Potatoes are grown as annual plants, with fields planted each spring, and harvested in the fall. All potato plants look something like the picture here. The part of the plant we call a “potato” grows underground. It grows on a specialized underground stem called a stolon. So, although potatoes grow underground, they are stems, not roots, and are known as “tubers.”
It is surprising to many people that potato plants produce flowers – sometimes very attractive ones. The fruits that grow from these flowers look a lot like green tomatoes, but they are not edible.
There are many different kinds of potatoes grown in Washington. Most of them are brown-skinned with white flesh, and are called “russets.” Russet Burbank is the most common variety of russet potato. It is grown for French fries and for the fresh market. Other russet varieties include Ranger Russet (grown for French fries) and Russet Norkotah (grown for fresh potatoes sold by supermarkets and baked by restaurants). Other varieties grown in Washington produce yellow, red, and blue potatoes. Other varieties are grown specifically to be made into potato chips. For more information on potato varieties in Washington, see the Recipes section of our web site.
Most potatoes grown in Washington are grown in large fields of over 100 acres (there are 640 acres in a square mile). Almost all potatoes grown in Washington have to be watered with sprinklers, known as “irrigation.”
There just isn’t enough rain in Washington during the summer to grow potatoes without irrigation. Many fields in Washington are irrigated using sprinkler systems called “center pivots.” These machines pivot around a center point, and can irrigate fields well over 100 acres in size. Electricity is used to drive motors on each pair of wheels. Complicated computerized control panels ensure an even water supply for all the plants in the field.
Many plant diseases can infect potatoes. To help minimize their loss to diseases, farmers usually grow potatoes in each field once in every four years. The three seasons between potato crops allow time for potato-infecting diseases to die out in that field.
Prior to planting, growers determine how fertile the soil in their field is. They have the soil analyzed to determine how much nitrogen, phosphorous, and other essential plant nutrients it contains. Then the grower orders a fertilizer mix specially tailored to the needs of their particular field. This will allow them to grow a high-quality potato crop without wasting fertilizer (which costs them money).
Potatoes are grown from special potatoes called “seed potatoes” which are cut into pieces (sometimes left whole) and planted in the ground. These pieces of potato grow stems and roots from the “eyes.” Because they are able to feed off the energy in the seed piece, potato plants grow very fast right from the start.
Seed potatoes are cut using specialized equipment that are carefully tuned to get seed pieces of nearly uniform size. Potato growers like to have all the plants in a field about the same size. This helps maximize harvested tubers, and allows most of the tubers produced in a field about the same size. Seed size is very important in starting a uniform planting of potatoes.
After cutting, seed pieces are usually treated with a “seed treatment” which helps prevent infection with bacteria and fungi that can cause rot. Cut seed pieces are loaded into trucks and carried to the field. There they are loaded into potato planting machines pulled by tractors. Many potato planters are guided by GPS systems to keep the rows straight and seed potatoes planted uniformly.
Potatoes are planted in Washington starting the last week of February, and ending in early May. The earliest areas are near Pasco, and the latest areas are northeast of Moses Lake.
Potato plants emerge from the ground 2-6 weeks after planting, depending on weather, location, and time of year. The plants grow quickly, and will begin to grow tubers just a few weeks after emergence.
After the plants emerge, the grower has to be very careful to keep his field thoroughly watered. Potato plants can survive periods of dry soil, but drought-stressed plants produce fewer potatoes of low quality. During the hottest weeks of summer, center pivot machines may run 24 hours a day, watering an entire field every 20-30 hours.
After emergence, growers must carefully keep track of the health and growth of their potato plants. They regularly take leaf samples and have them chemically analyzed for the amounts of essential plant nutrients. When the fertilizer they applied before panting starts to get used up, they see this in their leaf sample tests, and begin applying fertilizer to their growing plants. Toward the middle of the growing season, growers stop all fertilizing. This encourages the plants to put most of their growth into tubers for the second half of the season. Too much fertilizer late in the season causes plants to grow huge above ground, with very small tubers.
Many pest diseases and insects attack potatoes. Growers carefully watch their fields for disease symptoms and insects. Scientists track disease and insect populations throughout Washington’s potato-growing region, and provide helpful information to the growers. One of the most important pests of potatoes is late blight. This is the disease that caused the Irish potato famine of the 1840s. This disease can completely destroy a potato field, and can quickly spread to neighboring fields; everybody in the region must control it. Growers treat their crops to protect them from late blight using fungicides that stop late blight infection. They follow the recommendations of scientists who have studied and understand the biology of the late blight fungus.
The green peach aphid is the most important insect pest for potato growers. It is so important not because it directly damages plants, but because it carries a devastating potato disease called “potato leafroll virus” (like people, potato plants can get sick from viruses – but potato viruses cannot hurt people). This virus weakens potato plants and causes the tubers to have brown internal markings called “net necrosis.” Supermarkets and restaurants will not buy potatoes with net necrosis, so it is very important to growers to prevent it in the field. Similarly to late blight, scientists keep track of aphid numbers throughout the region, and keep the growers informed. When growers find aphids in their field, they have the field treated with insecticides. Some modern aphid-insecticides are incredibly targeted – affecting only aphids and a few close relatives, and are less toxic to humans than table salt. These insecticides preserve beneficial insects in the field, and are extremely safe for workers and consumers.
One important pest that many people will never have heard of is the plant-parasitic nematode. Nematodes are minute worms that infect plant roots and potato tubers. Nematode-infected potatoes are definitely not attractive, and growers cannot sell potatoes infected with nematodes. Many fields must be treated with nematicides to control these pests. Until consumers can accept these unnattractive, nematode-affected potatoes, growers will have to control nematodes.
Potato harvest starts in Washington around the 4th of July, and extends through October, depending on location and variety of potato. Some potatoes are harvested during the summer and processed immediately into French fries or chips, or packed in bags and boxes and sent to supermarkets and grocery stores.
Other potatoes are grown well into the fall and stored for up to 11 months in huge temperature-controlled storage buildings.
If a field is planned to be sold on the fresh market (to supermarkets and restaurants), the field will be “killed” prior to harvest. The plants are killed by either stopping all irrigation or by spraying with special chemicals that kill the leaves and stems. The potatoes are then left in the ground for at least 10 days. During this time the skins thicken. Thicker skins allow the potatoes to be harvested without being badly damaged and made unattractive to consumers. Fields planned for storage are also usually killed before harvest. In this case, thicker skins help prevent infection by diseases, which can destroy thousands of tons of potatoes in storage.
Potato harvesters are complicated machines that must dig the potatoes out of the ground, separate potatoes from other plant material, dirt, and rocks. Harvesters must do all this while being gentle enough to prevent bruising.
Another reason potatoes have become so popular around the world is the fact that they can be stored for several months. Pioneers in North America stored their potatoes in root cellars near their houses. Commercial potato growers store their potatoes in huge buildings built especially for storing potatoes. Potatoes in storage are piled up to 20 feet deep. Specialized air circulation systems keep the temperature and humidity as uniform as possible in the pile.
The success of the Washington potato growers has been built on their adoption of new practices and technology developed through research programs in the public and private sectors. From horse-drawn equipment of 100 years ago, potato growers have advanced to using laser-guided planters, tractors that navigate fields using satellites (GPS), and irrigation equipment that delivers exactly the needed amount of water and no more. Growers continue to transition to safe and effective pesticides, having moved from elemental pesticides like lead arsenate, through DDT, and now to pesticides that are less toxic than salt and kill only intended pests. Finally, detailed research has shown growers how to manage soil and nutrients to produce high yields of high quality potatoes.