Linda Geist

COLUMBIA, Mo. – The admonition for gardeners to plant their potatoes on St. Patrick’s Day is probably good advice for some parts of our country, but not for Missouri, said University of Missouri Extension horticulturist David Trinklein.

“It is a few weeks too early,” he said. Wait to plant potatoes until soil temperatures warm to above 45 degrees Fahrenheit.

Missouri weather presents challenges to potato growers, said Trinklein. Potato is a cool-season crop; the optimum air temperature for tuber formation of 78 F. Missouri’s quick transition from springtime to summer temperatures prompts some gardeners to plant potatoes early to take advantage of the cooler weather. Unfortunately, this leads to crop loss from late spring freezes or seed piece decay due to wet, cool soil.

The potato’s association with St. Patrick’s Day undoubtedly pays homage to the important role this vegetable played in 19th-century Ireland, Trinklein said. According to some estimates, a typical Irish laborer of that era ate as much as 14 pounds of potatoes daily. Potatoes were such an essential part of the Irish diet that the Great Potato Famine of the 1840s cut the country’s population in half through starvation and emigration.

In the U.S., however, most people in the mid-1800s considered potatoes more suitable for the feed trough than the dinner table. A farmers manual at that time stated that potatoes should be grown near the hog pens as a convenience towards feeding the hogs, said Trinklein. Americans began to incorporate potatoes on their menus toward the latter half of the 1800s. Today, the average American consumes about 140 pounds of potato each year.

Potato (Solanum tuberosum) is a member of the nightshade family. It is native to the Andean region of South America. Incas grew it as long as 4,000 years ago. Potato plants prefer 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. Till 8 to 12 inches deep and level the soil to allow furrows for planting.

Good yields need liberal amounts of fertilizer. Test soil for best results. Use a fertilizer with more phosphorus and potassium than nitrogen (such as 5-10-10). Apply 2-3 pounds per 100 square feet. Band the fertilizer about 6 inches deep and 2 to 3 inches on either side of the area where potatoes will be planted. Use a side dressing of garden fertilizer once or twice when tubers begin to form. Apply about 1 pound per 25 feet of row to boost yields.

Rather than seeds, pieces of potatoes called “seed pieces” are used to start new plants. Seed pieces are made by dividing disease-free potato tubers so that each piece has at least two “eyes” (dormant nodes). Divide seed pieces the day before planting to allow cut surfaces to dry. Plant seed pieces 9-12 inches apart in shallow trenches about 4 inches deep and cover with an inch or two of soil. Space rows 28-34 inches apart.

After potato plants emerge, apply an organic mulch to control weeds, conserve moisture and cool the soil. Irrigate as needed. An erratic moisture supply often results in “knobby” tuber growth. Most water use occurs during active plant growth and early tuber development. To prevent tubers from decaying, reduce watering when plants begin to die back.

Major insect pests of potato include Colorado potato beetles, flea beetles, leafhoppers and aphids. Wireworms, white grubs and other soil insects can damage tubers. Insects are most likely to appear if sod was turned under before planting. Foliar diseases include scab, early blight and late blight. The latter was the cause of the Great Potato Famine in Ireland. Fortunately, Missouri’s climate is not conducive for late blight to develop on a regular basis. Use shallow cultivation for weed control. Deep cultivation may injure tubers.

When plants are 6 to 8 inches tall, mound soil around the plant base to form a ridge or hill, unless you used mulch. By the end of the growing season, ridges should be 4 to 5 inches high. This helps to control weeds and prevents greening of shallow tubers due to light exposure. The green portion tends to develop the toxin solanine. Cut it off before cooking or discard the whole potato.

Harvest new potatoes as soon as they reach a usable size. Dig potatoes that you plan to store about two weeks after plants die back naturally. This allows skins to mature and reduces peeling and bruising, which lead to storage rot. Place tubers in a dark place immediately after harvesting to avoid greening.

Potatoes can be stored for several months if cured properly. Put them in a dark place for about 10 days at 60 to 65 F and a relative humidity of at least 85 percent. After the tubers cure, store them in a cool (40 to 45 F), dark location with high relative humidity. You can store potatoes in a refrigerator for many months. However, refrigerated potatoes tend to convert their starch into sugar. This decreases table quality. You can reverse this by taking potatoes out of refrigeration several weeks before use.

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