All tubers produced on a potato plant come from above the seed piece. Since the seed piece is planted only 3 inches deep, soil must be pulled toward the plant as it grows. This allows a place for the tubers to form. Some gardeners use a thick mulch for this purpose. Tubers formed in a soft mulch often are smoother and better shaped than those grown in soil. This is especially true if the soil is heavy. (see Figure 6).
As the tubers enlarge they must be protected from sunlight. Exposure to sunlight causes them to turn green. A thick layer of mulch applied when the plants are 8 to 10 inches tall can reduce soil temperature and increase yield and quality.
The soil moisture supply should be kept constant during growth. The plant must have adequate moisture and fertilizer when the tubers are forming. This usually occurs when the plants are 6 to 8 inches tall. Apply 1 cup of fertilizer for each 30 feet of row beside the plants when they are about 4 inches tall. Water the fertilizer into the soil. This is especially important on sandy soils.
Moisture stress followed by irrigation or rainfall can cause growth cracks and second growth. If the rainfall is accompanied by hot weather, the rest period of developing tubers can be broken and can cause the tubers to sprout in the soil (see Figure 7). Too much water causes enlarged pores on tubers and makes them rot easily in storage.
Potato plants usually produce flowers and sometimes produce fruits. The fruits bear the true seed of the potato plant. They look like small tomatoes but cannot be eaten. Potato plants do not cross with tomato plants (see Figure 8).
Harvesting and Storing
Potatoes are ready to harvest when the tops begin to die and the skin becomes firm on the potato. The skin is set when it does not scrape easily when rubbed with the thumb. Skin set can be speeded by cutting back the tops to the plants. Most of the potatoes should weigh 6 to 12 ounces at harvest. Harvest small, "new potatoes" during the growing season by carefully digging beside the plants with the fingers.
To harvest potatoes, dig under the plants with a shovel or spading fork. Keep the fork 8 to 10 inches away from the plant to prevent cutting the potatoes. Raise the plants and shake away the soil. Potatoes should be dug when the soil is moist. If it is too wet, the soil will stick to the potatoes. If too dry, clods will bruise the potatoes. Pull the potatoes from the vines and handle them carefully to prevent damage since damaged potatoes do not store well.
Allow the potatoes to dry, then store them in a cool spot with plenty of air movement. Most potato varieties are ready to dig 95 to 110 days after planting.
