When to Plant Onions

• Generally speaking, plant onion sets outdoors when the weather is cool—not cold. Ideally, outdoor temperatures shouldn’t dip below 28°F (-2°C) after planting.
  • In regions with a frigid winter, plant onions as soon as the ground can be worked in the spring—usually late March or April.
  • In milder regions, onions can be planted in the late fall or winter. They will sit dormant during the cool season, but will be primed and ready to grow as soon as the longer, milder days of spring arrive. 
• If planting 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.

Preparing the Planting Site

• Select a location with full sun, where your onions won’t be shaded by other plants.
• Soil needs to be well-drained, loose, and rich in nitrogen; compacted, rocky, or clay-heavy soil affects bulb development.
• Add aged manure or compost to the soil in early spring, before planting. Onion plants are heavy feeders and need constant nourishment to produce big bulbs.
• At planting time, mix in some nitrogen fertilizer.
• 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.

How to Plant Onions

• Because they mature much faster (and are less work overall), we recommend growing onions from onion sets (i.e., small onion bulbs) rather than from seeds. However, in mild regions with a long growing season, seeds are an option as well.
  • Tip: 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.
• If planting from seed, bear in mind that onion seeds are short-lived, so start with fresh seeds each year. Start seeds indoors about 6 weeks before transplanting to the garden.
• When planting onion sets, plant them between 2 and 6 inches apart, and don’t bury them more than 1 inch under the soil.
• When planting transplants into the garden, space plants 4 to 5 inches apart in rows 12 to 18 inches apart.
• Add straw mulch between rows of onions. This will help retain moisture and stifle weeds.
• 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.