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

Hello, I’m Yun, the founder of The Planting Key. Of all the vegetables I’ve researched, the humble onion is perhaps the most essential. It’s the foundation of countless recipes, and growing your own—whether for sharp, pungent storage onions or sweet, mild slices—is incredibly rewarding.

I know that onions can seem a little mysterious. They have a unique growing cycle, and I’ve heard from many new gardeners who feel frustrated when their plants fail to produce large bulbs. The truth is, their success hinges on one key secret (day-length!) that is often overlooked. But once you understand it, the whole process clicks into place.

I promise this guide will walk you through everything, step-by-step. From selecting the perfect variety (Long-Day vs. Short-Day) to choosing the best planting method, I’ve distilled all my research to give you the confidence to finally harvest those beautiful, papery-skinned bulbs. This is just one of the many topics I cover in my complete library of Vegetable Growing Guides, and I’m excited to share my findings with you.

Core Summary (TL;DR)

For those in a hurry, here are the critical points for success:

• Choose the Right “Day-Length”: This is the #1 rule. Northern gardeners must plant “Long-Day” varieties. Southern gardeners must plant “Short-Day” varieties. Planting the wrong type is the most common reason for failure.
• Choose Transplants for Reliability: While sets are common, planting “transplants” (young seedlings) is my top recommendation for beginners. They are less prone to bolting and produce larger, better-storing bulbs than sets.
• Feed and Water Well: Onions are heavy feeders with shallow roots. They need consistently moist, very fertile soil and zero weed competition to swell into large bulbs.
• Harvest When Ready: Don’t pull your onions too soon. The plants will give you a clear signal when they are done—the green tops will turn yellow and fall over on their own.

Onion Quick Facts Table

When to Plant Onions

Getting the timing right is the single most critical factor for onions. Because they are triggered by day length, planting at the wrong time—even by a few weeks—can result in zero bulbs.

The ideal planting window depends entirely on your USDA Zone and whether you are planting in Spring or Fall.

• 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.

Get Your Calculated Planting Date: I have created a dedicated guide with a precise, calculated calendar for every zone. It explains exactly when to put your transplants in the ground based on your specific frost dates.

Read the Authoritative Guide: When to Plant Onions (Zones 4-9)

Advanced Growing Tactics

Before we get to the step-by-step instructions, here are three pro-techniques to maximize your harvest.

1. Ideal Temperatures for Growth While day-length triggers bulbing, temperature drives growth.

• Soil: As mentioned, ensure your soil is at least 50°F (10°C) before planting for the fastest establishment.
• Air: Onions do their best leaf growing in cool weather (55°F – 75°F). High heat slows down leaf production, which is why establishing your plants early is so important.

2. Succession Planting (For Green Onions) While you plant your main crop of storage onions all at once, you should use “succession planting” for green onions (scallions). To get a continuous harvest, plant a small new batch of seeds or sets every 2-3 weeks, starting from early spring all the way until early fall.

3. Extending Your Season

• Spring Row Covers: Using a floating row cover in early spring is my top tip. It warms the soil faster and protects newly planted transplants from hard freezes.
• Fall Mulching: If you are overwintering onions (Southern strategy), applying a thick 3-4 inch layer of straw mulch after the first frost is essential to insulate the bulbs.

How to Plant, Grow, and Harvest Onions: A Step-by-Step Guide

This is the complete chronological process I follow, from the most important decision to the final, critical step of curing.

Step 1: Choose the Right Variety

This is the most important step. If you get this wrong, your onions will fail to form bulbs, no matter how well you care for them.

Onions are “photoperiodic,” meaning they are triggered to form a bulb by the number of daylight hours.

1. Long-Day Onions: Trigger at 14-16 hours of daylight. Essential for Northern gardeners (Zones 4-6). Examples:’Walla Walla’ (The classic northern sweet onion), ‘Yellow Sweet Spanish’, and ‘White Spanish Sweet’ (Often listed in university trials).

2. Intermediate-Day Onions: Trigger at 12-13 hours of daylight. These are the “safety players,” perfect for Transition Zones (Zones 5-7). They are highly adaptable. Examples: ‘Candy’ (The gold standard for adaptability), ‘Super Star’, and ‘Caballero’ (A reliable yellow variety).

3. Short-Day Onions: Trigger at 11-12 hours of daylight. Essential for Southern gardeners (Zones 7-9). Examples: ‘Texas SuperSweet’ (Also known as ‘1015Y’), ‘Yellow Granex’ (This is the variety grown as “Vidalia”), and ‘Red Creole’.

When I first started researching this for my friend in North America, I was amazed. This concept of day-length is the key I mentioned. Here in Guangzhou, my climate is subtropical; we only grow short-day varieties. A northern gardener trying to grow ‘Texas SuperSweet’ would get lovely green onions, but never a bulb!

You also must choose your planting method:

1. Transplants (Recommended for Beginners): These are live seedlings (about pencil thickness) started in a greenhouse.

Why I love them: Research from UC Agriculture & Natural Resources indicates they are less prone to bolting than sets. They also allow you to buy the specific Long-Day or Short-Day variety you need, avoiding the “mystery bulb” problem of sets.

2. Sets (Fastest & Easiest): These are small, dormant bulbs grown the previous year.

Pros/Cons: They are incredibly easy to plant—just push them into the soil. However, they are biologically older and generally more prone to bolting (flowering) than transplants.

My Pro-Tip: If you use sets, be picky. Research from Utah State University warns that sets larger than 5/8 inch (about the size of a dime) have a significantly greater tendency to bolt. I recommend sorting your bag: use the big sets for green onions, and only plant the small ones for bulbs.

3. Seeds (Most Economical) This is the most economical but also the slowest and most advanced method. To get a head start, seeds must be started indoors 8-10 weeks before your last frost date. This method also offers the widest selection of varieties.

Step 2: Prepare the Perfect Soil and Location

Onions are demanding. They will not perform in poor, compacted, or weedy soil. Success begins before you plant.

Location

Choose your sunniest spot. 8+ hours of direct sun is non-negotiable for large bulbs.

Crop Rotation

This is a key to success, and it’s my number one secret for preventing the buildup of soil-borne pests and diseases. You must avoid planting onions in the same spot where you’ve grown onions, garlic, or leeks in the last 3 years.

Soil Texture & Drainage

Onions need loose, fertile, and well-draining soil. Heavy clay soil is their worst enemy because it holds water and restricts bulb growth.

My Pro-Tip for Clay: If you have heavy clay, the ideal solution is to plant in raised beds. This guarantees the excellent drainage your onions crave.

Soil Fertility & pH

Onions are heavy feeders that require rich soil. The ideal pH is slightly acidic to neutral (6.0 – 7.0).

While a soil test is the best way to know exactly what you need, my general rule is to amend the bed thoroughly.

Before planting, I work a generous layer (about 1 inch) of rich, well-rotted compost or aged manure into the top 6-8 inches of soil. (Note: As the University of Minnesota Extension confirms, never use fresh manure, as it may contain harmful bacteria and increase weed problems).

A Weed-Free Bed

You must clear the bed of all weeds, as onions cannot compete effectively with weeds for nutrients and water.

Pro Technique (The “Stale Seedbed”): Here is a professional method I use, based on research from University of California (UC ANR).

1. Prepare your bed (amend and rake it smooth).

2. Water the empty bed thoroughly and wait a week or two.

3. This “false watering” sprouts all the surface weed seeds.

4. Gently hoe or rake off this new flush of tiny weeds.

5. Now you can plant your onions into a truly clean bed.

Step 3: Sowing Seeds (or Planting Starts/Sets)

How you plant depends on the method you chose in Step 1. I’ll cover my recommended method first.

1. Planting Transplants (Recommended Method) These are the live, green seedlings. They establish very reliably.

My Pro-Tip (Trimming for Success): Before planting, I trim the plants to reduce transplant shock. Trim the green tops back to about 4 inches tall, and trim the roots to about 1/2 inch long.

Depth: Plant about 1 inch deep (2.5 cm), just enough to cover the white root zone.

Spacing: Space them 3-4 inches apart.

2. Planting Sets (Alternative Method) If you chose sets for convenience, plant the small, dormant bulbs into your prepared soil.

How: Press each bulb gently into the soil, pointy-end facing up.

Depth: Plant them about 1 inch deep, just deep enough to firmly cover the bulb tip.

Spacing: Space them 3-4 inches apart.

3. Sowing Seeds (Advanced Method)

As I mentioned, this is the most challenging method, as seeds usually must be started indoors 8-10 weeks before your last frost to get a head start.

Depth: Plant your seeds about 1/4 to 1/2 inch (0.5-1.25 cm) deep.

Spacing: Sow seeds about 1/2 inch apart in rows 12 to 18 inches apart.

You will need to thin them aggressively later, which I cover in the next step.

Step 4: Thinning, Watering, and Mulching

This step is about setting your plants up for success during the main growing season. It starts with giving them space, then building strong roots, and finally protecting the soil.

Part A: Thin Your Seedlings (If You Grew from Seed)

This is a mandatory and (I know!) heartbreaking task, but it must be done.

Once your seedlings are a few inches tall, you must thin them. You must thin before the plants start to compete with each other or begin to bulb.

Final Spacing: Thin them so the remaining plants are 3-4 inches apart. If you don’t, you will get a carpet of tiny, unusable onions.

Pro Tip: You can eat all the thinnings! I toss them into salads or eggs; they are delicious, tender green onions.

Part B: Establish Roots with Consistent Water

Here is a critical difference between onions and other hardy vegetables. Onions have very shallow roots and a high water requirement.

They must not be allowed to dry out.

Unlike plants that you can “toughen up,” any drought stress on an onion will decrease yield, reduce bulb size, and can even make the final flavor hotter or more pungent.

Therefore, for the first few weeks after your plants are in place (either newly thinned or newly transplanted), your goal is to keep the soil evenly moist, like a wrung-out sponge. This encourages the shallow roots to establish quickly without stress.

Part C: Apply Mulch

After this establishment period (about 2-3 weeks in) and once the plants show new, healthy growth, it’s time to apply mulch.

This is your reward for being patient! Spread a thin layer (1-2 inches) of organic mulch, like straw, shredded leaves, or grass clippings (that haven’t been treated with chemicals).

Why is mulch so important for onions? It’s the single best way to solve two problems at once:

Weed Control: This is the #1 reason. Weed control is critical in the first two months, and mulch smothers new weeds.

Moisture Retention: This is the #2 reason. It locks in that even moisture you just established, keeping their shallow roots from drying out.

For detailed advice on the ongoing watering and feeding schedule your onions will need after this establishment phase, I cover that in the “Expert Guide” section below.

Step 5: Harvesting for Peak Flavor

Your onions will tell you exactly when they are ready. Here is the process I follow:

Stop Watering (The First Signal)

As soon as you see the first few onion tops starting to turn yellow and fall over, stop watering completely. This signals the onion to stop growing and to prepare its outer skins for storage.

Watch for the “Green Light”

Your main harvest signal is when the leafy tops have collapsed. I wait until about half (50%) or more of the plants have fallen over on their own. This is the “green light” from your garden.

Lift, Don’t Pull

Once your harvest signal is met, it’s time to act.
Wait for a dry day (ideally cool weather). Harvesting on dry, cool days helps limit post-harvest diseases.

Use a garden fork to gently loosen the soil beside the bulbs. Then, lift them from the ground. Never just pull them by their tops; this can damage the neck and lead to rot in storage.

Step 6: Post-Harvest Handling & Storage

You’re not done yet! To store your onions for months, you must cure them properly. Curing is a two-part drying process.

Part A: The Field Cure (1-2 Weeks, Weather Permitting)

If the weather is dry and sunny, I leave my harvested onions right in the garden for 1-2 weeks.

Pro Technique: It’s a step I never skip. Don’t just leave them exposed; I arrange the onions so the green leafy tops of one row cover the bulbs of the row next to it. This uses the leaves as shade to prevent the bulbs from getting “sunscald” (a sunburn) while allowing the necks to start drying.

If rain is forecast, move them immediately to Part B.

Part B: Complete the Cure Indoors (if needed)

After field drying (or immediately if weather is damp), move the onions to a warm (75°F-90°F), dry, well-ventilated location out of direct sunlight. The total curing time is 2 to 4 weeks from harvest. You’ll know they’re ready when the necks are completely dry and the outer skins are papery.

How: Spread them in a single layer on a screen or a slatted rack so air can circulate all around them.

When is it Done? Let them cure for 1 to 3 weeks (it can take up to four weeks in some conditions). You’ll know they’re ready when the necks are completely dry and shriveled (you can’t feel any moisture), and the outer skins are papery, tight, and dry.

Prep for Storage

Once fully cured, use scissors to snip the roots off the bottom and cut the tops back to about 1 to 2 inches (2-5 cm) above the bulb.

Store

Place them in mesh bags, a wicker basket, or even old nylon stockings (my personal favorite for airflow!). Hang or store them in a cool, dark, and dry place.

Temperature: Aim for 32°F to 40°F (0°C to 4°C). This is the precise range recommended by university sources. As the University of Minnesota warns, onions will start to sprout if stored above 40°F.

Location: A cellar, cool basement, or unheated (but non-freezing) garage is perfect.

Note: Store onions in a cool, dry area rather than in the refrigerator, as the higher humidity may affect storage quality (cause them to spoil or sprout).

Expert Guide to a Healthy Harvest: Tips & Troubleshooting

Here is my advice for the ongoing care and “what-if” scenarios that pop up during the season.

Ongoing Care: Mastering Watering, Mulching, and Feeding

Watering

Onions have shallow roots, so my main rule is consistency. They do not like to swing from bone-dry to soaking wet, water stress or infrequent watering makes onions develop a hot or pungent flavor.

Here is my watering method:

• The 1-Inch Rule: I aim to keep the soil lightly and evenly moist, like a wrung-out sponge. This usually averages out to about 1 inch of water per week (including rain).
• Check Your Soil: This rule changes based on your soil. Sandy soil drains fast and needs water more often. Always check the soil with your finger first.
• Prevent Pungency: Keeping moisture consistent isn’t just for size. As UC ANR research points out, water stress or infrequent watering is what makes sweet onions develop a “hot” or pungent flavor.
• My Pro-Tip (Water in the Morning): I always water in the morning. This allows the plant’s leaves to dry off before nightfall, which is my single best defense against the fungal diseases that thrive on damp leaves.

Mulching

As I explained in Step 4, your mulch is your best friend. I make sure to keep a 3-4 inch layer maintained throughout the season for effective weed suppression.

This one action solves two of the biggest onion problems:

• It blocks weeds, which onions hate competing against.
• It keeps the soil moisture and temperature consistent.

Feeding

Here is my core feeding strategy, which I’ve refined into the onion’s life cycle. Onions are heavy feeders, but when you feed them is the secret to success.

Phase 1: Leaf Growth (Planting to Mid-Summer)

For the first half of their life, onions need a steady supply of Nitrogen (N) to grow as many big, green leaves as possible. (Remember our rule: more leaves = bigger bulbs!)

• What I Do: During this phase, I “side-dress” (apply fertilizer alongside the plants) every 3-4 weeks.
• What to Use: I use a balanced all-purpose organic fertilizer or one rich in nitrogen, like a liquid fish emulsion. University sources recommend high-nitrogen options for side-dressing, such as Urea (45-0-0) as suggested by University of Minnesota, or Ammonium Sulfate (21-0-0) as mentioned by Utah State University.
• How to Apply: Apply the fertilizer about 6 inches away from the plants and gently scratch it into the soil, then water it in.

Phase 2: The Critical Cut-Off (Mid-Summer)

This is the most critical change and the #1 secret to onions that store for months.

Stop all nitrogen fertilizers by mid-summer. As Utah State University recommends, do not fertilize after mid-July in northern climates (adjust timing earlier in warmer regions).

Continuing to feed nitrogen—even a little—into late summer will give you large bulbs with thick, soft necks. These “thick-necked” onions will never cure properly and are the ones that rot in storage.

You must signal to the plant that it’s time to stop growing leaves and start preparing for dormancy. Stopping the fertilizer is the “off-switch.”

Natural Pest and Disease Control

My entire approach to pests and diseases is built on one word: prevention. A healthy, unstressed plant is its own best defense. Most problems can be solved before they start by following a few core principles.

My Core Prevention Strategy (This Solves 90% of Problems)

• Water Smart: As I mentioned in the last section, I always water in the morning. This allows leaves to dry before nightfall, which is a key defense against fungal diseases.
• Space Plants Out: Good airflow is essential. Make sure your plants have the 3-4 inches of space I recommended in Step 3. Crowded plants trap humidity and invite disease.
• Keep it Clean: Remove weeds! They are a “hotel” for pests like thrips. Also, if I see any plant that looks truly sick, I pull it out immediately to prevent it from spreading.
• Rotate Your Crops: As I said in Step 2, never plant onions where onions (or garlic) grew last year. This is the single best way to stop soil-borne pests and diseases (like maggots and rot) from building up.

Common Pests I Watch For

Thrips: They are tiny, slender insects that scrape the leaves, leaving silvery-white streaks. You’ll find them hiding near the base of the leaves.

• My Solution: First, I try a strong blast of water from the hose to knock them off. If they persist, I use an organic insecticidal soap.

Cutworms: These grubs live in the soil and chew through the stems of young seedlings at night.

• My Solution: Prevention! When I plant my sets or transplants, I place a 3-inch “collar” (like a toilet paper tube or a ring of aluminum foil) around the base of each seedling, pressed 1 inch into the soil. The cutworm can’t climb it.

Onion Maggots: These are the larvae of a fly that lays eggs at the base of the plant. The maggots then bore into the stem and bulb, causing the plant to yellow, wilt, and die.

• My Solution: Prevention is the only reliable solution. If you’ve had them before, covering your new plants with a floating row cover immediately after planting. This prevents the adult flies from ever laying their eggs.

Common Diseases (And How to Avoid Them)

Fungal Leaf Issues (Purple Blotch, Botrytis Leaf Blight):

• The Cause: This is almost always caused by too much moisture, poor air circulation, or watering too late in the day (which is why my core strategy is so important!).
• My Solution: Prevention is key. Follow the “Core Prevention Strategy” above. If I see it starting, I remove the affected leaves immediately.

Basal Rot (Growing Season):

• The Cause: Caused by fungi that attack the base of the plant, often due to soggy, waterlogged, or compacted soil.

• My Solution: There is no cure for infected plants; they must be removed. Prevention through long crop rotations and avoiding over-watering is essential.

Neck Rot (Storage Period):

• The Cause: A fungal disease that occurs during storage, causing watery decay inside the bulb.

• My Solution: Prevention is key: avoid excess moisture near harvest and cure bulbs well before storing. A well-cured, dry neck prevents rot from entering.

A Final Note on Pests & Diseases: Remember that pest and disease pressures can vary greatly by region. If you encounter a problem you can’t identify, or if the organic methods above are not effective, your best resource is your local University Extension Service. They often provide free, region-specific advice and diagnostics tailored to home gardeners in your area.

Solving Common Problems with Onions

Problem: My onions sent up a flower stalk (bolted)!

Cause: This is a stress response, but it’s almost always triggered by specific, preventable planting errors:

• Planting Too Early: If young plants are exposed to prolonged temperatures below 50°F, they can be “tricked” into flowering. (Check my When to Plant Onions guide to ensure you are hitting the safe window for your zone).
• Planting Large Sets: You used onion sets that were too large. As I noted in Step 1, sets larger than 5/8 inch in diameter are significantly more prone to bolting.
• Temperature Swings: A sudden, prolonged swing in temperature later in the season (like a hot spell followed by a surprise cold snap) can also be a trigger.

Solution: I’m sorry to say, once an onion bolts, its mission changes. It stops trying to make a big, storable bulb and puts all its energy into that flower. You must pull that onion immediately. You can’t store it, but you can (and should!) still eat it. Use it within a day or two.

Tip: I’ve found that some varieties are more prone to bolting than others. For example, many yellow onion varieties are more likely to bolt under stress than most red onion varieties are. If this is a constant problem for you, try a different, more bolt-resistant variety next year.

Problem: My bulbs are tiny.

Cause: This almost always has one of three causes:

• Wrong Day-Length: You planted a “short-day” variety in the North (or vice-versa). This is the #1 mistake.
• Too Crowded: You didn’t thin your plants (Step 4), so they fought for space and resources.
• Not Enough Food/Sun: You planted in a shady spot or in poor, un-amended soil.

Solution: Unfortunately, you can’t fix this in the current season. But the good news is, you’ve now diagnosed the problem for next year! For now, just harvest and enjoy what you have as “pearl onions.”

Problem: My onions have “thick necks” and rotted in storage.

Cause: This is a classic case of too much nitrogen (feeding) too late in the season.

The high-nitrogen fertilizer kept the leaves green and “juicy” when they should have been drying down. The neck never cured properly, allowing rot to get in.

Solution: As you’re harvesting, identify any onions that feel “thick” or “juicy” at the neck. Set these aside and use them first. They will not store. This is exactly why I am so strict about stopping all nitrogen fertilizer as soon as the bulbs start to swell.

Frequently Asked Questions (FAQ)

References & Further Reading
This guide is based on research from leading agricultural extension services:
Cornell University Cooperative Extension
“Growing Guide: Onions“
University of Minnesota Extension
“Growing onions in home gardens“
Utah State University Extension
“How to Grow Onions in Your Garden“
UC Agriculture and Natural Resources (UC ANR)
“Cultural Tips for Growing Onions and Garlic“
Texas A&M AgriLife Extension
“Onions“
Regional Note: Different universities provide recommendations optimized for their specific climate zones. This guide synthesizes research from cool-climate sources (Cornell, Minnesota) to explain Long-Day onion strategies, and warm-climate sources (Texas A&M) to explain Short-Day onion strategies. This ensures the advice provided is accurate for gardeners across USDA Zones 4-9.