Potato tubers form best in cool soils.

Potatoes are a cool-season vegetable that ranks with wheat and rice as one of the most important staple crops in the human diet around the world. The white potato is referred to as the "Irish potato" because it is associated with the potato famine in Ireland in the 19th century.

Potatoes are not roots but specialized underground storage stems called tubers. Maximal tuber formation occurs at soil temperatures between 60° and 70°F.

The tubers fail to form when the soil temperature reaches 80°F. Potatoes withstand light frosts in the spring and can be grown throughout most of the country in the cooler part of the growing season, but they prefer the northern tier of states for maximal yield and quality.

Recommended Varieties

There are more than 100 varieties of potatoes. White-skinned (actually very light brown) and red-skinned varieties with white flesh are the most common in home gardens.

Some russets and yellow-fleshed types are also grown.

• Russet Burbank is the most important commercial variety produced in the United States, but the weather over most of the country is too warm and the moisture fluctuation is too great for the production of smooth tubers and good yields. Common garden varieties offer better taste, texture, and cooking quality for home use anyway.

If possible, use northern-grown seed potatoes that are certified disease free.

• Green Mountain is an old semi-rough white variety noted for its great taste. Due to a fairly high number of misshapen tubers, it has all but disappeared from commercial production. For dependable production in all seasons and the greatest-tasting baked potato ever, Green Mountain is worth the effort to find certified seed.
   
• Yukon Gold is the most famous of the yellow-fleshed varieties now available. Long popular in Europe, these have good flavor and moist flesh, which many people claim requires less of the fattening condiments required by dry-as-dust Russet Burbanks. Yukon Gold is a very early bearer of large, round, attractive tubers with a hint of pink around the eyes. Many grocery stores around the country now feature some name-brand version of "golden" potatoes, usually this variety. If the flavor of these market potatoes suits you, look for seed of Yukon Gold.

Growing Potatoes

When to Plant

Potatoes are among the earliest vegetables planted in the garden.

• Early, midseason, and late varieties all may be planted in March or early April.
• Planting too early in damp, cold soils makes it more likely that seed pieces rot before they can grow.
• Potatoes planted in March also may be frozen back to the ground by late frosts. Plants usually recover fully, but the blackened shoots are always demoralizing to the gardener.
• Medium-early plantings, when soils have dried and warmed, may do as well as extremely early, winter-defying plantings.
• Midseason and late varieties may be planted as late as the first of July. Late potatoes are best for winter storage.

Spacing & Depth

Potatoes are started from "seed pieces" rather than from true seed. These seed pieces may be small whole potatoes or potatoes that are cut into 1-1/2 to 2-ounce pieces.

• Plant the pieces soon after cutting.
• Be sure that there is at least one good "eye" in each seed piece.
• Some garden centers and seed suppliers sell "potato eyes" that weighs less than an ounce. These may be too small for optimal production.

Small, whole, certified seed potatoes are often the best choice for home gardeners.

• Plant seed pieces 10 to 12 inches apart and cover in a furrow between 1 and 3 inches deep.
• Space rows 24 to 36 inches apart. The 24-inch spacing is often beneficial because the plants shade the soil and prevent high soil temperatures that inhibit tuber development.

"Straw Potatoes"

Potatoes grown by a special cultural method in that they are not hilled or cultivated after planting are called "straw potatoes."

• The seed pieces and rows should be spaced the same as for conventional cultivation, but the seed pieces are planted at the soil surface.
• Place loose straw 4 to 6 inches deep over the seed pieces and between the rows.
• Potato sprouts should emerge through the straw cover.
• Cultivation should not be necessary.
• Pull any weeds that manage to emerge through the straw cover and add more straw through the season if decomposition starts to thin the layer.
• Harvest by carefully removing the straw and picking up the tubers that lie on the soil surface.

In addition to weed control, strawing has several other advantages. The straw keeps the soil temperature more uniform and about 10°F cooler, reduces water loss, and results in better-shaped tubers. It is usually more rewarding to straw late varieties than early ones because there is a longer period for tuber development. Many gardeners who grow potatoes for competition in exhibits and fairs use the strawing method because the potatoes are of excellent size, color, shape, and smoothness.

Care

The soil should be fertile and well-drained. Clay soils should be improved with organic matter and plowed deeply in the fall. If space allows, a cover crop such as clover, buckwheat, or winter rye grown in the potato bed the year before potatoes are planted improves soil structure, organic-matter content, and subsequent potato production.

Mulch is usually beneficial in growing potatoes. After the potato plants have emerged, organic mulch can be applied to conserve moisture, help keep down weeds and cool the soil. Some gardeners cover rows of early potatoes with clear plastic film at planting to warm the soil and promote early growth when the soil temperature is low. When the plants emerge, remove the film to allow the plants to grow unrestricted.

After the potatoes break the surface of the ground, gradually build up a low ridge of loose soil by cultivation and hoeing toward the plants. This ridge, which may become 4 to 6 inches high by summer, reduces the number of "sunburned" (greened) tubers. The object of potato cultivation is to eliminate competition from weeds, to loosen and aerate the soil, and to ridge the row. Misshapen potatoes develop in hard, compact soil. Use extreme caution when hoeing near potato plants because developing tubers are easily cut and ruined.

Irrigate to assure uniform moisture while the tubers are developing. A uniform moisture supply also helps to cool the ground and eliminate knobs caused by secondary growth.

Common Problems

Flea Beetles are shiny, usually black, beetles that often are not seen due to their small size (1/16 inch) and ability to jump quickly from plants when disturbed. They attach cabbage, Chinese cabbage, eggplant, radish, spinach, sweet corn, turnip, and potato. Flea beetles scratch holes or leave white streaks in green foliage in late spring. Intense feeding results in wilting and dying of leaves and decreased yield.

Leafhoppers are up to 3/8 inch long, green in color, and wedge-shaped. They may migrate from one area of the garden to another and hop away in large numbers when foliage is disturbed. They attack bean, carrot, cucumber, Irish potato, and muskmelon. Symptoms of leafhopper damage include curled or crinkled foliage and "hopper burn" (caused by leafhoppers' feeding, indicated by brown edges on leaves).

Colorado Potato Beetles are larger beetles about 3/8-inch long with a wide oval shape. Their wing covers are yellow-white with ten black stripes. Their larva are damaging to potato plants along with others crops in the Solanaceae family. Colorado potato beetle is difficult to control and resistant to most home garden insecticides. Azadirachtin and spinosad are two organic insecticides that these beetles have not yet developed resistance to and give marginal control.

Harvesting Potatoes

• Harvest potatoes after the vines have died. Handle as gently as possible during harvest. Because the tubers develop 4 to 6 inches beneath the soil surface, a shovel or spading fork is a useful tool for digging potatoes.
   
• Potatoes for use in early summer ("new potatoes") may be dug before the vines die (usually in July). When the potatoes reach 1 to 2 inches in size, you may wish to dig a few hills to use for soup or to cook with creamed peas or to butter and roast.
   
• Late potatoes are usually dug in August or early September. They keep in the garage or basement for several weeks in their natural dormancy. Store over the winter in a dark room at a temperature between 38° and 40°F with high humidity. Check periodically for spoilage. Temperatures below 38°F cause internal damage to the tubers.

Selection & Storage

Potatoes are the most popular vegetable in the United States. Although there are more than 100 known varieties, about six varieties make up the entire commercial market. Home gardeners can taste some of the wonderful flavors and textures unknown to the average person. Some varieties are not considered marketable because they do not ship well or are prone to disease.

Potatoes are generally classified as round red, round white, oblong white, and yellow-fleshed. New potatoes are any variety of freshly dug young potato that hasn't been stored. Potatoes can be harvested at any stage of development from marble-size to full maturity. Potato size at maturity depends on the variety planted. Potatoes should be firm, free of soft spots, and free of disease when harvested.

Even stored under the best conditions, potatoes lose some quality the longer they are stored.

• For best results, store in a cool, dark place with good air circulation.
• Do not refrigerate potatoes. Cold temperatures convert starch to sugar, giving potatoes an uncharacteristic sweet taste.
• The sugar caramelizes during cooking producing brown potatoes and an off flavor.
• Potatoes can be stored for a week or two at room temperature (65 to 70 degrees) with good results.

If potatoes start to sprout, they can still be eaten.

• Remove the sprouts and discard.
• If the potato is still firm, it is good to eat. Shriveled, wrinkled, sprouting potatoes should not be eaten.
• Green-skin potatoes have been exposed to too much light. A mildly toxic alkaloid called solanine forms in the skin. The green skin can simply be peeled away. Although the remaining potato is safe to eat, it will not be at its best.

Questions and Answers

Q. Should I save some of my potatoes for seed?

A. No, unless you are saving seed of an heirloom variety not commercially available. Saving your own seed potatoes can lead to a buildup of viruses and diseases. Whenever possible, plant seed potatoes certified to be free from certain viruses and diseases.

Q. My potato plants flowered and formed green fruits that resemble small tomatoes. What are they?

A. These small seed balls are the fruits that contain the true seeds. They are not edible. Except for breeding purposes, growing potato plants from the true seeds in these fruits is a troublesome and unrewarding exercise.

Q. What causes green skin on my potatoes?

A. The green areas on tubers develop where the potato was exposed to the sun. This condition occurs when the potatoes were not planted deeply enough or not covered with straw. The green portions taste bitter because they contain a moderately poisonous alkaloid. These green areas should be cut off and discarded. Exposure of potato tubers to fluorescent light or sunlight causes greening during storage.

Q. How should potatoes that are cut into seed pieces be cured?

A. They can be cured by holding them for a week at 60° to 65°F with high humidity (85 percent or higher). This treatment is of questionable value for the home gardener.

Q. Can I make chips from homegrown potatoes?

A. Yes. Almost any potato variety can be used to make chips when the potatoes are freshly dug and starchy. Commercial chips are made from selected varieties that are naturally high in solids, carefully handled, and properly stored to preserve starch and avoid the buildup of sugars. Chips made from potatoes stored at low temperatures for long periods are brown or have a dark ring because they contain excessive amounts of sugar.

Q. Can I use grocery store potatoes for planting?

A. Probably not. They may have been treated with a sprout retardant, in which case, they will not grow. Even if they are sprouting, they have not been inspected and certified free of disease. While results occasionally may be acceptable, the risk of introducing a nematode, disease, or other pest is much higher than from quality-certified seed potatoes.