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University of Illinois Extension
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Irish Potatoes

James C. Schmidt
Extension Specialist, Home Horticulture/4-H
University of Illinois Extension

The white potato (Solanum tuberosum) – often referred to as the Irish potato because of its association with the potato famine in nineteenth century Ireland – is a staple in the American diet and is the fourth most important food crop worldwide after wheat, corn, and rice.  Potatoes are also a favorite and quite productive crop for the home garden, but they require a relatively large growing space.  Despite the fact that potatoes grow beneath the soil, the edible part is not a true root but a specialized storage stem called a tuber.

Soil Requirements

Potatoes grow best in a fertile, well-drained soil that is high in organic matter.  Soil that stays too wet and compact has a reduced oxygen level and produces misshapen potatoes and low yields.  If your garden has a heavy soil, you can use well-rotted manure, compost, or a green manure crop to add organic matter and thus improve the drainage and tilth.

Varieties for Illinois

More than 100 varieties of potatoes are available to the home gardener.  White-skin/white flesh varieties are often used for baking, French fries, and chips.  Red-skin/white flesh varieties are usually boiled and used for mashed potatoes.  Yellow-flesh varieties have been popular in Europe and have only recently caught on in the U.S.  The variety ‘Yukon Gold’ has large, round tubers with flavorful, moist flesh. Other novelty colors exist; a few specialty types have blue to purple outer skin and either white or blue flesh.

Always purchase certified disease-free seed pieces from a garden center or seed store. This is your best assurance of productive, healthy stock.  To save money, many gardeners use potatoes from the supermarket produce section for seed pieces.  This practice is discouraged because many of the potatoes sold for food have been treated with chemical sprout inhibitors and will not grow well.  Also, they may be contaminated with disease organisms or be varieties that are not well adapted for Illinois.  The following are potato varieties recommended for our state:

Early
‘Norland’ – an attractive red variety with oblong tubers. Moderately resistant to scab.

Midseason
‘Superior’ – a red variety with semi-round to slightly oblong tubers.  
‘Red Pontiac’ – a popular, high yielding, smooth-skinned variety.

Late
‘Kennebec’ – a popular white variety with rough, light brown tubers that develop early in the season close to the soil surface; resistant to some viruses and late blight.

Planting

To produce a crop of potatoes, seed pieces rather than seeds are planted.  The seed pieces may be small whole potatoes or potatoes that are cut into pieces about the size of a medium egg – 1½-  to 2 ounces each, with at least one eye.  In general, for each pound of seed pieces you plant, expect about 20 pounds of potatoes.  Whole, small potatoes are preferred because they result in more vigorous growth and decay is less likely because there are no cut, exposed surfaces where infection can occur.  If tuber pieces are used, cut up the potatoes a few days before they are to be planted and allow the cut surfaces to callus, thus reducing chances of spoilage.  A cool cellar or basement, kept at 50o to 65 o F and about 90 percent humidity, is recommended for this process.

Potatoes are a cool-season crop.  If soil conditions permit, potatoes can be planted in March or early April.  Disregard the popular myth that potatoes should be planted on a specific calendar date such as Washington’s Birthday, St. Patrick’s Day, or Good Friday.  Planting on such a predetermined date may be unwise because the soil at that time can be too wet or the weather unfavorable.

Plant the seed pieces 10 to 12 inches apart in a furrow and cover with 2 to 3 inches of soil.  Space the rows 2 to 3 feet apart.  After the potato sprouts break the surface of the soil and begin to grow, gradually build a ridge of loose soil over the row.  By midsummer this ridge should be 4 to 6 inches high and will be helpful in reducing the number of sunburned and green tubers.  If seed pieces are not planted deep enough or covered with straw, the tubers develop green areas that contain the poisonous alkaloid solanine.  Affected areas should be cut out and discarded before cooking the potatoes.

An alternative planting method is to grow the potatoes in straw.  With this method, the seed pieces and rows are spaced as described above, but the tops of the seed pieces are planted at or within 1 inch of the soil surface.  When the green sprouts appear, a 4- to 6- inch layer of straw is placed over the row.  Although potatoes planted in straw generally do not yield as high as those grown by traditional methods, the straw keeps the soil temperature cooler and it might help avoid problems with heavy soils.  This cooler soil temperature reduces water loss and decreases the need for cultivation, resulting in more regularly shaped tubers that are also easier to harvest.

Summer care

Begin hoeing or pulling weeds early in the season to control weeds.  An organic mulch applied over the plants emerge will not only prevent weeds but will also conserve moisture and keep the soil cooler.  Avoid extensive hoeing and cultivation that can damage tubers and roots and possibly reduce the yield.  Remember, too, that applying an organic mulch before the soil has warmed up in the spring will delay growth.  

Uniform moisture is one of the most important factors that contribute to good potato yields and quality.  Adequate and uniform soil moisture should be maintained from planting until tuber development is complete to keep the soil cool and to eliminate misshapen tubers.  Dry periods alternating with very wet periods cause knobs, growth cracks, and “hollow heart.” 

Sometimes potato plants flower and form small, marble-like green fruits that resemble tiny tomatoes.  These small fruits, which contain the seed, should not be eaten.  Likewise, the seed should not be saved and planted.  The crop produced from the seed would not be very satisfactory, and there is the chance of virus and disease carryover.

Harvesting

“New” potatoes can be dug in early summer when the tubers have reached 1½-2 inches in size.  Keep in mind, however, that this practice will reduce total yield.  To harvest the small tubers, carefully dig away the soil from the sides of the ridge and replace it after the tubers have been removed.  The bulk of your potato crop will likely be harvested after the vines turn yellow and die in late summer.  Attempting to harvest earlier can result in bruised tubers and poor keeping quality.  Use a spading fork for digging the potatoes.  Potatoes grown in straw can be harvested by carefully removing the mulch and pulling up the tubers, which are usually near the soil surface.

Storage

Potatoes will keep in a garage or basement for several weeks if the temperature is above freezing.  For winter storage, however, keep them in a dark room at 38 o to 40 o F and high humidity.  Extremely low temperatures (30 o to 37 o F) cause a chemical conversion of the starch to sugars and result in lower quality and poor flavor.  The high sugar content makes the potatoes unsuitable for chipping because the slices become too brown when deep-fried.

Pests

Although potatoes are usually a reliable crop for the home garden, it is essential to control insects and diseases.  One of the most important steps the home gardener can take is to prevent diseases.  Of the foliar diseases, early blight is the most serious.  Early blight is most serious in warm, humid weather.  The first signs are dark brown patches on the lower leaves.  Upon close inspection, the lesions have a concentric circle or ‘bull’s eye’ pattern.  It can be controlled by using certified seed potatoes; avoiding heavy, wet soils; rotating crops; avoiding excessively wet foliage; and using a recommended fungicide.  

Potato scab, caused by a soil-borne pathogen, occurs as brown, scabby, cracked areas on the surface of the tubers.  Although it can be prevented by maintaining an acidic soil, it is best controlled by selecting scab-resistant varieties, maintaining moisture during tuber set, and following crop rotation.

Insect pests include leafhoppers, flea beetles, and Colorado potato beetles.  Leafhopper damage first appears as small, brown “burns” at the tips of the leaves.  Eventually the entire leaf margins turn brown and roll upward.  Flea beetle feeding results in numerous pinholes in the leaves.  Adult Colorado potato beetles, which are round and yellow with black stripes, lay their eggs on the undersides of the leaves and feed on the foliage.  Check with your local extension office for specific chemical recommendations to control these pests.