When To Plant Onion Seeds

Knowing when to plant onion seeds is the single most important step for a successful harvest. Get the timing right, and you’ll be rewarded with beautiful, long-lasting bulbs. This guide gives you the clear, simple steps you need to succeed, whether you’re starting seeds indoors or sowing them directly in your garden.

Onions are a garden staple, but they can be tricky. They need a long growing season and are sensitive to day length. Starting from seed gives you the widest variety to choose from, often at a lower cost than buying sets. Let’s break down the timing and techniques to make your spring garden a success.

This is your core calendar. Timing varies significantly based on your climate and method.

Understanding Your Onion Type: Day Length is Key

Onions are categorized by how much daylight they need to form bulbs.

Long-Day Onions: Need 14-16 hours of daylight. Best for northern states (above latitude 37°). Planted in spring.
Short-Day Onions: Need 10-12 hours of daylight. Best for southern states (below latitude 35°). Planted in fall or winter.
Day-Neutral (Intermediate) Onions: Form bulbs with 12-14 hours of daylight. Adaptable to many regions.

Choosing the wrong type for your location is a common mistake. Check seed packets carefully.

Starting Onion Seeds Indoors

For most gardeners in colder climates, this is the best method. It gives onions the head start they need.

When to Start: Begin seeds indoors 8-10 weeks before your last expected spring frost date. For many, this is mid-to-late winter.

Step-by-Step Indoor Sowing:

Fill a seed tray or pots with a fine, sterile seed-starting mix.
Sow seeds about 1/4 inch deep, spacing them 1/2 inch apart.
Gently water and cover with a humidity dome or plastic wrap.
Place in a warm spot (65-70°F) until germination, which takes 7-10 days.
Once sprouted, move to a very sunny windowsill or under grow lights. Keep soil moist.
Trim tops back to 3 inches tall every few weeks to encourage strong root and stem growth.

Direct Sowing Onion Seeds Outdoors

In regions with milder winters, you can sow seeds directly into prepared garden beds.

For Spring Planting (Long-Day & Day-Neutral): Sow seeds as soon as the soil can be worked in early spring, usually 4-6 weeks before your last frost. The soil doesn’t need to be warm, but it should not be soggy.

For Fall Planting (Short-Day): In the South, sow seeds in fall so they establish roots before winter. They’ll resume growth in spring for an early summer harvest. Timing is usually 4-6 weeks before your first fall frost.

Reading Your Frost Dates and Zone

Your local frost dates are your best guide. Use your USDA Hardiness Zone to confirm which onion types grow best in your area. A simple online search for “frost dates [your town]” will give you the information you need.

Regional Timing Cheat Sheet

Northeast & Upper Midwest (Zones 3-6): Start long-day seeds indoors February to March. Transplant outdoors late April to May.
Pacific Northwest (Zones 7-8): Start long-day or day-neutral seeds indoors January to February. Direct sow in early spring.
South & Gulf Coast (Zones 8-10): Plant short-day seeds directly in the garden in October through November.
Southern California & Southwest (Zones 9-10): Plant short-day or day-neutral seeds in fall for a winter/spring harvest.

Preparing Your Garden Bed

Onions need loose, well-draining soil with plenty of nutrients.

Location: Choose a spot with full sun (at least 6-8 hours).
Soil: Work the soil to a depth of 8 inches. Remove any rocks or clumps.
Fertilizer: Mix in a balanced, granular fertilizer or well-rotted compost a week or two before planting. Onions are heavy feeders.
pH: Aim for a soil pH between 6.0 and 7.0. A simple test kit can tell you if you need to add lime (to raise pH) or sulfur (to lower it).

Transplanting Seedlings Outdoors

Hardening off is a non-negotiable step. About 4 weeks before your last frost, begin acclimating your indoor seedlings.

Start by placing them outside in a sheltered, shady spot for just an hour or two.
Gradually increase their time outside and exposure to sun over 7-10 days.
They are ready to transplant when they have 3-4 sturdy leaves.
Plant them about 1 inch deep, spacing them 4-6 inches apart in rows 12 inches apart. Firm the soil gently around them.

Water them in well to settle the roots and reduce transplant shock.

Essential Care Through the Season

Consistent care is what turns small starts into plump bulbs.

Watering: Onions have shallow roots. Provide 1 inch of water per week, more during dry spells. Consistency is key to prevent splitting. Reduce watering as bulbs mature and tops begin to yellow.
Weeding: Keep the area weed-free. Weeds compete fiercely for nutrients and water. Hand-pull carefully to avoid disturbing onion roots.
Fertilizing: Side-dress with a nitrogen-rich fertilizer (like blood meal) every 2-3 weeks until bulbs begin to swell. Then stop to allow the bulbs to firm up.
Mulching: A light layer of straw or grass clippings helps retain moisture, suppress weeds, and keep soil temperatures even.

Common Problems and Simple Solutions

Even with good timing, issues can arise. Here’s how to handle them.

Bolting: This is when an onion sends up a flower stalk. It’s often caused by temperature fluctuations. Choose bolt-resistant varieties and plant at the correct time. Once an onion bolts, use it quickly as it won’t store well.
Small Bulbs: Usually from planting too late, overcrowding, or insufficient water/nutrients. Stick to your schedule and give them space.
Pests: Onion thrips and onion maggots are the main culprits. Use floating row covers as a barrier. For thrips, a strong spray of water or insecticidal soap can help. Crop rotation is essential to prevent maggot issues from building up in the soil.
Diseases: Good air circulation and avoiding overhead watering prevents most fungal issues like mildew. Rotate where you plant your onions each year.

Harvesting and Curing for Storage

Your patience pays off! Harvest time typically comes in mid-to-late summer.

Signs of Maturity: The tops will turn yellow and begin to fall over. Gently bend over any remaining tops to signal the bulbs to stop growing.

How to Harvest: On a dry day, carefully lift bulbs with a garden fork. Brush off excess soil, but do not wash them.

Curing: This step is vital for storage. Lay bulbs in a single layer in a warm, dry, airy place out of direct sun (like a garage or covered porch). Let them cure for 2-3 weeks until the necks are tight and the outer skins are papery.

After curing, trim roots and cut tops back to 1 inch. Store in a cool, dry, dark place in mesh bags or a single layer in a box.

FAQ: Your Onion Seed Questions Answered

What is the best month to plant onion seeds?

It depends entirely on your location. For most northern gardeners starting indoors, January through March is ideal. For southern gardeners sowing outdoors, October through November is common.

Can I plant onion seeds directly in the ground?

Yes, if you have a long enough growing season. In the North, direct sow as soon as soil is workable in spring. In the South, direct sow in fall. Starting indoors is often more reliable for a head start.

How late is too late to plant onion seeds?

If you plant too late, bulbs won’t have time to mature before your growing season ends. As a rule, if you can’t get seeds started at least 100-120 days before your first fall frost, it’s better to use onion sets (small bulbs) for a quicker crop that year.

Do onion seeds need light to germinate?

No, they do not. Plant them 1/4 inch deep in soil. However, once they sprout, they need abundant light—more than most seedlings—to grow strong and stocky.

Why are my onion seedlings so thin and floppy?

This is usually due to insufficient light. They are stretching to find it. Move them to the sunniest possible window or provide supplemental grow lights placed just a few inches above the leaves for 12-16 hours a day.

Getting your timing right with onion seeds sets the foundation for your whole season. By understanding your onion type, respecting your local frost dates, and providing simple, consistent care, you can grow a bounty of onions from tiny seeds. The key is to start early, be patient, and enjoy the process of watching them slowly swell into a staple that will fill your kitchen for months to come.