Growing Onions

When to Plant Onions

• Onions can be planted in both the spring and fall seasons, depending on where you live. Generally speaking, plant onion sets outdoors when the weather is cool—not cold.
  • A fall-planted crop of onions needs at least 4 to 6 weeks of warm temperatures to become established in the ground. They will remain dormant during the cool season, but be all primed and ready to grow when spring arrives. Fall onions often grow much larger and better-tasting than same-year planted onions.
  • In regions with a frigid winter, it’s best to plant onion sets in the spring as soon as the ground can be worked—usually in March or April. Ideally, time your planting so that outdoor temperatures no longer dip below 28°F (-2°C) after the onions are in the ground.
• Fall-planted onions are a great way to enjoy an earlier and larger bulbed harvest from next year’s garden. Plant in the warm autumn soil so that they may establish a strong root system before winter sets in. As the cold chill of winter arrives, the crop goes dormant. Then, as the temperatures and soil warm again in early spring, the onions come back to life.
• If starting onions from seeds, start them indoors about 6 weeks before you plan to transplant them to the garden. Onion seeds need temperatures of at least 50°F (10°C) to germinate properly. 
  • Tip: Once the seeds have foliage that’s 

Preparing the Planting Site

• Growing great onions, whether in the spring or fall, all starts with soil that is well-drained, loose, and fertile. Compacted, rocky, or clay-heavy soil will affect bulb development and result in subpar onions.
  • Tip: If you live in an area with heavy clay or hard soil, add in aged compost (humus) to improve soil texture. Contrary to popular belief, adding sand will only make it worse! Read more about preparing soil for planting.
• Add aged manure or compost to the soil in the fall or early spring. Onion plants are heavy feeders and need constant nourishment to produce big bulbs.
• Select a location with full sun, where your onions won’t be shaded by other plants. The more energy they can get from the sunlight, the larger their bulbs can grow.
• At planting time, add about an inch of compost to the bottom of each row before planting. Or, dig a trench in the soil about 2 inches deep and 3 inches wide, and then fill the trench back in with about an inch of compost.
• Practice crop rotation with onions. In other words, don’t plant them in the same location year after year, as this can encourage the spread of diseases that affect the crop. Learn more about crop rotation.

How to Plant Onion Sets

• Choose onion sets that are around ¾-inch in diameter and no bigger. Larger ones may produce stiff necks and go to seed too quickly.
• When planting onion sets, plant them between 2 and 6 inches apart.
• When planting larger transplants into the garden, space plants 4 to 5 inches apart in rows 12 to 18 inches apart.
• Set the bulbs with the point end up and don’t bury them more than 1 inch under the soil. It’s important that onions aren’t planted too deep, as this can affect bulb development.
• Find more tips for planting onions in different regions and soils.

Can You Plant a Sprouted Onion? 

Yes, you can plant a sprouted onion, though you won’t get more onions from it. You will get lots of tasty green sprouts, however! Here’s how to do it:

1. Fill a pot with potting soil and make a hole in the middle that is about the depth and width of the onion.
2. Place the onion in the hole and cover with soil.
3. Water and put the pot in a sunny spot.
4. Harvest the green sprouts as needed for cooking.

If you get a sprout with a flower, wait until the flower goes to seed. Save the seeds for planting in the spring.

Check out this video to learn how to plant onions: