Harvest potatoes young or mature. Small, round, immature potatoes are often called “new potatoes.” New potatoes are usually eaten skin and all. Mature, full-size potatoes are called main crop potatoes. Main crop potatoes are often stored for winter use.
Harvest new potatoes when plants begin to flower and for another 2 to 3 weeks, starting about 60 to 70 days after planting. Large mature potatoes are ready for harvest about 15 weeks after planting.
Some potato varieties bloom late or do not bloom. If you do not see flowers 65 to 75 days after planting, check near the base of the plant for developing tubers. Potato tubers form on underground stems called stolons. Potato stolons are typically 12 to 18 inches long; so developing tubers will be within that circumference of the plant.
New Potatoes: To harvest new potatoes gently lift the plant with your hands or a garden trowel or hand multi-pronged garden fork. As you lift the plant, the surrounding soil and mulch will fall away. Take as many new potatoes as you need then set the plant back in place and firm the soil so that the plant and remaining tubers can grow on. (Do not leave the plant out of the soil for long; sun can damage exposed roots and tubers.) As an alternative, you can lift a whole plant for harvest and leave the neighboring plants undisturbed. The potatoes you leave in the ground will grow to become your main crop—your mature tuber crop, and the plant will produce new potatoes as well.
If the plant is growing in hills of very loose soil, mulch or straw, you can simply ease your hand into the tuber zone and remove new potatoes.
Main Crop Potatoes: Mature or main crop potatoes will be ready for harvest 2 to 3 weeks after plants turn yellow and die back—about 100 to 110 days after planting. Vines will either die back naturally or, to spur harvest, you can break off the stems at ground level to stop growth. Between the time the plant dies back and harvest, do not water potato plants. A dry period will allow skins to “set” or harden which is important for long storage.
Harvest mature potatoes using a spading fork. Work from the edge of the planting row or bed inwards. Insert your fork 10 to 18 inches away from the plant stem. Loosen and turn the soil carefully so the potatoes you lift are not damaged. Most of the crop will be on the same level in the top 4 to 6 inches of the soil. If you know how deep the tubers are growing, you can use a garden spade to lift the entire hill.
If the potato plant does not die back as the tubers mature, cut the plant off at soil level two weeks before you want to harvest the tubers.
It is easiest to dig potatoes when the soil is dry. If the weather has been rainy, wait a few days until the soil dries to begin your harvest.
It’s best to harvest potatoes on a warm, dry day after a few days of no rain or a cloudy day will do. Let the potatoes sit in the garden for an hour or so to dry. As the tubers and soil dry, soil will drop away from the tubers. If the soil does not drop away, use a soft brush to remove soil from the tubers. Do not wash just harvested potatoes; washing potatoes will shorten their storage life.
Drying and Curing Just Harvested Potatoes: Newly harvested potatoes do not have a tough skin so handle them carefully to avoid bruising which can lead to rot. Set the tubers on a screen or lattice where they can dry for an hour. If you leave them longer, set them in a dark, dry place where it is a bit humid.
New potatoes will be most flavorful if eaten almost immediately after digging.
Main crop otatoes that you want to store should be allowed to “cure” for one to two weeks after harvest. Curing will allow cuts, nicks, and bruises to heal.
Potatoes with deep cuts or bruises are best used right away and not stored.
Storing Potatoes: Store main crop potatoes in a dark, dry place for a week or two at 55° to 65° F with high humidity of 85 to 85 percent.
After two weeks, potatoes that you want to store longer for winter use should be moved to a much cooler– 35° to 40°F—dark room, basement, or root cellar with moderate humidity and ventilation. For long storing—as long as eight months, choose potatoes that are firm with no soft spots. Temperatures higher than 40°F will cause tubers to sprout and shrivel. Check stored potatoes often; if sprouts begin to form, knock the sprouts off with your hands.
Do not refrigerate potatoes; the air in a refrigerator is too dry for potatoes and can cause them to shrivel. Do not store potatoes with apples; picked apples expel ethylene gas which will cause potatoes to spoil.
Storage Problems: Some potatoes can become “sweet” when stored. Potatoes in storage may convert starch to sugar which is used in the tuber “breathing” process. The breathing process of potatoes stored in a cool place slows so that the starch converted to sugar is not used in full; the unused sugar will give the potatoes a sweet taste when cooked. To avoid a sweet taste, take the potatoes out of storage several days in advance of cooking so that the extra sugar can revert to starch—a process called “reconditioning.”
The skins of potatoes exposed to light can turn green. Greening is caused by a toxic alkaloid called solanine. Green potatoes taste bitter. Do not eat green potatoes; solanine can cause illness. If a potato skin is green, peel or cut away the green before cooking.
Be advised that the leaves of potato plants are poisonous to humans and animals.