by Tina Lee & Marcia Fein
Orin Martin, Manager of the Alan Chadwick Garden at UC Santa Cruz has the inside scoop on local potato growing. Here are some highlights of an excellent presentation from him.
Contrary to (some people) popular belief, the potato did not originate in Ireland. The potato is native to the Andes and was introduced to Ireland by Sir Walter Raleigh. The potato grew very well there and began to replace cereal and grain crops. Eventually the Irish became dependent on the potato, but since they grew only one vulnerable variety, late blight destroyed almost the entire crop and caused the infamous potato famine. This exposed the dangers of relying on a narrow gene base.
The botanical name for the potato is Solanum tuberosum. The potato belongs to the nightshade family that includes tomatoes, peppers, and eggplants. Although the potato tuber grows underground, it is actually an aberrated stem, not a root. The tuber develops from the swollen end of a modified underground stem called a stolon. The eyes on the potato are buds for new plants to sprout and develop.
It is a cool season crop and grows best when the day temperatures range from 60-65 degrees and night temperatures are between 45-55 degrees. Tuber production will stop if the temperatures exceed 85 degrees. The potato plant grows initially from the food reserves stored in the tuber.
The best times to plant are spring and fall. March is a good time to plant in Santa Cruz but if cool, wet weather persists, it's better to delay the planting until April. If moisture sits on the foliage, fungal diseases can develop. Potatoes are especially vulnerable to Phytophthora infestans (Late Blight) which is transmitted by spores in water; in water vapor on cool wind, and from live plant to live plant.
Potatoes prefer loose, loamy, well-drained soil. Rocky soil and heavy clay can result in a bumpy looking potato. Raised beds that are only 12-15 inches deep work well because potato roots grow wide but not deep. Most roots and all tubers will develop at the same level and above the level that the original seed potato was planted.
Use 2-4 oz. seed pieces when planting potatoes. Anything larger than that has a tendency to rot. Buy certified disease-free seed potatoes found at local nurseries or through mail-order catalogs. Don't try to use potatoes from the grocery store because they may be diseased, and they probably have been sprayed with a chemical to inhibit sprouting.
One technique to improve your success rate is to "chit" (pre-sprout) the potatoes before planting them. You can use small seed potatoes or cut bigger potatoes into 2 to 4 ounce pieces. Cutting the potato increases the tendency to rot, so leave the pieces out for 2-3 days so the cut ends will callous. The cuts exude subarin-a natural fungicide. You can dip the cut ends into lime, wood ash, or sulfur, which act as a desiccant, if you need to plant immediately. To chit, place the seed potatoes in indirect sunlight at 65-75 degrees. The pieces should be "rose-end" up - the end with the most eyes. After about 5-7 days, the seed pieces will begin to sprout. Rub off all but three sprouts and plant them.
The spacing between the plants depends on what kind of potatoes you want to harvest. On average, each plant will produce 2-4 pounds of potatoes. If the plants are spaced far apart, you will get bigger tubers but fewer of them. However, if the plants are spaced close together, you will get a greater number of smaller sized tubers. For new potatoes, you can plant as close as 8-10 inches apart. For an average size tuber, plant 12-15 inches apart. And if you want big, storage potatoes plant them 18-24 inches apart.
Plant potatoes in a trench 10-12 inches deep and cover with 2-4 inches of soil. As the plants grow taller, you gradually fill in the trench with soil around the plant. Called "hilling" this operation results in higher tuber production and prevents sunscald. It also increases the risk of disease entry through mechanical injury. It can disguise gopher hills as well. Gophers eat only the tubers, not the roots so the plants appear normal above ground. You may not be aware of gopher damage until you're ready to harvest.
Potatoes are heavy NPK feeders. They require a lot of organic nitrogen during the first month of growth. (Cautions against applying too much N assume chemical fertilizer.) Manure and fish emulsion are good organic sources of nitrogen. Potassium is also important for thickening the stems. Potatoes need moderate to heavy watering. The soil needs to be moist but not wet. Overhead watering will spread fungal diseases.
The plant growth cycle is as follows: From day 1-30, you'll see new plant growth above the soil. From day 30-60, new potatoes will form which have a low starch and high water content. From day 60-90, the existing potatoes will enlarge. From day 90-120, the plant begins to yellow and die back. During this time the skins will thicken, water content will decrease, and starch and other nutrients will increase.
Harvest at the appropriate time for the type of potato you want. Don't harvest according to when the plant flowers because different varieties flower at different times in the growth cycle, and not necessarily concurrent with tuber development. Use a spade or fork to dig up the potatoes. Eat immediately any injured potatoes. They will not keep. Store potatoes in a cool (40-50 degree), dark, humid place. Don't store them in the refrigerator because the starch will turn to sugar. Light will cause potatoes to turn green. They are producing solanin, which protects the potato but disrupts human digestion.
If you're interested in varieties to grow, Orin recommends the following: Red skinned potatoes such as All Red, Cherry Red, or Desiree; Yellow fleshed potatoes such as Yellow Finn, Bintje, or Yukon Gold; and Fingerlings e.g. Ruby Crescent and Russian Banana.