How to Grow a Red Onion From a Store-Bought Red Onion

What would the food world be like without onions? This underground bulb has been cultivated for over 5,000 years as an important food and medicine, and before that, wild onions were harvested and consumed. They were among the first crops to be cultivated because they are easy to grow, and they tolerate a variety of soils and climates.

Only about 8 percent of the onion crop in the United States is red, with the vast majority being yellow. Red onions taste sweet and look good on sandwiches and salads. They are no more difficult to grow than other types of onions.

Red onion plants, like other onions, are biennial plants. That means they require two years to complete their life cycle. The first year, the onion seeds grow, forming tiny, underground bulbs. The next year, those red onion bulbs grow and mature until they are ripe and ready. This can be compressed into one year by planting onion sets, which are the small red onion bulbs.

Red onion tastes sweeter, brighter, and more subtle than the typical yellow onion. While yellow onions are often cooked or caramelized, red onions are almost always served raw or grilled. Their attractive color makes them popular for salads and sandwiches, while yellow onions do the heavy lifting in stuffings and meat dishes.

Most onions (​Allium cepa​) are grown from onion sets or onion seeds. While it takes an onion two years to go from seed to mature onion, this can be short-circuited by using onion sets. These small bulbs produced during an onion's first year of growth are pulled out, dried, and sold for spring planting. Once planted in your garden, the sets begin to grow again.

This method of growing onions has the advantage of convenience. Since the sets have a head start, they will mature in the same year that they are planted. On the other hand, growing onions from seeds will result in larger mature onions that store longer than set-grown onions.

Both sets and seeds can be planted as soon as the soil is workable in the spring. Both require a site with full sun. If you opt for seeds, sow them about 1/2 inch deep in a 2-inch-wide band. Thin the seedlings to 3 inches apart. Provide regular water — at least an inch a week but more if your soil is sandy. Stop watering when the bulbs are mature and the tops begin to flop over.

Although you wouldn't want to try growing a crop of onions from roots, it's a fun project to try with a child. Buy a firm red onion at the grocery store and slice off about 2 inches on the bottom — the section where you see roots growing. Place the root end in a shallow dish of water so that the top of the onion slice is above water. Alternatively, suspend it above a glass of water with toothpicks. Place the container with the onion roots in the sun, replenishing the water as needed.

It will take three or four days to notice any growth. When you see the center of the onion slice starting to grow, transplant the onion scrap to a small container filled with potting soil. Keep the soil moist and in the sun. A new onion will develop.