Step-by-Step Guide to Starting Onion Seeds Indoors Successfully
Starting onion seeds indoors gives you a head start on the growing season and helps produce strong, healthy plants ready for transplanting. With over 30 years of experience growing onions from seed, I’ve developed a simple method to ensure your seedlings thrive.
Step 1: Choose the Right Seeds
Select varieties suited to your climate and growing season (short-day, intermediate-day, or long-day onions). Buy fresh, high-quality seeds from reputable suppliers.
Step 2: Prepare Seed Starting Containers and Soil
Use seed trays or small pots with drainage holes. Fill with a well-draining, sterile seed-starting mix to prevent damping-off and promote root health.
Step 3: Sow the Seeds
- Plant seeds about ¼ inch deep.
- Space seeds evenly to avoid overcrowding.
- Lightly cover with soil and gently water.
Step 4: Provide Optimal Light and Temperature
- Place trays in a warm location (65–75°F) to encourage germination.
- Once seedlings emerge, provide 12–16 hours of bright light daily using grow lights or a sunny window.
Step 5: Water Carefully
Keep the soil moist but not soggy. Use a spray bottle or bottom watering to avoid disturbing seeds or seedlings.
Step 6: Thin Seedlings
When seedlings develop their first true leaves, thin them to about 1–2 inches apart by snipping weaker seedlings at soil level.
Step 7: Harden Off Before Transplanting
About 1–2 weeks before planting outdoors, gradually expose seedlings to outdoor conditions—start with a few hours daily and increase exposure to reduce transplant shock.
My Experience Tip
I’ve found that starting seeds 10–12 weeks before your last frost date gives onions plenty of time to develop strong roots and sturdy leaves, improving transplant success.
Onion Seed Starting Checklist and Timeline
| Weeks Before Last Frost | Task | Details |
|---|---|---|
| 12 weeks | Choose seeds and prepare supplies | Select varieties; get seed trays and soil mix |
| 11.5 weeks | Sow onion seeds indoors | Plant ¼ inch deep; keep soil moist |
| 10–9 weeks | Provide light and maintain temperature | 65–75°F for germination; 12–16 hours light/day |
| 8 weeks | Thin seedlings | Space seedlings 1–2 inches apart |
| 3 weeks before transplant | Harden off seedlings | Gradually expose outdoors for 1–2 hours/day, increasing time daily |
| Transplant date | Move seedlings to garden | Plant when soil is workable and frost risk has passed |
Additional Tips:
- Use a spray bottle for gentle watering.
- Keep seedlings in bright light to prevent legginess.
- Monitor for pests or fungal issues and treat early.
Onion Growing Hub
Start here: The Ultimate Onion Growing Guide: From Seed to Harvest
🌱 Planning & Varieties
- How to Choose the Best Onion Varieties for Your Region – Match flavor, storage, and climate to your garden.
- Daylength and Onion Growth: Understanding Short, Intermediate, and Long-Day Varieties – Pick the right type for your latitude.
- Growing Bulb Onions: Pick the Right Variety for Your Garden – Choose the right variety for where you live.
- Onion Sets vs. Seedlings vs. Seeds: Which Is Best for Your Garden? – Pros and cons of each planting method.
🌿 Planting & Starting
- Step-by-Step Guide to Starting Onion Seeds Indoors Successfully – Timing and troubleshooting for strong transplants.
- Planting Onions Seeds and Sets – Here’s how to get started.
- How to Grow Onion Sets – You can grow your own onion sets from seed for next year.
- Growing Onions in Containers: Soil, Pots, and Care Tips – Small-space planting success.
- How to Grow Green Onions, Spring Onions, and Scallions – Learn how these alliums differ.
- How to Plant, Grow, and Harvest Welsh Onions – Learn to grow these mild-flavored members of the onion family.
🌞 Care & Maintenance
- Watering Onions for Optimal Growth: Techniques and Tips – How much and how often to water.
- Feeding and Fertilizing Onions: Organic and Synthetic Options – Nutrient needs for strong bulbs.
- Companion Planting with Onions: Plants to Grow With and Avoid – Beneficial pairings and plants to avoid.
- Onion Family Growing Problems: Pest, Disease, and Problem Controls – Natural control of thrips, maggots, and more; identifying and treating fungal or bacterial problems.
🧅 Harvest & Storage
- How to Harvest Onions at Peak Maturity for Best Storage – Signs onions are ready and how to cure.
- How to Harvest and Store Onions – Here are the basics for harvesting and storing onions.
- Onion Storage Techniques: Keep Your Harvest Fresh for Months – Best environments for long-term keeping.
🍳 Using Your Onions
- Cooking with Onions: Recipes and Tips to Bring Out Flavor – From fresh salads to caramelized dishes.
- How to Make Onion Soup with No Recipe – Enjoy this tasty classic!
