University of Vermont Extension 
Department of Plant and Soil Science

Fall News Article
line
HARVESTING SWEET POTATOES
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
           
Sweet potatoes are becoming increasingly popular as they’re tasty and provide much nutrition.  One cup of sweet potatoes provides beta-carotene, over 4 times the daily need for vitamin A, and other nutrients. One sweet potato plant can yield on average 2-1/2 pounds of potatoes, or 100 pounds or more from a 20-foot row.  If you tried growing your own, as I did this year in gro-bags (large 15-gallon fabric bags, filled with a topsoil and compost mix, good for patios and decks), you may be wondering when to harvest and how to store them properly.
           
Sweet potatoes will continue to grow, as long as soil temperatures on average remain above 65 degrees (F), or tops are killed by frost.  You can check soil temperatures with an expensive thermometer for soils or compost, available at many complete garden supply stores or online (good to have around the garden anyway). You can dig around and check roots for proper size, too.  Even if leaves begin to yellow, leaving roots in the ground helps to improve the yield and nutrition. 
           
Regardless of timing and size, if tops have been killed by frost, you should harvest roots at once or they’ll quickly rot from disease entering from the dead tops.  If you can’t get the roots dug right after a frost, cut the killed tops off just above the soil line, and you can then leave the roots in the ground for a few days. 
           
Take care when harvesting, as the sweet potato roots can be damaged very easily.  They don’t have a protective hard layer as do the tubers of white potatoes.  Wounds on sweet potato roots aren’t good, as they often lead to rotting in storage. 
           
You can use a spade to dig roots, but many prefer a spading fork as there is less chance to damage the roots.  Loosen the soil, and dig around with your hands (I like to use gloves) for the roots.  Since roots can grow a foot away from the plants, allow for this when digging.  It’s easier to harvest in dry soil than wet.  You can gently wipe or brush excess soil off, but don’t wash the roots until you’re ready to use them.  Don’t leave roots in direct sun more than a few hours.
           
For best storage, and to protect against any rots from abrasions and wounds, roots should be “cured.”  To do this, keep roots dark, warm (80 to 85 degrees) and humid (85 to 95 percent relative humidity) for 4 to 7 days. 
           
You should resist the temptation to dig and immediately eat sweet potatoes, as fresh ones are more starchy than sweet, and don’t bake as well as cured ones.  Wait at least three weeks before eating, so the starches can convert to sugars. 
           
Sweet potatoes can last six months or more in storage, if held properly.  Ideal conditions are the same as for winter squash—just slightly cool (55 to 60 degrees) and with a moderate relative humidity (60 to 75 percent).  Much below 55 degrees and sweet potatoes (similar to winter squash) will suffer chilling injury. Such injury appears as sunken, dark areas on the surface and blackening on the insides. 
           
If roots have holes, as if damaged from feeding, there could be a couple causes.  Voles love sweet potatoes, as do larvae of wireworms in the soil.  Adults lay eggs in the spring, which lead to the larvae.  Since they can live for several years in the soil, waiting for the right crop to come along and before feeding, crop rotation to non-susceptible crops (not grasses, corn, potatoes of whatever kind, or other root crops) helps to control wireworms and other problems.


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