooler climates, this will be before or after the first frost.
How Do You Know When Your Sweet Potato Is Ready to Harvest?
Sweet potatoes roots usually have creamy white flesh with light or purple skin or orange flesh and coppery-orange skin. It is surprisingly easy to grow sweet potatoes, especially in tropical regions.
They are sensitive to changes in soil temperature and grow best when it is warm. They are normally planted in spring and harvested in summer, 3-4 months (12-16 weeks or up to 120 days) after planting.
If grown in dry conditions, or during a dry season, they may only be ready to harvest 20-22 weeks after planting.
Root maturity can differ between varieties, and when the weather is cool it can slow down root development.
Commercially grown sweet potatoes range in weight from about half to 2 lbs (¼-1 kg). Leaving them in the ground for too long results in them becoming oversized in commercial terms.
You might be proud of huge sweet potatoes, but they have a reputation for getting pithy the bigger they get. They also become more cumbersome to prepare and cook.
Signs to Look Out For
Sweet potato plants thrive in warm weather and will continue to grow until frost kills the vines. But you don’t have to wait until this happens – and in warm climates, it won’t!
Whatever your climate, yellowing vines are a sure sign that your sweet potatoes are ready to harvest.
The Best Time to Harvest Sweet Potatoes
It mustn’t be too hot or too cold when you harvest your sweet potatoes. And it’s best to harvest them when the soil is dry.
I’m going to talk about the dangers of chilling injury in the next section. This can damage newly harvested sweet potatoes in cold weather.
Linked to the risk of chilling injury, Integrated Pest Management (IPM) at the University of Missouri maintains that sweet potatoes are normally harvested after the first frost of the fall season. But they recommended the roots be dug out as soon as possible after the first frost.
If it’s too hot, your newly harvested root vegetables run the risk of sunscald. This reduces the quality of sweet potatoes and can happen after only 30 minutes in the sun.
If you have to harvest your crop on a sunny day, move the roots to a shady spot as soon as you can. Don’t leave them lying in the sun.
What Happens If You Don’t Harvest Sweet Potatoes?
It is very easy to grow sweet potatoes, but they don’t like cold temperatures. While the roots of sweet potatoes won’t be damaged by the first fall frost, as the soil temperature continues to fall, they won’t survive and will start to rot.
What Happens in Cold Climates
Even if you get your sweet potatoes out of the ground intact and unharmed, if the temperature is below 50°F (10°C), there will be a strong risk of chilling injury. They can be damaged if the temperature drops to 45°F (7°C) and can freeze at 30°F (-1°C).
According to Postharvest Handling of Sweet Potatoes, a book published by the North Carolina Cooperative Extension Service, chilling injury can be difficult to diagnose.
The most common symptoms include:
- Surface pitting
- Loss of dry matter
- Susceptibility to decay
- Internal voids
- Failure to sprout
- Darkening of the flesh
- Reduced color, texture, taste, and smell
The authors recommend harvesting sweet potatoes as soon as possible after frost has killed the vines to avoid chilling injury. And once you have harvested them, don’t leave them exposed outdoors overnight.
What Happens in Warm Climates
If you live in a warm climate that doesn’t experience frost (Zone 8 and above), it’s a different story.
Sweet potatoes are perennial in warm climates and will often grow back from roots that are left in the ground through winter. So, in this environment, it isn’t critical to harvest all your sweet potatoes.
How to Harvest Sweet Potatoes?
Sweet potatoes have very thin, almost delicate skin that is easily damaged during the harvesting process. The roots also bruise very easily.
For this reason, it is important to be careful how you harvest your crop.
How to Dig Up Sweet Potatoes
Home gardeners use a spading fork to dig up the mature roots by hand. Use the fork to loosen the soil around the roots so that you can pull up the primary crown of the sweet potato plants.
Wear gloves when you dig them up.
You want to remove the soil on the roots once you have dug them up. Do this by shaking them gently. Don’t wash the dirt off.
If the temperature is warm (around 60 degrees) then leave the newly harvested roots outside in the sun for a few hours.
If any of the sweet potatoes roots are badly damaged or cut, separate them from the rest of the crop. They won’t store well, though you can still eat them.
If there is only minor damage, the curing process (see Curing Sweet Potatoes, below) will enable them to heal. If you don’t cure the sweet potatoes, they won’t be very sweet, but they make a good addition to casseroles and stews.
Get New Plants From Your Harvested Sweet Potatoes
Whatever the size of your crop, you might like to try producing slips for planting in spring. It’s more work than buying them for a garden center or farmers’ market, but it can be rewarding and a lot of fun.
About 3 months before the last spring frost, remove a few of the harvested roots from your store. Place them together in a seedbed on top of about 3 inches (7.6 cm) of light, well-draining soil.
Cover with a few inches more soil and leave them to sprout. It’s an easy, although not a prolific, way of propagating new sweet potato plants.
Water periodically to keep the roots damp – not wet. Each sweet potato will produce 3-6 slips in about six weeks.
Snip or twist the stems off the swollen root with any new roots that have developed. If they haven’t got new roots, put the slips in a jar of water to develop roots.
Then, in spring, you can plant the slips in your home veggie garden.
Should You Wash Sweet Potatoes after Harvesting?
As mentioned above, don’t wash the dirt off your sweet potatoes after harvesting. Shake as much soil off the roots as you can and wipe with a dry cloth if the sand bothers you.
Washing the roots before you store sweet potatoes can result in microbes developing, which can result in the roots rotting.
But of course, if you’re going to cook them right away, without going through the curing process, then you must wash them. The same applies to any roots that you have cured as well as those in storage.
Even if they aren’t sandy, it’s essential to wash them.
Can You Eat Sweet Potatoes Immediately after Harvesting?
You can cook and eat newly harvested sweet potatoes, but to maximize their flavor, they need a rest period after harvesting and before you eat them. During this time, you need to let them cure in a warm, humid environment for about 10 days.
Curing sweet potatoes in cool temperatures can take a lot longer – up to 4-6 weeks.
Curing Sweet Potatoes
Curing sweet potatoes converts some of the starch in the sweet potatoes roots into sugar. It also allows minor wounds to heal and improves their storage quality.
The curing process is simple, but temperature and humidity levels are vitally important. If you don’t allow your sweet potatoes to cure in a sufficiently warm (up to 85 degrees F), humid environment, they will dry out and they won’t taste the way they should.
Purdue University’s extension service for home gardeners recommends curing sweet potatoes in temperatures between 80 and 85°F (26.6-29.4°C). Aim for 85-90% relative humidity.
Cover the harvested roots in your home cure store with paper or heavy cloth to maintain ideal humid conditions. Don’t let them get too hot or cold, and protect them from light.
Storing Sweet Potatoes
Unlike most root vegetables, you do not store sweet potatoes in a root cellar. Rather, store them in perforated plastic bags in temperatures between 55 and 65°F (12-18°C) – 60 degrees is perfect.
You can store good quality, unblemished sweet potatoes for 6-10 months. The perforated plastic allows for adequate gas exchange and lets the air circulate.
Do not store them at temperatures below 50°F (10°C).
It’s easy to identify when sweet potatoes are ready to harvest.
You can be guided by the length of time the variety you have planted takes to mature. You can decide based on when you expect the first fall frost.
Or you can simply wait for the vine leaves to turn yellow and start to die.
On average they take about four months to be ready to harvest, you can store sweet potatoes for up to 10 months after the curing process is complete. This means you’ll still have roots ready to cook when your next batch of sweet potato plants is growing.
You’ll have a constant supply!
So, if you like sweet potatoes, why not try growing them at home? It’ll undoubtedly be worth the effort.