Growing Onions From Seed

Why Grow Onions From Seed?

Growing onions from seed costs about $3 for a seed packet that produces 150+ plants versus $6–10 for a bag of 50 sets. More important, seeds unlock hundreds of specialty varieties — sweet Spanish, Italian reds, storage types, cipollini — that sets and transplants never carry.

Onion sets (the small dormant bulbs sold in bags) are convenient but limited. Most US garden centers carry only two or three varieties as sets: a generic yellow onion, sometimes a red, occasionally a white. Seeds from specialty suppliers like Johnny’s Selected Seeds, Territorial Seed Company, and Southern Exposure Seed Exchange offer dozens of varieties selected for specific US climates, flavor profiles, and storage performance.

Seed-grown onions also store longer than set-grown onions, because they produce a firmer, more complete cure layer. Many experienced US onion growers consider set-grown onions an easy shortcut and seed-grown onions the superior product.

The tradeoff is time. Onion seeds need 10–12 weeks indoors before transplanting — a longer lead time than almost any other vegetable. Starting onions in January for April planting requires commitment and proper grow light setup.

Choosing the Right Onion Variety

Onion variety selection by day length is the most critical decision in onion growing. Plant the wrong type for your latitude and the onions will bolt without forming full bulbs, regardless of how well you grow them.

Examples of each type:

Short-day varieties for southern gardens: Granex (base for Vidalia onions), Red Creole, White Bermuda, Texas 1015Y. These form bulbs when spring days reach 10–12 hours — which happens early in the South.

Long-day varieties for northern gardens: Walla Walla, Candy, Copra, Ailsa Craig, Patterson. These need the long summer days of northern latitudes before they begin to bulb.

Intermediate-day varieties for the middle US: Cabernet, Talon, Superstar. These work across a wide range of US climates.

For companion planting that benefits nearby crops while onions develop, see what to plant with onions and companion planting chart.

Starting Onion Seeds Indoors

Start onion seeds 10–12 weeks before your last frost date. Sow into moist seed-starting mix, cover lightly with just 1/8 inch of mix, and place under grow lights at 65–75°F. Germination occurs in 7–14 days.

Equipment needed:

• 72-cell or 128-cell seed trays, or soil blocks
• Seed-starting mix (not regular potting soil)
• Grow lights — at least 30–40 watts per square foot of growing space
• A timer set for 14–16 hours of light per day
• A heat mat (optional, speeds germination)

Step-by-step:

1. Fill cells with moist seed-starting mix
2. Make a shallow indent, 1/8 inch deep, in each cell
3. Place 2–3 seeds per cell — you will thin to one later
4. Cover seeds lightly with mix and mist the surface
5. Cover tray with a clear dome to retain moisture
6. Set on a heat mat if available, or near a heat source
7. Place under grow lights as soon as the dome goes on
8. Check daily for germination — remove the dome the moment the first sprout appears

Grow lights should be 2–4 inches above seedlings. Onions are heavy light demanders. Insufficient light produces spindly, weak seedlings that perform poorly after transplanting.

For how these plants fit into your broader garden plan, see vegetable garden layout.

Caring for Onion Seedlings

Trim onion tops to 3 inches every 10–14 days when seedlings grow taller. This is counterintuitive but essential — it forces root development and prevents the energy-drain of excessive top growth. Do not let tops grow beyond 6 inches without trimming.

Onion tops are hollow. They flop over when they grow faster than the root system can support. This is normal and expected. Trimming keeps the plant balanced and focused on roots rather than leaves.

After trimming, seedlings look like stubby green tubes for 3–5 days before new growth resumes. By the time transplant date arrives, a well-trimmed seedling should be:

• 6–8 inches tall with the pencil-thick base
• Showing a firm, not wobbly, stem
• Roots filling the cell or soil block

Feed seedlings every 2 weeks starting at 4 weeks old. Use a balanced liquid fertilizer at half strength. Onion seedlings grow slowly — do not overfeed or you risk burning the roots.

If growing multiple onion types, label trays carefully. Long-day and short-day seedlings look identical. Mixing them up results in poor bulb formation in the wrong climate.

Hardening Off and Transplanting

Begin hardening off seedlings 10–14 days before your last frost date. Set trays outside for 1–2 hours on day one, adding an hour each day. Onion seedlings tolerate light frost — they can handle temperatures down to 28°F briefly once hardened.

Onions are one of the most cold-tolerant vegetable crops. Mature seedlings can survive a light frost, which means you can often transplant them a week or two before the official last frost date in your zone. Check your local extension service’s last frost probability chart — “last frost” dates typically represent a 50% probability, so there is often room to push earlier with protection.

Transplanting guidelines:

1. Prepare beds 2 weeks before transplanting — work in 2 inches of compost
2. Soil pH should be 6.0–7.0 (see how to test soil ph)
3. Space transplants 4–6 inches apart in rows 12–18 inches apart
4. Set transplants 1 inch deep — no deeper
5. Firm soil around roots and water in well
6. Mulch lightly with straw to conserve moisture

Do not plant too deep. Onion bulbs develop above the soil surface or just at the surface. Deep planting restricts bulb expansion. If in doubt, plant shallow — the onion will tell you what it needs.

Watering and Fertilizing During Bulb Development

Water onions 1 inch per week through most of the growing season. Reduce watering when the tops begin to fall over — this signals bulbing completion. Stop watering 2 weeks before harvest to improve cure quality.

Onion water needs are moderate but consistent. Irregular watering — drought followed by heavy irrigation — causes cracked bulbs and split necks. A steady 1 inch per week (from rain or irrigation combined) produces the most uniform harvest.

Fertilizer timing is important:

• Planting time: Work 1 pound of 10-10-10 per 25 square feet into the bed before transplanting
• 4 weeks after transplanting: Side-dress with nitrogen — apply 1 tablespoon of blood meal or 2 tablespoons of balanced granular fertilizer per plant, 4 inches away from stems
• When bulbing begins (tops reach 12 inches): Stop nitrogen entirely. Excess nitrogen at this stage delays bulbing and reduces storage quality.

An easy visual check: when the tops begin to flop naturally (not from water stress but from their own weight), the plant is redirecting energy to bulb development. This is the signal to stop nitrogen feeding.

Knowing When to Harvest Onions

Harvest onions when 50–75% of the tops have fallen over naturally. Do not wait for all tops to fall — late-harvested onions have thin necks that do not cure well and store poorly. Pull on a dry day and cure for 3–4 weeks.

Push the remaining standing tops over gently with your foot or the back of a rake if 75% have fallen but some have not. Then let the onions sit in the ground 2 more days for the skins to tighten.

Curing steps:

1. Pull onions on a dry, sunny day
2. Brush off loose soil — do not wash
3. Spread in a single layer in a shaded, well-ventilated location
4. Let cure for 3–4 weeks until the necks feel dry and papery
5. Clip roots and trim tops to 1 inch before storage
6. Store in mesh bags or open crates at 40–50°F with low humidity

Sweet onions (Walla Walla, Vidalia types) have high water content and do not store long — use within 1–2 months. Storage onions (Copra, Patterson, long-day types) keep 6–9 months under proper conditions.

For more on timing and harvest cues, see when to harvest onions and how to grow garlic for a companion crop with a similar seed-to-harvest process.

Common Problems with Seed-Grown Onions

Most seed-grown onion failures trace to one of three causes: wrong variety for day length, inadequate light during seedling stage, or planting too late to complete the growing season before heat triggers bolting.

For related pest and problem management in vegetable beds, see companion planting for natural pest deterrent strategies.

Resource: Johnny’s Selected Seeds at johnnyseeds.com publishes a comprehensive US onion variety guide that includes day-length requirements by region and last-frost-date planting calculators. The resource is free and updated annually.