A couple of weeks ago, I planted potatoes for the first time by sticking 15 store bought potatoes about 4 inches down in the ground. Many sprouted nicely. But then yesterday I read that potatoes should be planted deep and gradually covered over with dirt as they grow. So, I dug up my potatoes and replanted them 8 inches down using the trench method:
Trench method: "Dig a shallow trench about 4 inches wide and 6-8 inches deep. Plant your potato seeds (8) inches apart in this trench. Place the potato seeds into the trench (cut side down) and then cover them with 3-4 inches of soil. After the sprouts emerge, add another 3-4 inches of soil. The potatoes will form between the seed piece and the surface of the soil. For this reason, when the stems are about 8 inches high, again add enough soil to bring the level half way up the stem of the plant. Another hilling will be needed 2-3 weeks later, at which time you again add soil half way up the stem of the plant. After these initial hillings, it is only necessary to add an inch or two of soil to the hill each week or so, to ensure there is enough soil above the forming potatoes that they don't push out of the hill and get exposed to light. (If the new potatoes are exposed to sunlight while they are developing, they will turn green and may be toxic.) The hilling process is necessary to create sufficient space for the potatoes to develop large tubers and an abundant crop. Don't get carried away with hilling though... If you cover up too much of the foliage, you may end up reducing your final crop yield."
One of my trenches (the one in the picture above) is more like a round hole (maybe 1'x 2'), but that was about the only free space that I had left in the garden.