Here's how -
If you keep it inside, let it dry out for a day to reduce the chance of mold. Place toothpicks into both sides of the potato piece, then place it on top of a glass of water. The toothpicks will hold some of the potato above water, but the eyes should be submerged. The roots will grow into the water and the leaves will grow toward the top. When there are enough roots, transplant into the garden.
Ginger: Use a piece of ginger with a knob, or one with an arm sticking out of the main body of the root. Soak the root overnight, then bury it in a pot of rich soil with good drainage. Be sure to keep the soil moist. Ginger does best in the shade or as a houseplant. It cannot tolerate cold weather or drought. If given the right conditions, ginger is easy to grow.
Green Onions: Cut off the bulb and roots (the white end) and place it roots down in a glass of water overnight. Replant outside or in a large pot the next day, and it will start growing quickly.
Leeks: Cut off the bulb, just like you would with green onions, and place it in a shallow glass of water. Change the water and wash the roots once a week. Harvest fresh leeks when they’re big enough to eat.
Romaine Lettuce: Cut off the bottom inch of a head of romaine lettuce. Place it roots down into a shallow glass of water, and new leaves will grow from the top.
Garlic: Plant a single clove of garlic root side down (flat end) into a few inches of potting soil. Once shoots start sprouting, cut them back so the bulb will be able to grow, and eventually you will have a fresh garlic bulb. Use a clove off the new bulb to start growing another for later.