Growing Onions

How to Grow Onions: freshly harvested

Freshly harvested onions

Growing onions is simple: If you can poke a hole into the ground, you can grow an onion from a little plant. Many of our onion varieties are sold as little seedlings in bare-root bundles rather than as plants in biodegradable pots; each plant will start growing within days after you plant. If you can’t plant your onions right away, remove their bindings and place them in a bucket with 2 inches of moist soil in the bottom. Keep them in a cool, bright place but out of direct sun until you are ready to plant. A sunny basement is ideal.

Soil, Planting, and Care

Red onion transplants are easy to plant and grow.

Uniform and easy to grow, onion plants will get off to a fast start. Separate before planting.

Most gardeners want sweet onions, and the sweetness of an onion is determined by both nature and nurture. For the mildest onions, start with a variety known to produce sweet, mild-flavored bulbs such as Texas Sweet (at southern latitudes) or Walla Walla (at northern latitudes). Get the plants off to a strong start by mixing an organic or timed-release fertilizer into the soil before your plant your onions. This fertilization technique, called “banding,” places nutrients right where young onion roots will find them. They will use up the nutrient supply as they grow. This is good because later, as onions form under lean conditions, they tend to taste sweeter.

Growing onions requires abundant sun and good drainage, and they grow best when the soil pH ranges between 6.0 and 6.8. Raised beds or raised rows made by mounding up soil are ideal, especially if your soil is heavy clay. Mix a 2-inch layer of compost into the soil before placing an organic or timed-release fertilizer into planting furrows, following label rates. Set plants 1 inch deep, so that their roots are well covered with soil but the top of the plant’s neck is not buried too deeply. You don’t want the part of the neck where the leaves grow away from the clear sheath to collect soil or water down between the young leaves, or they can rot. Space plants 6 inches apart in furrows 12 inches apart. Plants will appreciate a starter solution of liquid fertilizer after planting.

Onions planted in raised beds have optimal growing conditions.

Onions can be grown in raised beds, which offer superior drainage.

Onions roots are shallow and not very efficient at taking up moisture, so they need a steady supply of water to grow without interruption. Although they actually recover well from drought and start growing again when watered, it is best to keep the soil consistently moist until the bulbs enlarge.

You may mulch with a light layer of weed-free and herbicide-free grass clippings or another fine mulch. Onions naturally push toward the surface as they form bulbs, and it’s best if the tops of the bulbs are allowed to bask in dry sun. Remove mulch that might keep the expanding bulbs excessively moist.

Seedlings that are about the diameter of a pencil produce the biggest, most beautiful bulbs, so some gardeners sort seedlings by size before planting. Plant the largest ones together only 2 inches apart to start enjoying as green onions in just two or three weeks. Very small seedlings set at close spacing can serve as a second crop of scallions. Use the pencil-sized plants to grow full-sized onions that will produce extra-juicy slices.

Troubleshooting

Create mounded rows for planting onions.

These young onions are growing on a mounded row.

As onions leaves expand, they may be found by tiny black onion thrips, which suck sap from onion leaves. These are hard to see because they hide down in the folds and neck of the leaves. Also be on the lookout for aphids. Finally, weak plants that slowly wilt may be infested with onion root maggots, the larvae of a common fly.

Harvest and Storage

Pull onions before they form bulbs to use as scallions.

Plant extra if you want to harvest some early (before they form bulbs) to use as scallions.

How to Grow Onions: mature onion bulb pushing out of ground

As onions mature, the bulbs begin to push out of the ground. Leave them exposed.

You can harvest young onions just a few weeks after planting if you want to use them as “spring onions” or scallions. There is no perfect size, just pull when they are big enough to suit you.

For full-sized bulbs, let onions grow and mature. They are ready to harvest when the bulbs are big and the tops begin to turn yellow and fall over. Pull them up, shake off the soil, and lay them out to cure with the tops still attached. Any warm, airy location is a good place to do this; you can even sling them over a fence as long as they aren’t rained upon. Bulbs must stay dry and have good air circulation. As the onions cure, the roots will shrivel and the necks above the bulbs will slowly dry – a natural process that helps to seal the top of the bulb, making the onions less likely to rot. After 7 to 10 days, clip off the tops of the onions and the roots with pruning shears, remove as much dry dirt as possible without taking off the papery outer skins, and store your onions in a cool place. Very sweet, juicy onions may be stored, wrapped in newspaper or paper towels, in the fridge.

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FAQs

Which onion will make my eyes water less, white or yellow onion?

Even though white onions have a milder flavor than yellow, the sulfur content is still high. So expect to tear up just the same with both. Refrigerating an onion to make it cold before you cut it helps keep the fumes down.

Which onions are sweeter? Yellow or white?

White onions are notably sweeter than yellow. Walla Walla and Granex types are the sweetest type available.

How often should onions have water?

The soil should feel slightly moist when you poke your finger in at a depth of 1 inch. Be careful not to over-water onions or they can rot.

How do you judge the quality of an onion?

Good-quality onions are firm and free of blemishes and mold. The skin is dry and has an even color. They should also be near the full-size characteristic to the variety that you grow.

Where should onions be stored?

Onions should be stored in a dry, dark, well-ventilated place. They should also be separated from each other so as not to touch. Only sweet onions should be stored in the fridge.