Potato: Oft-maligned; Seldom Appreciated

Published: April 1, 2013

For those who might have resolved to lose a bit of weight, potato (most likely) was one of the first foods eliminated from their diet. After all, everyone knows that “potatoes are fattening”. Fortunately, such is not the case. An eight ounce potato (boiled or baked) contains only 100 calories. It is the way we prepare or embellish potatoes that packs on the calories. High in carbohydrates but low in fat, potatoes contains significant amounts of vitamin C and other essential nutrients, making them somewhat of a “health food”. Indeed, it has been speculated by some that humans could survive on a diet that included only potatoes and milk. April is a popular month for potato planting in Missouri and a good opportunity to take a closer look at this valuable food staple that (arguably) does not get the respect it deserves.

Potato (Solanum tuberosum) is a member of the Solananceae, or Nightshade, family and native to the Andean region of South America. It was known to be cultivated by the Incas as many as 4000 years ago. There are more than 150 wild species of potato most of which contain significant levels of a glycoalkaloid called solanine, a bitter-tasting, toxic compound associated with the members of the Solananceae family. Today, solanine content still is a concern when developing new potato cultivars.

The Spanish explorer de Leon probably was responsible for introducing potato to Spain from the New World. From there, it was introduced to England and Italy in 1585, to Belgium, Germany and Austria around 1887 and to France around the turn of the 17th century. Wherever it went it was slow to be accepted as a food source because of its association with the poor and the fear that it might be poisonous or cause diseases such as leprosy.

The potato is said to have found its way to Ireland thanks to a Spanish ship carrying potatoes that wrecked off the Irish coast in 1588. Potato thrived in Ireland and, in short order, the Irish became greatly dependent on the potato as food and usually consumed it at every meal. It was estimated that an average Irish laborer of the 1800s consumed 14 pounds of potatoes daily. This dependence on a single source of food was a primary reason for the Great Potato Famine of the 1840s that occurred when late blight (Phytophthora infestans) destroyed most of Ireland’s potato crop for several years in succession. It is estimated that Ireland lost 1.5 million people to starvation and related illness during the famine, cutting the country’s population in half through starvation and emigration.

The potato was first introduced into what is now the United States in 1621. It was introduced on several other occasions throughout the 17th century but was slow to gain acceptance. Indeed, as late as the mid-1800s most people considered potatoes more fit for animal than human consumption. A quote from the Farmer’s Manual at that time stated that potatoes should “be grown near the hog pens as a convenience towards feeding the hogs”. The potato became more accepted as a human food source toward the latter half of the 1800s and it was during that period that vast improvements in potatoes were made in both productivity and table quality. Today, the United States produces nearly 46 billion pounds of potatoes and the average American consumes about 140 pounds of them each year.

Perhaps the primary challenge to successful potato production in Missouri revolves around the weather. Potato is a cool-season crop that has an optimum (air) temperature for tuber formation of 78 degrees F. Missouri is notorious for its quick transitions from springtime to hot summer temperatures. As a result, gardeners often plant their potatoes early in an attempt to subject them to cooler temperatures for a longer period of time. Unfortunately, early planting can lead to crop loss from late spring freezes or seed piece decay due to wet, cool soil. Waiting to plant until soil temperatures warm to 50 degrees F is advisable.

Potato prefers a sunny location in a well-drained garden loam high in organic matter. The ideal soil pH is relatively low (5.3-6.0), since scab, a troublesome disease of potato, favors soil with a high pH. Areas to be planted in potato should be tilled 8 to 12 inches deep. After tilling, level the soil so that furrows can be made for planting.

Liberal amounts of fertilizer are required for good potato yields. Soil testing prior to planting will help to determine the amount of fertilizer to be added. Normally, application of about 2-3 pounds per 100 square feet of a fertilizer with higher amounts of phosphorus and potassium than nitrogen (e.g. 5-10-10) is sufficient initially. Banding the fertilizer about 6 inches deep and 2 to 3 on either side where the seed pieces will be placed is helpful. Later, when tubers begin to form, side-dressing once or twice with a garden fertilizer at the rate of about 1 pound per 25 feet of row will help boost yields.

Genetically, potato is a tetraploid and, as such, is highly sterile. Therefore, instead of planting seeds, seed pieces are used to start potato. Seed pieces are made by dividing certified (disease free) potato tubers so that each piece has at least two “eyes” (dormant nodes) remaining. It is best to cut seed pieces the day prior to planting to allow cut surfaces to dry. Plant the seed pieces 9 to 12 inches apart in shallow trenches about four inches deep and cover with an inch or two of soil. Rows should be spaced 28 to 34 inches apart.

After potato plants emerge, mulching with an organic form of mulch will help to control weeds, conserve moisture and cool the soil. If natural rainfall is insufficient, potato should be irrigated to provide uniform moisture. Erratic moisture supply often results in undesirable, “knobby” tuber growth. Maximum water use occurs during active plant growth and early tuber development. The amount of water supplied should be reduced when plants begin to die back in order to prevent tubers from decaying.

Major insect pests of potato include Colorado potato beetles, flea beetles, leafhoppers and aphids. Wireworms, white grubs and other soil insects can damage potato tubers. These insects are most likely to occur if sod was tuned under before planting the area. Scab, early blight and late blight can be problematic foliar diseases. Fortunately, the climate in Missouri is not conducive for the latter to develop on a regular basis. If weed control is necessary, it should be accomplished by shallow cultivation since deep cultivation may injure tubers.

Unless mulch has been applied, when plants are 6 to 8 inches tall, begin to mound soil around the bases of the plants to form a ridge or hill. By the end of the production season, the ridges should be 4 to 5 inches high. This practice not only helps to control weeds but is necessary to prevent greening of shallow tubers. Green skins are caused by exposing potato tubers to light. Since the green portion tends to develop the previously-mentioned toxin solanine, it should be cut off before cooking, or (better) green-skinned potatoes should be discarded totally.

New potatoes can be harvested as soon as they reach a useable size. Potatoes destined for storage should be dug about two weeks after the plants have naturally died back. This allows the skins to mature and reduces peeling and bruising during harvest. The latter tends to lead to storage rot. Place tubers in a dark place immediately after harvesting. As previously mentioned, tubers exposed to sun (and high temperatures) tend to turn green.

Potatoes can be stored for several months if the tubers are cured properly. The latter involves placing them in a dark place for about 10 days at a temperature of from 60 to 65 degrees F and a relative humidity of at least 85 percent. After the tubers are cured, store them in a cool (40 to 45 degrees F), dark location with high relative humidity. Those having the luxury of an extra refrigerator can store potatoes for many months. However, under refrigerated storage potato tubers tend to convert their starch into sugar, decreasing their table quality. This can be reversed by taking potatoes out of refrigerated storage several weeks before they are used.