When to Plant Onions: The Authoritative Day-Length & Zone Guide (Zones 4-9)

Quick Answer: When to Plant Onions for the Best Harvest?

For the highest success rate, transplants (seedlings) are recommended. However, Southern gardeners have a special “Fall Seeding” option.

• Northern Gardeners (Zones 4-6):
  • Best Method: Plant Transplants in Spring, 2-4 weeks before the last frost.
• Southern Gardeners (Zones 7-9):
  • Best Method (Easiest): Plant Transplants in Late Winter (Dec–Feb).
  • Alternative (Cheapest): Direct sow Seeds in Fall (Sep–Nov) to overwinter.
• Days to Harvest: 85-100 days from transplants; 110-140 days from fall seeds.

(Scroll down to the Onions Planting Calendar for exact dates based on your frost cycle.)

Hello, I’m Yun, founder of The Planting Key. Of all the vegetables I’ve researched, none has a more fascinating or misunderstood “secret” than the onion.

For many new gardeners, planting onions is a gamble that ends in frustration—either the plants “bolt” (flower) prematurely or they produce bulbs the size of a marble. The truth is, their success or failure was decided the moment they planted.

Why? Because the #1 rule for planting onions is not just about temperature. It’s about a counter-intuitive biological trigger called Day-Length.

I have synthesized research from top agricultural universities to create this guide. Unlike generic calendars, the dates below are calculated specifically based on the frost dates of your zone.

The #1 Secret: Why “Day-Length” Beats Temperature

This is the “Ah-ha!” moment. Onions are “photoperiodic,” meaning they only start to form a bulb when the number of daylight hours hits a specific trigger.

Here is the twist that confuses everyone: Most people assume the South has “more sun.” But strictly speaking of hours of daylight, summer days are actually significantly longer in the North than in the South. This simple geographical fact is exactly why Northern gardeners need “Long-Day” onions (to match their long summer days) and Southern gardeners do not.

Long-Day Onions (North: Zones 4-6)

Trigger at 14-16 hours of daylight. You must plant in early spring to maximize growth before the summer solstice.

What if you plant the wrong one? If a Northerner plants a Short-Day onion, the bulb will trigger too early in spring while the plant is still tiny, resulting in a puny bulb.

Short-Day Onions (South: Zones 7-9)

Trigger at 11-12 hours of daylight. You must plant in Winter (before the days get too long in spring) to establish roots.

What if you plant the wrong one? If a Southerner plants a Long-Day onion, the 14-16 hour signal never arrives (southern summer peaks at only ~14 hours). You will get lush green tops, but zero bulbs.

Pro Tip: What about “Intermediate-Day” Onions?

If you live in the transition zones (Zone 5b, 6, or 7), you might see “Intermediate-Day” varieties. These trigger at 12-13 hours. They are an excellent “safe bet” for gardeners who struggle with timing, as they are more adaptable than the extreme Long or Short types.

Method 1: The “High Success” Strategy (Transplants)

Recommended for: Beginners & Zones 4-9

This guide’s calendar focuses on Transplants (young seedlings about the thickness of a pencil). While you can grow from seeds or sets, transplants are the superior choice for beginners for three reasons:

1. Avoiding the “Bolt”: UC Agriculture & Natural Resources warns that much of the energy in onion sets “seems to go into producing seed stock instead of bulbs.” They also note that unless sets are stored perfectly, they “tend to bolt” rather than produce a large onion. Transplants avoid these risks.
2. The “Wrong Variety” Trap: A crucial insight from the University of Minnesota reveals that most onion sets available in their region are actually “Short-Day” varieties. Planting these generic sets in the North is biologically doomed to fail. Transplants are usually sold with accurate variety names.
3. Easier Management: In the South, growing from seed requires careful weeding through the winter. For home gardeners, transplants offer a simpler, more reliable start than seeds.

The Northern Strategy (Zones 4-6)

Goal: Maximize leaf growth in cool spring, before the Summer Solstice triggers bulbing.

• Variety Type: Long-Day (or Intermediate-Day)
• The Challenge: You must plant early, but planting too early in frozen soil can stunt growth.
• The Rule: According to Cornell University, you should plant your transplants 2 to 4 weeks BEFORE your average Last Frost Date (LFD), as soon as the soil is workable.

The Southern Strategy (Zones 7-9)

Goal: Establish a strong root system in winter so the plant can explode with growth in early spring.

• Variety Type: Short-Day
• The Challenge: Do not wait for “Spring.” If you wait until your Last Frost Date to plant, it is too late. The days will already be too long, and the heat will stunt the bulbs.
• The Rule: Data from Texas A&M AgriLife Extension specifically recommends planting transplants 4 to 10 weeks BEFORE your average Last Frost Date.
  • What This Means: In practice, for most Southern zones, this calculation places your optimal window in Late Winter (December – February). This timing allows the roots to establish in cool soil before the heat of spring arrives.

Method 2: The “Budget” Strategy (Direct Seeding)

Recommended for: Southern Zones 7-9 Only

If you live in the South, you have a second option used by commercial farmers: Fall Seeding.

• The Logic: You plant seeds directly in the garden in the fall. The onions germinate, grow slowly as small “grass-like” seedlings through the winter, and explode with growth in spring.
• The Pros: Extremely cheap.
• The Cons: You must weed the bed carefully all winter.
• The Rule: According to Texas A&M, sow seeds 8 to 10 weeks BEFORE your First Fall Frost Date.

Quick Reference: Planting by State

Not sure of your Zone? While exact zones vary by zip code, use this cheat sheet for common locations, or check the official USDA Zone Map here for precision.

• Texas, Georgia, Florida, & Louisiana: Most gardeners here are Zones 8-9.
  • Strategy: Follow the Southern Strategy. You must plant Short-Day varieties in Winter (Dec–Feb).
• North Carolina (NC), Tennessee, & Arkansas: You are likely in Zone 7 or 8.
  • Strategy: You are on the edge! Stick to the Southern Strategy (Short-Day or Intermediate-Day) and plant in Late Winter.
• Missouri, Virginia, & Kentucky: You are often Zone 6 or 7.
  • Strategy: Look at the Zone 6 or Zone 7 rows in the calendar below. Intermediate-Day onions planted in Late Winter/Early Spring often perform best here.
• Ohio, Pennsylvania (PA), Illinois, & New York: You are primarily Zones 5-6.
  • Strategy: Follow the Northern Strategy. You must plant Long-Day varieties in Spring (March–April).

Onion Planting Calendar (Transplants)

How to Read This Calendar

The calendar below provides key planting dates based on two strategies: Safe (to minimize risk) and Aggressive (to maximize your bulb size).

• Aggressive Dates: These dates push the planting window to the limit.
  • In the North: It means planting as early as possible to maximize leaf growth before the summer solstice. Risk: A hard freeze could damage young transplants.
  • In the South: It means planting in early winter to establish massive root systems. Risk: An extreme freeze could damage plants without protection.
• Safe Dates: These dates prioritize plant survival. The window is more conservative, reducing the risk of frost damage, but potentially resulting in slightly smaller bulbs due to a shorter growing period.

Onion-Specific Timing Rules

• Spring Columns (Transplants): This is the gold standard for most gardeners.
  • North (Zones 4-6): Dates are set 2 to 4 weeks before the Last Frost Date.
  • South (Zones 7-9): Dates are set 4 to 10 weeks before the Last Frost Date. Note that for the “Aggressive” strategy, this often places the planting date in Winter (Dec-Feb). This is correct for Short-Day onions.
• Fall Columns (Seeds):For Southern Zones 7-9 Only.
  • These dates are for direct seeding to overwinter. Dates are set 8 to 10 weeks before the First Fall Frost Date. This ensures seedlings are established enough to survive winter but not so large that they bolt in spring.

✔ Accuracy is my priority. The key planting dates in this calendar are calculated based on representative Aggressive and Safe frost date baselines established for each zone. Because your garden’s microclimate may vary, I strongly recommend that you verify your local frost dates for the most accurate timing. Learn My Methodology & Find a Frost Date Tool Here.

Understanding the Planting Windows & Risks

You might wonder: Why are the planting dates so specific? It comes down to one “Core Rule” of onion growing: The more leaves the plant has before bulbing starts, the larger your final onion will be.

Every leaf produces a ring on the onion bulb. If you have a huge plant with 12-15 leaves when the day-length trigger arrives, you get a colossal onion. If you have a tiny plant with 4 leaves, you get a tiny onion.

This creates a race against time: You must plant early enough to grow those leaves, but not so early that you kill the plant with cold.

The Danger Zone: The Risk of Bolting

“Bolting” is when an onion sends up a flower stalk instead of swelling its bulb. Once an onion bolts, the bulb stops growing and becomes tough/inedible. It is a stress response usually triggered by temperature swings, not day length.

Risk 1: The “False Winter” (Planting Too Early) While we want to plant early, there is a danger. If young plants (especially sets or large transplants) are exposed to prolonged temperatures below 50°F (10°C) and then suddenly warm up, the plant is “tricked.” It thinks it has lived through a winter and matured, so it decides to flower (bolt).

• The Fix: Stick to the “Safe” dates in the calendar if your spring weather is wildly unpredictable. Use row covers to buffer temperature swings.

Risk 2: The Set Size Trap If you choose to use sets (small bulbs) instead of transplants, size matters. Utah State University research found that sets larger than 5/8 inch in diameter (about the size of a dime) are significantly more prone to bolting.

• The Fix: If you buy a bag of sets, sort them. Plant the smallest ones (dime-sized or smaller) for bulbs. Use the larger ones only for green onions (scallions), as they will likely flower before making a bulb.

A Note on “Fall Planting” (Southern Strategy)

In Zones 7-9, the “Fall Planting” columns in the calendar refer to Direct Seeding. The goal here is distinct: you are planting seeds in the fall to establish a small, pencil-thin seedling before growth stops in winter. These tiny plants overwinter and are poised to explode with growth the moment temperatures rise in February, giving you the earliest possible harvest.

Timeline: How Long Until Harvest?

• From Transplants: 85 – 100 days.
• From Fall Seeds: 110 – 140 days (overwintering process takes longer).
• Green Onions: 50 – 70 days.

Key Factors for Success

1. The Soil Temperature Signal

While onions are cold-hardy, they go dormant if it’s freezing. Cornell University suggests waiting until the soil temperature reaches at least 50°F (10°C) for the fastest vegetative growth.

2. Fertility Matters

Onions are heavy feeders. Because they have shallow roots, they need consistent nitrogen to grow those big green leaves.

• Pro Tip: Stop fertilizing when the bulb starts to swell (when the ground cracks around the stem). Fertilizing too late prevents the bulb from curing properly for storage.

3. Protection

• North: If a hard freeze (below 28°F) is threatened after planting, cover your transplants with a floating row cover.
• South: Onions planted in Jan/Feb are generally hardy, but if temperatures drop into the low 20s°F, mulch or cover them to prevent cold damage to the young tips.

Frequently Asked Questions (FAQ)

Your Next Step: Learn How to Grow

You have now mastered the “when”—the most critical step for success. You know your regional planting window and why Transplants are your best bet.

Now, you are ready to master the “how.”

My complete, pillar guide covers everything from preparing the perfect soil and spacing your transplants to mastering the “Neck Check” for harvest time.

Ready to grow? Read my pillar guide:

How to Grow Onions: The Complete Guide from Seed to Harvest