Timing Your Sweet Potato Harvest
Usually sweet potatoes are harvested in the week that the first frost typically occurs in your region, so those in the colder half of the country will have harvested and those in warmer regions, maybe not. For those who haven't, here are some pointers about harvest: Aim to harvest on a mild day, when the air is above 50 degrees Fahrenheit (10 degrees C), to avoid chilling injury.
In drought, irrigate the field before harvest, to avoid scratching the skin with chunks of dry soil. If the days (and the soil) are warm, a couple of light frosts will not harm your crop. Despite myths, there is no toxin in frozen leaves that goes down into the roots. Sweet potato leaves are completely edible. If frost does strike, waste no time — get them harvested within a few days.
The danger is not from frost itself, but from cold temperatures. Don’t wait till soil temperatures get below 55 degrees. A permanent chilling injury (hard core) can happen to sweet potatoes. The potatoes remain hard no matter how long you cook them, and are useless. Cold injury can ruin the crop — if roots without leaf cover are exposed to cold air temperatures, they lose their method of pulling water up out of the soil, and get chilling injury. Additionally, cold wet soil can quickly rot sweet potatoes.
How to Harvest Sweet Potatoes
Before you start your harvest, if your winters are relatively short, you could consider propagating sweet potatoes for next year by taking vine cuttings in the fall, rooting them in water, then potting them up as house plants for the winter, to provide cuttings for early slips next spring. But perhaps, like me, you don’t “do” houseplants, because you appreciate an indoor space where you don’t need to think about keeping plants alive!
Remove the vines from the plants to be harvested that day. If there is more than one day’s digging, leave intact vines to protect the rest of the crop. Clip the vines, leaving stumps to show where to dig. Roll the vines into the gaps between the rows — if you have close rows you may need to roll the vines further away. Digging forks can be useful tools for this job, rakes don't really "grab" enough.
Mowing isn’t recommended, as the roots sometimes stick up out of the ground. Potato digging machines usually do too much damage to sweet potatoes. We dig ours by hand, and it is a much-enjoyed task.
Using digging forks, carefully dig up the tuberous roots, which grow in the ground in a bunch-of-bananas shape. Begin digging 12-18” from the center of the plant to avoid damaging them. Go straight down about 6 inches, then angle toward the center and gently lift the potatoes out of the ground by hand. It’s important not to drop, throw or in any other way bruise the roots. Avoid any abrasion of the skin, which is very fragile at this stage.
Set the potatoes out beside the spot they’ve grown, one clump per plant, so it’s easy to identify the most productive plants, for seed stock.
Field-Sorting and Crating
Let the tubers dry in the sun for up to an hour, unless the weather is unsuitable. Don’t leave roots exposed to temperatures higher than 90 degrees for more than ½ hour or they get sun-scald. And below 55 degrees, they’ll get chilled.
If you want to grow your own slips next year select seed potatoes first, while grading and crating the roots in the field. After gathering enough seed tubers, sort the storable from cull or “Use First” roots.
Large, open, broken surfaces will cure and can be stored, but any roots with soft wet damaged areas or deep holes (whether from bugs or fork tines) will not store and should be graded out, for composting or immediate use at home. We sort into wood flats for curing and buckets for the “Use First” category.
Selecting 'Mother Root' Seed Sweet Potatoes
Save about one sweet potato tuber (root) per 10 slips wanted. We save 60 roots for 600 plants. Choose plants with a high yield and no string (rat-tail) roots. From these plants, choose medium sized (1½” diameter) sweet potatoes with good shape and color.
Do not save for seed any roots with disease symptoms. Damage due to poor growing conditions can look like a disease, but as it isn’t, it will not carry over to the next crop. See the commercial growing page of the North Carolina Sweet Potato Commission for lots of good information, including photos of problems.
Don’t save jumbo sweet potatoes for seed, they’re harder to deal with, and will not produce more or better crops. Each sweet potato produces about the same number of slips (shoots) regardless of size. If you want to be sure to avoid saving roots with color breaks (also known as viral streaking or chimeras) you can cut a small slice from the distal end (the end distant from the plant) for examination. All the sprouts will grow from the stem end, so don’t cut there! The cut surface will heal over during curing. Discard any roots with streaks or dots bigger than a pencil lead. This test could instead be done in the spring, just before the conditioning stage, if you'd rather just keep 10% extra and leave them be for the winter.
The best time to select seed roots is at harvest, as you can then choose roots from the highest-yielding plants. If you didn’t do that, retrieve some from your stored potatoes before too many get eaten, following the selection guidelines in the previous section.
If you didn't grow sweet potatoes this year, but want to start your own slips and grow them next year, buy roots from a local grower now. If you buy locally grown you can get a variety that does well in your area, while supporting the local economy. If you are in a cold area with a short summer, choose a variety with a short number of days to maturity. Sometimes natural food stores sell named varieties.
Failing all of the above options, you could buy slips next year, and save your own roots for the following year’s slips. I wrote a blog post on Growing Sweet Potatoes.
Storing Sweet Potato Seed Roots During Winter
Ideal storage conditions for sweet potatoes are 55-60 degrees at 85-90 percent humidity with one air change each day. Above 60 degrees F, shrinking and sprouting may occur, and below 55°F, as mentioned, you get permanent chilling injury. Do not ever let the temperature drop below 50 degrees.
Sweet potatoes do not need to be in the dark. Dormancy is generally broken by moisture and warmth, not daylight. Green sprouts are not toxic, as are those of white potatoes. (See my post Green Potato Myths and 10 Steps to Safe Potato Eating).
Pam Dawling manages the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at MOTHER EARTH NEWS Fairs. Pam also writes forGrowing for Market magazine. Her book, Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, is available in the MOTHER EARTH NEWS Store and at Sustainable Market Farming. Pam's blog is on her website and also on Facebook, and you can read all of her MOTHER EARTH NEWS posts here.
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