Grow Lights for Onion Seedlings in Late Winter
Grow lights for onion seedlings hold late winter starts sturdy until they go outside. University of Minnesota Extension advises starting onion seed indoors 10 to 12 weeks before planting out. Hang lights 2 to 4 inches above the seedlings, run them 12 to 16 hours a day plus trim the tops.
The short answer
Hang lights no more than 4 inches above the seedlings, with 2 inches ideal, plus run them 12 to 16 hours a day, as University of Minnesota Extension advises. Iowa State University Extension suggests clipping seedlings back to 4 inches once they reach 5 inches tall for stockier plants.
When should onions be started indoors?
University of Minnesota Extension advises starting seed indoors 10 to 12 weeks before planting outside plus lists early February for onions in Minnesota. Iowa State University Extension gives eight to 10 weeks before outdoor planting, with bright light plus 55°F to 65°F for growth.
| Week | Task | Light setup |
|---|---|---|
| 10 to 12 weeks before planting | Sow seed in one flat | Lights 2 to 4 inches above seedlings |
| As seedlings grow | Raise the lights | 12 to 16 hours a day |
| At 5 inches tall | Clip back to 4 inches | Same schedule |
| Two weeks before planting | Harden off outdoors | Gradually more sun outside |
Source: University of Minnesota Extension plus Iowa State University Extension.
How close should lights be to onion seedlings?
University of Minnesota Extension advises lights no more than 4 inches above seedling tops, with 2 inches ideal, plus notes lack of light is the major cause of elongated, skinny stems. Iowa State University Extension notes a two tube fluorescent shop fixture works for seedlings when hung 4 to 6 inches above them for 12 to 14 hours a day.
Why trim onion seedling tops?
Iowa State University Extension notes that clipping seedlings back to 4 inches when they reach 5 inches tall, sometimes two times or more, gives stockier plants that are more vigorous at planting. University of Minnesota Extension advises trimming tops to 4 inches plus roots to one half inch at transplant.
How to: step by step
- Sow onion seed in a flat in late winter.
- Hang lights 2 to 4 inches above the seedlings.
- Run lights 12 to 16 hours a day on a timer.
- Lift lights as seedlings rise.
- Clip tops back to 4 inches when they reach 5 inches.
- Harden off seedlings for two weeks before planting out.
Troubleshooting
Seedlings flop: tall soft tops. Shear them to 4 inches.
Pale thin seedlings: low light. Set lights 2 to 4 inches above.
Seedlings collapse at the base: damping off. Use bottom heat plus pour off extra tray water.
For related reading see grow lights for cannabis, cannabis seedling light schedule plus hardening off cannabis seedlings. Kind Seed Co also stocks seeds for indoor growers.
Sources
- Growing onions in home gardens, University of Minnesota Extension
- All about onions, Iowa State University Extension, Yard plus Garden
- Starting seeds indoors, University of Minnesota Extension
- Will a fluorescent light fixture provide sufficient light for seedlings?, Iowa State University Extension
FAQ
Do onion seedlings need darkness at night?
Yes. University of Minnesota Extension advises against leaving lights on continuously, since many plants need some dark period each night to develop properly. It suggests 12 to 16 hours of light daily on a simple timer. Iowa State University Extension suggests 12 to 14 hours for seedlings under fluorescent lights.
How long do onions stay indoors?
University of Minnesota Extension advises starting seed 10 to 12 weeks before planting outside, while Iowa State University Extension gives eight to 10 weeks. Iowa State University Extension lists early April to early May as the time to plant onion seedlings in Iowa gardens, so count back from your own date.
Can onion seedlings go out before frost ends?
University of Minnesota Extension notes onion transplants tolerate light frosts plus can be planted when temperatures reach 50°F. Harden them off first. The same source advises two weeks of gradually longer time outdoors, bringing plants in before temperatures drop at night.
Should I start onions from seed or sets?
Each works. Iowa State University Extension notes seed is the least expensive method plus offers more cultivar choice, but is the most difficult because of poor germination plus long lead times. Sets are easy but are usually sold only by color, so the exact cultivar is unknown.
Why do my onion seedlings fall over?
Tall tops get heavy plus flop. Iowa State University Extension advises clipping seedlings back to 4 inches once they reach 5 inches tall plus notes trimmed plants are often stockier. University of Minnesota Extension notes lack of light is the major cause of elongated, skinny stems, so check light height too.
