Start indoors or direct sow?

Start bulb onions indoors and transplant them, especially in regions with shorter seasons. Onion seedlings tolerate transplanting well, while green onions can also be direct sown.

When to start Onions seeds

Start seeds indoors about 8 to 10 weeks before the average last spring frost. Transplant outdoors 2 to 4 weeks before that frost date once the soil can be worked.

How to sow the seeds

Sow seeds 1/4 inch deep in a sterile seed-starting mix. Use open trays or cells at least 1 1/2 to 2 inches deep, and keep the mix evenly moist. Provide bottom heat if your growing area is cool.

Germination

Seeds usually germinate in 7 to 14 days. Soil temperatures around 65 to 75 F encourage good germination. Keep the mix moist but never waterlogged.

Seedling care

Give seedlings strong light for 14 to 16 hours daily, keeping grow lights close above the leaves. Water from below or gently at soil level, and provide airflow to reduce damping off. Trim tops to about 3 inches if they become floppy.

Transplanting & hardening off

Harden seedlings off for 7 to 10 days before planting. Set them outside in fertile, well-drained soil when they are pencil-thick or close to it, spacing according to the variety. Plant them at the same depth they grew in the container.

Tips

Use fresh seed because onion seed loses viability relatively quickly. Start more plants than needed, since small seedlings can be used as scallions.

General seed-starting guidance for US home gardens; timing depends on your local frost dates and variety. Page updated Aug 4, 2026.