How to Grow Onions

Learn how to ensure a successful onion harvest.

wooden basket with onions in dirt with gloves

Treehugger / Christian Yonkers

The onion is a member of the Allium family and is related to lilies but, to no one's surprise, far better tasting. The part we eat is not a root even though it grows underground; rather, it is a bulb encasing pre-sprouting leaves. Onions are excellent crops for the small grower, take up little garden space in return for a plentiful harvest, and attract few pests—but they do require a bit of feeding. Learn how to grow your own onions with our planting guide below.

Botanical name Allium cepa 
Common name  Garden onion
Plant type  Biennial, commonly grown as an annual 
Size  The flowering stalk may up to 2-3 feet
Sun exposure  Full sun
Soil type  Nutrient-rich, well-drained
Soil pH  Slightly acidic (6.2-6.8)
Hardiness zones   2-9
Native area  Wild onion are found throughout North America; cultivated bulbing onions are thought to have originated in Central Asia.  
Toxicity  Toxic to dogs and cats 

How to Plant Onions

hand plants onion bulb in black dirt
Treehugger / Christian Yonkers

There are three options for starting onions: seeds, sets, and transplants.

Growing From Seed

While you can grow onions from seed, save seeds from heirlooms, and choose from a wider selection of varietals, this is a very slow process. Still, a few ounces of seed can turn into hundreds of pounds of food. Start seeds in flats indoors in February or March, 10-15 weeks before last frost, keeping them warm and moist. Be patient—they take a couple of weeks to germinate. They will be ready to go outdoors in April or May. Bunching onions started from seed may have a better outcome than the large, bulbing types.

Growing Sets

Space them about ¼ inches apart in the fall, water normally, and, bend the tops over before the first frost. After about a week, the bulbs will be ready to dig up to cure and later store in a cool, dry, dark environment of about 40 degrees F, as advised by the National Gardening Association. In the spring, plant your sets with a head start on forming a large bulb that will store well.

If you have a very long growing season, you can plant seeds directly in the soil, ¼ inches deep and just a few inches apart. Soaking in warm water for few hours may hurry germination along. Before they start forming a bulb, harvest every other one to use as spring onions and make some room.

Transplanting Starts

Onions can be transplanted from trays or purchased as pencil-thin seedlings. These are usually sold in bunches that are pre-ordered from a seed vendor. They will look dry but are simply dormant and ready to go in the soil. While you want to get these into the ground as soon as possible, they are not dried out and dying. If you can’t plant them immediately, however, separate them and keep them in a cool, dark place.

For either type of transplant, plant 4 seedlings per hole, cover the roots quickly, and space the bunches 6 inches apart. They don’t need to be planted very deep—just one inch, enough so they can stand up.

Onion Care

planting onions in open forest
Treehugger / Christian Yonkers

Onions are heavy feeders and need plenty of nitrogen. But don’t plant them with legumes—they are the opposite of companion plants, as beans and onions are likely to inhibit each others’ growth. Keep in mind that each leaf represents a ring of the onion bulb, so feed and water the plants to stimulate vegetative growth.

Light

Onion varietals have different “day-length” requirements, meaning the hours of daily sunlight required to form bulbs. In other words, during the growing season, northern latitudes (think Alaska) have extra-long days that suit varietals like Walla Walla, while southern latitudes don’t have that extended summer daylight, so short-day onions like Bermudas or Vidalias work well in those locations. Intermediate or neutral day onions work best across middle latitudes and can often grow in both the north and the south. The sweet, short-day onions are best eaten fresh, as they don’t store as well.

Soil and Nutrients

Onions prefer well-drained, slightly acidic, nutrient-rich soil. A raised bed of about 4 inches works well. You may consider working in an all-purpose fertilizer and compost into the soil before planting, if you feel your soil requirements more nutrients. After adding the fertilizer, apply a side-dressing of fertilizer in a narrow stripe 2-3 inches from the base of the onion at a rate of one pound per 100 feet of row. 

Since onions do not fare well if they must compete with weeds for soil nutrients, remove weeds around the plants carefully, keeping in mind the onion’s roots are not far down. 

Water

Onions need a good amount of water, but because their roots are very shallow, soil should be consistently moist but never soggy. On the other hand, if the soil at the surface is dry and compacted, it will restrict the growth of the bulb. Drip irrigation and mulching around the plants can help. Sprinkler or hose watering should only be done early in the morning, as water on the leaves can lead to disease.

Temperature and Humidity

Fluctuating warm and cold weather can cause the onion plant to flower. It tricks the plant to go dormant and restart, as if it had gone through a winter and a spring, so it believes it has completed its biennial cycle. You can protect your plant with a fabric cover such as Agribon.

Common Pests and Problems

white chicken pecks at dirt outside
Treehugger / Christian Yonkers

Birds are known to lift onion sets by pecking at the skins. To avoid this, remove loose skin at the top of the set before planting.

In addition, thrips are the most common onion pest and can damage leaves by sucking out chlorophyll, reduce yield, and leave scars on the post-harvest onion. Smaller growers should take the following actions to avoid pests: 

  • Put down compost and/or mulch.
  • Surround onion and garlic plots with trap crops, such as chrysanthemum and wildflowers.
  • Hose off plants to wash thrips away.
  • Avoid toxic insecticides and encourage natural enemies that keep onion thrips populations low. 

Onion Varieties

Scallions
Scallions. Kerin Gould / Treehugger

Typical red, white, or sweet yellow onions such as Vidalias and Walla Wallas have about a million culinary uses. But don’t hesitate to branch out and try these other delicious types:

  • Scallions: Also known as spring onions or bunching onions, these are relatively easier and faster to grow and can be planted in several successions throughout a season.
  • Shallots: Shallots are like garlic in that one clove divides into many, but they are like onions in that they have layers inside. In terms of flavor, they also land right between onion and garlic.
  • Torpedo: These bright purple, football-shaped onions are sweet and especially flavorful, without too much bite.
  • Cipollini: These little onions are the size of a flattened ping-pong ball and are great in roasted veggies or on a veggie-kabob skewer, as they caramelize delightfully.
  • Egyptian/Walking Onions: Most unusual, these plants are perennials that produce small bulbs where you'd expect flowers and seeds. They eventually weigh the stem down until they reach the ground and plant themselves, thus "walking" across the garden. Mini-bulbs, leaves, and the main bulb are all edible.

How to Harvest Onions

basket of onions in dark pantry
Treehugger / Christian Yonkers

How do you know when to harvest onions? When the leaves are turning dry and yellow and begin to fall over, push a little soil away from the base of the plant to see if the bulb is forming. Once the leaves die, no more growth will take place, so go ahead and loosen the soil, then pull them out gently, gripping at the base of the leaves. Cure storage onions by laying them in the sun, on the warm soil, or on a pallet. If it's extremely hot, move them to a place that is cooler but still dry, warm, and well-ventilated. You can also hang them to dry. After a couple of weeks check to see if the stems have become totally dry and whether the outer skin is papery. At this point, trim away stems and roots and store the onions in a cool, dry place.

How to Store and Preserve Onions

cutting up onions on board with other ingredients around
Treehugger / Christian Yonkers

Sweet onions with thin skin will have a shorter storage period. Storage onions should be kept in a cool, dry, dark place. Onions and scallions that are intended to be eaten fresh can be kept in the crisper of your fridge. You can also dehydrate chopped or sliced onions to add to recipes later or pickle small onions.

Sprouting Onions

You can eat the greens of any onion you pull when thinning your rows. If they sprout in the pantry, these greens can be eaten, as well. Sprouting is a signal, however, that the rest of the onion is absorbing moisture and will spoil soon.

View Article Sources
  1. English, Jean. "Start Onions From Seed in February March." Maine Organic Farmers and Gardeners, 2009.

  2. "Growing Onions." The National Gardening Center Association.