Irish (or white) potatoes grow on a vining annual plant that reaches 30 inches wide. Potatoes are actually stolons, or underground stems, that become swollen with stored starch. They are generally round or oblong, and can be white, yellow, red, or blue. Depending on the variety you grow, they should mature anywhere 90 to 180 days. You can expect 25 to 50 pounds of potatoes from a 25-foot row.
Before you begin growing your potatoes, make sure that your garden soil is in full sun, with good drainage and a porous texture. Work in some humus (peat moss, leaf mold, etc.) to improve the soil, and add 6 pounds of any balanced garden fertilizer (such as 8-8-8) per 100 feet of row. If you know or can test the pH of the soil, anything between 4.8 and 6.5 is ideal for potatoes.
There are two methods of starting potato plants: from "seed potatoes" or from true seed. Each is explained below.
Method 1: Starting with Seed Potatoes
(if growing from true seed, scroll down to Method 2)
Most potatoes are grown either from pieces of the potato, referred to as "seed pieces," or from whole small potatoes. Be sure to use only "certified seed potatoes" for best result. Trying to grow from store-bought potatoes is usually unsuccessful, because these varieties have often been treated to prevent sprouting.
Plan to plant the seed potatoes when the soil temperature reaches about 45 degrees F and the soil is no longer soggy from winter moisture. This may occur around the time of the last frost date for your area.
About a week before you plant the seed potatoes, set them all out in a sunny spot, such as a bright windowsill, and let them bask in the sun. If they begin to sprout, all the better.
Before you plant, dig furrows 6 inches deep. Then set the seed potatoes 12 inches apart, covering each completely with 2 to 4 inches of soil. (The furrow will be a few inches lower than the rest of the soil, like a very shallow trench.) Space furrows 2 to 3 feet apart.
Scroll down to Growing the Potatoes for further information about caring for and harvesting your crop.
Method 2: Starting from True Seed
Sow true seed indoors in late winter, 4 to 6 weeks before last scheduled spring frost. Sow in a shallow seed flat, covering the seeds with 1/16-inch of soilless medium. Keep temperature between 60 and 70 degrees F, and never allow the medium to dry out. The seeds should germinate in 10 to 15 days, and will be ready to transplant into small pots when they have at least 4 true leaves.
After danger of frost (but before the roots fill the pots), set the pots outdoors in a shaded, wind-free area for 3 days. This is called "hardening off," and it eases the transition from indoors to outdoors for the seedlings.
After the seedlings have been hardened off, dig furrows 6 inches deep. Then set the seedlings 12 inches apart in the furrow, covering only their roots. (The furrow will be a few inches lower than the rest of the soil, like a very shallow trench.) Space furrows 2 to 3 feet apart.
Growing the Potatoes
The potatoes grow right below the surface of the soil, so to prevent them from becoming exposed (and turning green), mound the surrounding soil around the potato plant several times as it grows, leaving only about 6 inches of the top growth visible. This is called "hilling."
Keep the plants well watered and free of weeds.
Harvesting the Potatoes
Whenever harvesting potatoes, make sure that you handle them gently to avoid bruising. Do not expose them to the sun or strong wind longer than necessary.
If you want "new" potatoes, harvest them as soon as they reach an inch or more in diameter. (You can experiment by digging up one or two sample potatoes to see their size.)
For mature potatoes, harvest them according to the days to maturity listed for each variety. And if you want to store them, do not dig the potatoes until the plants have died down for 2 to 3 weeks after the first fall frost. Then wash them or brush off the dirt and store them in a dark, humid place at about 45 to 50 degrees F.