1. TIMING IS EVERYTHING.
As soon as they reach your desired size, sweet potatoes tubers can be harvested for fresh use. (I often pick large ones that start poking out of the soil well before the first frost.) But the longer you let your potatoes stay in the ground, the sweeter they’ll tend to get.
Generally you want to harvest your main crop when the tubers are 6”-8” long (as larger ones may get woody) or immediately after the first hard frost—whichever comes first—as frosted plants soon start to rot the tubers. If frost catches your plants and the leaves are turning black in the morning, clip the vines off at ground level as soon as possible. This will prevent rot spreading from the dying vines into the tubers. You can then safely leave the tubers in the ground for a few days before digging them (as long as the ground isn’t actually freezing).
2. HARVEST YOUR SWEETS WITH CARE.
Dig your sweet potatoes with a garden fork, starting 18” away from the central stem and working in, and gently lift the tubers out of the loosened soil. While most tubers form right below the center of the plant, some varieties form individual tubers further away or quite deep, and you may even find the occasional tuber where a vine rested on the soil and sent down some roots.
Shake off any excess soil but don’t wash your sweet potatoes yet, since doing so encourages rot. You can save that for when you’re ready to cook them up. Spread your harvested sweets out, and allow them to dry away from the sun and somewhere that is at least 45° F for a couple of hours. Handle the potatoes very gently, setting them down rather than tossing them, as they bruise easily. Once the surface of the tubers is dry, sort out any very small or badly damaged ones to eat first; they may not be all that sweet—more like Irish potatoes—so you may want to use them in recipes calling for those. Gently place the rest into cardboard boxes or baskets for curing and storage.
3. IF YOUR HARVEST IS SMALL, SUPPLEMENT IT.
My local farmers’ markets are rich in sweet potatoes in the fall, but they tend to sell out quickly. On years when I don’t grow enough sweet potatoes of my own, I like to stock up (curing them if necessary). When shopping for sweets, look for firm tubers with smooth skins that feel heavy for their size. Avoid any with sunken areas or ones that feel less than totally firm. If you’re buying from a farmer, ask if the sweets have been cured or not (a blank stare means “no”).
4. YOU CAN MAKE A CURING CHAMBER AT HOME.
These tropical tubers like it HOT to start. Ideal curing conditions are dim, hot (80°F-85°F), and humid. Curing allows the tubers to callus over where the stem or roots were snapped off and heal any other damage. It also triggers the development of the sugar-creating enzymes that will allow your sweets to get sweeter in storage, and may help to prevent premature sprouting.
You can cure a modest-sized harvest in your oven—no special equipment required! Place an open pan of water on the floor of the oven, put a light with an incandescent 40 watt bulb on another shelf as a heat source, and set a thermometer on the middle shelf where you will be able to read it through the window. Then, close the door almost all the way (I put a wooden ruler flat in the crack), and turn on the light. Check the temperature after an hour or so. Change to a thinner or thicker spacer if the temperature is below 80°F or above 85°F. I’ve also used a large picnic cooler and a heating pad to create a curing chamber. Once the temperature of your chamber remains in the magic zone, gently place the sweet potatoes on the shelves and close the door on the proper spacer. Check the temperature twice a day, making adjustments if necessary, and cure your sweets for seven days.
5. STORED RIGHT, YOU CAN ENJOY YOUR SWEET POTATOES FOR MONTHS AND MONTHS.
Sweet potatoes will stay sound and good for up to a year if you cure and store them under ideal conditions. Storing for at least 6-8 weeks before cooking makes them sweeter and the flesh more tender. This isn’t to say you have to wait that long to sample your sweets, but if you can stand to wait you’ll be glad you did. Store sweets in a cool (55°F-60°F) well-ventilated area with a relative humidity of 75-80 percent. Temperatures below 55°F can cause the flesh to darken and change texture, so don’t put sweets in a cold place or—perish the thought—the fridge!