When to Plant Onions: Sets, Seeds and Transplants by Region

Onion timing trips people up because the plant is reading two clocks. It needs cool weather early to grow leaves, and it starts forming a bulb once days reach a certain length. Each leaf it grows before then becomes a ring of the bulb, so the goal is a big, leafy plant by the time bulbing starts. Plant too late, or pick a variety built for a different latitude, and you get small bulbs or plants that never bulb at all. This guide covers both clocks, using University of Minnesota, Texas A&M and Kansas State extension advice.

Step 1: pick the right day length

Onion varieties are grouped by how many hours of daylight trigger bulbing. Texas A&M lists the groups below, and Kansas State gives the latitudes where each does best. The ranges overlap, so treat them as a guide and check the seed or plant catalog.

As a rough guide, if you live north of about the Kansas–Nebraska line or the Mason–Dixon line, long-day onions are usually the safe choice, and in the Deep South and Texas short-day onions are. University of Minnesota Extension notes that short-day onions tend to stay small in Minnesota, while long-day storage onions need 14 hours or more of daylight. Kansas State recommends intermediate-day varieties for all of Kansas.

Step 2: spring planting dates (cold-winter areas)

Most of the US plants onions in early spring. UMN says sets can go in, and seed can be direct-sown, as soon as the soil can be worked; transplants handle light frost. Our planting calendar uses about 3 to 4 weeks before the last spring frost for setting out onions, and about 12 to 14 weeks before it for starting seed indoors, which leaves the 10-to-12-week indoor head start UMN and K-State recommend. Here is that schedule for common frost dates:

These are planning dates, not guarantees. Wait if the soil is waterlogged or still frozen, and don't worry if a light frost follows planting. For a sense of scale, Kansas State plants onions in mid to late March, and in Minnesota, UMN describes starting seed indoors in February and setting transplants out in spring.

Step 3: fall and winter planting (mild-winter areas)

Where winters are mild, short-day onions grow through the cool months and bulb as days lengthen in spring. Texas A&M's schedule for Texas: For everything else that goes in during the cool months, see what to plant in winter.

• Seed: sow October through December, about ¼ inch deep and 1 inch apart. Thin to one plant every 2 to 3 inches when the plants are about 6 inches tall, and eat the thinnings as green onions.
• Transplants or sets: plant from mid-January to early February, about ¾ inch deep and 3 inches apart. Texas A&M warns not to plant them deeper than 1 inch.
• Harvest: bulbs mature in late spring and early summer, from about May through July depending on variety and region.

Short-day onions are often sweet and mild, and they generally don't store as long as long-day storage types. If you're in between, such as the upper South, intermediate types planted in late winter or very early spring are usually the safer choice. Check with your local extension office for exact dates.

Seeds, sets or transplants?

K-State suggests rows as close as 15 inches, with plants 2 to 4 inches apart depending on the bulb size you want. Onions have shallow roots and don't compete well with weeds, so give them full sun, loose, well-drained soil and steady water; UMN suggests about an inch a week.

When onions are ready

UMN recommends harvesting bulb onions when about half of the tops have fallen over. Pull them, let them dry in a warm, airy, shaded place for two to four weeks until the necks are tight and the skins papery, then store them cool and dry. Bulbs that send up a flower stalk usually don't store well, so use those first.

Where onions fit in the garden

• Good neighbors: extension sources list the onion family as companions for tomatoes, carrots and cabbage-family crops. See what to plant with onions.
• Keep apart: beans and peas, which several sources list as poor neighbors for onions and garlic.
• Rotate: avoid planting onions where garlic, leeks or other onions grew recently, since they share pests and diseases. For garlic timing, see when to plant garlic.

Frequently asked questions

When should I plant onion sets?

In cold-winter areas, plant onion sets in early spring as soon as the soil can be worked, about 3 to 4 weeks before your last spring frost. In mild-winter areas such as Texas, sets and transplants go in from mid-January to early February.

When do I start onion seeds indoors?

About 10 to 12 weeks before you plan to set the transplants out. With onions planted out 3 to 4 weeks before the last frost, that means starting seeds about 12 to 14 weeks before your last frost date.

Can you plant onions in the fall?

In mild-winter areas, yes. Texas A&M suggests sowing short-day onion seed from October through December. In cold-winter areas, onions are usually planted in spring; garlic is the allium that goes in during fall.

What is the difference between short-day and long-day onions?

The hours of daylight that trigger bulbing. Short-day onions bulb at about 11 to 12 hours and suit the South. Long-day onions need about 14 to 16 hours and suit the North. Intermediate types bulb at about 12 to 13 hours.

How deep do you plant onions?

Texas A&M suggests about ¾ inch deep for transplants and sets and no deeper than 1 inch, and about ¼ inch deep for seed. Kansas State plants sets 1 to 1½ inches deep.

Can onions survive a frost after planting?

Yes. Onions are a cool-season crop, and transplants handle light frost, which is why they can be planted several weeks before the last spring frost.