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Key takeaways
- Onions and leeks need a long head start. University of Minnesota Extension suggests sowing onions indoors in February, about 10 to 12 weeks before planting out.
- Pick the right onion day-length type for your latitude. Northern growers need long-day onions for good bulbs.
- Trim the tops as they grow so plants stay sturdy and upright. UMN Extension says to trim onion tops to about 4 inches at transplant time.
- Bright, cool conditions make stocky seedlings. A greenhouse beats a windowsill for this crop.
- Harden off for about a week before transplanting. UMN suggests five to seven days in a cold frame for leeks.
Why start onions and leeks in a greenhouse?
Onions and leeks are slow. From seed, they need months to reach transplant size, and bulb onions need to be well grown before summer days trigger bulbing. Starting them under cover in late winter gives them that head start.
A greenhouse suits them better than a windowsill. They like bright light and cool temperatures, and in a warm, dim house they stretch into weak, floppy threads. A greenhouse bench in January and February gives them full daylight and cool nights. If you're starting seeds indoors under lights instead, GardenGear's guide to starting seeds indoors covers that setup. Our starting seeds in a greenhouse guide covers the general principles under glass.
Timing: when to sow
Timing depends on your last frost date and when you want to plant out. Extension guidance for a northern climate gives a useful baseline.
| Crop | When to sow (UMN Extension, Minnesota) | How long before planting out | Planting-out conditions |
|---|---|---|---|
| Bulb onions | February | About 10–12 weeks | May, once temperatures reach around 50°F |
| Leeks | Late February or March | Transplants no older than 10–15 weeks | Once early spring weather calms and days are at least 45°F |
Adjust for your region. In milder climates with earlier springs, you may sow in January. In the South, short-day onions are often grown on a different calendar entirely, sometimes sown in fall. Check your state extension service for local dates.
Worked example
Say your planting-out target is the first week of May. Counting back 10–12 weeks lands you between about mid-February and late February. Sowing a week or two earlier is common in a greenhouse, because growth on a cool bench is a little slower than under warm indoor lights.
For leeks, if you plan to transplant in early May and want transplants no older than 15 weeks, sow no earlier than about mid to late January. Late February or March sowing keeps them within UMN's 10–15 week window.
Pick the right onion type
Bulb onions form bulbs in response to day length. Choose the wrong type and you'll get small bulbs no matter how well you grow them.
- Long-day onions start bulbing when days are long. UMN Extension says Minnesota growers should choose long-day types that need 14 or more hours of daylight. They're the choice for northern states.
- Short-day onions bulb on shorter days and suit southern regions. In the north, UMN notes they generally develop small bulbs.
- Intermediate-day (day-neutral) onions sit between the two and suit middle latitudes.
Seed catalogs list the day-length type for each variety. Leeks and bunching (green) onions don't form bulbs, so day length matters much less for them.
Sowing: step by step
- Choose containers. Many growers sow onions and leeks thickly in open trays or deep cells, then transplant clumps. Soil blocks also work well. See soil blocks vs seed trays.
- Use a fine, free-draining seed mix. Moisten it before sowing.
- Sow thinly across the tray or several seeds per cell or block. Onions are often multi-sown, with a few seeds per cell planted out as a clump.
- Cover lightly with mix or vermiculite and firm gently.
- Keep evenly moist. Don't let the surface dry out during germination.
- Provide gentle bottom heat if needed. In a cold greenhouse, a heat mat speeds germination. Follow the mat manufacturer's instructions. Best heat mats and thermostat combos covers options.
- Move off the heat once most seeds are up. Seedlings grow sturdier in cooler conditions.
Growing on: light, temperature and water
- Light: Put trays on your brightest bench. In late winter, every hour of light counts. See greenhouse lighting science if you're considering supplemental light.
- Temperature: Cool conditions after germination keep them stocky. Protect them from hard frost on the coldest nights with frost cloth or a heated propagator. Track lows with a min-max thermometer.
- Water: Keep the mix moist but not soggy, and water in the morning so foliage dries before night.
- Feeding: Once seedlings have been growing a few weeks, a diluted liquid feed helps, following label rates.
- Airflow: Damp, still air invites damping off and mold. Vent on mild days and keep air moving. Seedling problems covers what to watch for.
Trimming the tops ("haircuts")
Onion and leek seedlings grow long, thin leaves that flop over and tangle. Many growers trim them back with scissors every few weeks during growth. The goal is sturdier plants with thicker necks that are easier to handle at transplant. Trimming also helps keep the leaves from lying across wet mix.
At transplant time, UMN Extension says to trim onion tops to about 4 inches long. For leeks, UMN notes you can trim roots to about an inch if necessary to make transplanting easier.
Compost the trimmings or use them in the kitchen like chives.
Hardening off
Greenhouse seedlings need to adjust to outdoor wind, sun and cold before planting. For leeks, UMN Extension suggests hardening off for five to seven days in a cold frame. If you don't have one, set plants outside for longer each day and bring them in at night. The same approach works for onions.
- A cold frame makes this easy. Open it by day, close it at night.
- Start in a sheltered, partly shaded spot and increase sun exposure over the week.
- Keep them watered. Wind dries small trays quickly.
GardenGear's hardening off guide goes deeper if you're new to it.
Transplanting
Onions
Transplant when conditions allow (UMN gives May in Minnesota, once temperatures reach about 50°F). Plant in fertile, well-drained soil in full sun. Set single seedlings or small multi-sown clumps at the spacing your seed supplier recommends, then water in.
Leeks
For long white shanks, UMN Extension suggests setting transplants at the bottom of a six-inch-deep furrow and gradually filling it with soil as plants grow. Some growers drop each seedling into a deep dibbed hole and water it in instead of filling with soil. Either way, the goal is to blanch the lower stem. Our transplanting from greenhouse to garden guide covers the general process.
Common mistakes
- Sowing too late. Onions sown in April rarely make big bulbs. Late winter is the window.
- Choosing the wrong day-length type. Match variety to latitude.
- Growing them too warm. Warm, dim conditions make weak, stretched seedlings.
- Skipping haircuts. Long, tangled tops are hard to separate and plant.
- Rushing out unhardened seedlings. Sudden wind and sun can scorch them.
- Forgetting the labels. Varieties look identical until they bulb.
From harvest to storage
If you're growing storage onions, plan ahead for curing. Our curing and storing guide and best curing and drying racks cover the end of the season.
Frequently asked questions
When should I start onion seeds in a greenhouse?
University of Minnesota Extension suggests sowing onions indoors in February, about 10 to 12 weeks before planting out. Adjust for your last frost date: milder regions often start earlier.
When should I start leeks from seed?
UMN Extension suggests late February or March in northern climates, and says transplants should be no older than 10 to 15 weeks when moved outside.
Should I trim onion seedlings?
Yes. Many growers trim onion and leek tops every few weeks to keep plants sturdy. UMN Extension says to trim onion tops to about 4 inches at transplant time.
What type of onion should I grow?
Choose by day length. Northern growers need long-day onions, southern growers use short-day types, and intermediate-day onions suit middle latitudes. Seed catalogs list the type for each variety.
Do onion seeds need a heat mat?
Not always, but gentle bottom heat speeds germination in a cold greenhouse. Move trays off the heat once most seedlings are up, because cooler conditions produce sturdier plants.
How do I harden off onion and leek seedlings?
Give them about a week of gradually increasing outdoor exposure. UMN Extension suggests five to seven days in a cold frame for leeks, bringing plants in or closing the frame at night.