Minnesota’s late frost dates push onion seed starting to a precise indoor schedule. Knowing the best time to start onions seeds in minnesota is the first step to a big harvest of bulbs or scallions. Onions take a long time to grow from seed, so getting the timing right indoors makes all the difference.
Starting onions from seed gives you way more variety than buying sets. You can grow sweet Walla Wallas, storage-friendly Copras, or red onions. But if you start them too early, they get leggy. Too late, and they won’t size up before winter.
This guide walks you through the exact timeline for Minnesota. We will cover indoor sowing, hardening off, and transplanting. You will also learn how to adjust for your specific zone in the state.
Best Time To Start Onions Seeds In Minnesota
The short answer is: start onion seeds indoors 8 to 10 weeks before your last spring frost date. For most of Minnesota, that means late February to mid-March. The exact date depends on where you live in the state.
Southern Minnesota, around the Twin Cities, has a last frost date around May 10 to May 15. That means you start seeds around March 1 to March 10. Northern Minnesota, near Duluth or the Iron Range, has a last frost date around May 20 to May 25. So you start seeds around February 25 to March 5.
Why this window? Onion seedlings grow slowly at first. They need about 8 weeks to develop into sturdy transplants with pencil-thick stems. If you start them too early, they get root-bound and stressed. Too late, and they won’t have time to form big bulbs.
Understanding Minnesota’s Growing Zones
Minnesota spans USDA zones 3a to 4b in the north, and 4b to 5a in the south. Your zone affects your last frost date and your onion seed starting schedule. Check your specific zone using the USDA map or your local extension office.
Here is a quick breakdown by region:
- Northern Minnesota (zones 3a-4a): Last frost around May 20-25. Start seeds February 25 to March 5.
- Central Minnesota (zone 4b): Last frost around May 10-15. Start seeds March 1 to March 10.
- Southern Minnesota (zones 4b-5a): Last frost around May 5-10. Start seeds March 5 to March 15.
These dates are averages. A late frost can hit in late May even in the south. Always watch the 10-day forecast before transplanting outdoors.
Why Onion Seeds Need An Early Start
Onions are photoperiod-sensitive plants. They form bulbs based on day length. In Minnesota, we grow long-day onions because our summer days are long. These varieties need 14-16 hours of daylight to bulb up.
Starting seeds indoors gives the plants a head start. If you direct-sow seeds in the garden in April, the onions will not have enough time to mature before fall. The growing season in Minnesota is only about 140 to 160 days. Onions from seed take 100 to 120 days to reach full size.
By starting indoors, you add 8 to 10 weeks of growth. That means you can transplant strong seedlings in early May, and they will be ready to harvest by late August or September.
Step-By-Step Indoor Seed Starting Schedule
Follow this timeline for the best results. Adjust the dates based on your region.
- Week 1-2 (Late February to Early March): Gather supplies. You need seed starting mix, trays or pots, grow lights, and heat mats. Onion seeds need warmth to germinate, around 65-75°F.
- Week 3-4 (Mid-March): Sow seeds 1/4 inch deep in moistened soil. Cover with a humidity dome or plastic wrap. Place on a heat mat. Seeds germinate in 7-10 days.
- Week 5-6 (Late March): Remove the dome once seedlings appear. Move trays under grow lights, 2-3 inches above the tops. Keep lights on for 14-16 hours a day.
- Week 7-8 (Early April): Thin seedlings to one per cell or pot. Fertilize weekly with a diluted liquid fertilizer. Keep soil moist but not soggy.
- Week 9-10 (Mid-April): Harden off seedlings. Start by placing them outside in a shaded spot for 1-2 hours. Increase time and sunlight over 7-10 days.
- Week 11 (Late April to Early May): Transplant outdoors after the last frost. Space plants 4-6 inches apart in rows 12-18 inches apart.
Choosing The Right Onion Varieties For Minnesota
Not all onions grow well in Minnesota. Stick with long-day varieties. These are bred for northern climates with long summer days. Short-day onions, which need 10-12 hours of daylight, will not bulb up here.
Here are some top picks:
- Yellow Sweet Spanish: Large, mild bulbs. Good for fresh eating. Matures in 110 days.
- Redwing: Deep red color, stores well. Matures in 110 days.
- Copra: Excellent storage onion. Keeps for 6-8 months. Matures in 105 days.
- Walla Walla: Sweet and mild, but does not store long. Matures in 110 days.
- Patterson: Disease-resistant, stores well. Matures in 115 days.
You can also try scallions or bunching onions. These do not form bulbs and are ready in 60-70 days. Start them at the same time as bulb onions.
Common Mistakes When Starting Onion Seeds
Even experienced gardeners make errors. Here are the most common ones and how to avoid them.
Starting too early: If you start seeds in January, the seedlings will be huge and root-bound by transplant time. They will struggle to establish and may bolt (flower early). Stick to the 8-10 week window.
Not enough light: Onion seedlings need bright light. A sunny windowsill is not enough. Use grow lights placed 2-3 inches above the plants. Keep them on for 14-16 hours a day. Without enough light, seedlings get leggy and weak.
Overwatering: Onion seeds rot easily in wet soil. Keep the soil moist but not waterlogged. Use a spray bottle to water gently. Once seedlings are up, water from the bottom to avoid damping off.
Skipping hardening off: Do not move seedlings straight from indoors to the garden. They need to adjust to sun, wind, and temperature changes. Harden off over 7-10 days. Start with a few hours of shade, then gradually increase.
Planting too deep: Onion transplants should be set at the same depth they were growing in the pot. Burying the stem too deep can cause rot. The roots should be covered, but the green tops should be above soil.
Transplanting Onion Seedlings Outdoors
Once the danger of frost has passed and the soil is workable, it is time to transplant. In Minnesota, that is usually early to mid-May. Soil temperature should be at least 50°F.
Prepare your garden bed. Onions need full sun and well-drained soil. Work in compost or a balanced fertilizer before planting. Onions are heavy feeders, so good soil prep is key.
Dig a shallow trench or use a dibber to make holes. Space plants 4-6 inches apart. If you want larger bulbs, give them more space. For scallions, you can plant them 2-3 inches apart.
Water the transplants well after planting. Mulch with straw or grass clippings to keep the soil moist and suppress weeds. Onions have shallow roots, so they dry out fast.
Fertilize every 2-3 weeks with a nitrogen-rich fertilizer. Stop fertilizing once the bulbs start to swell. That is usually in late June or early July.
Harvesting And Storing Onions
Onions are ready to harvest when the tops fall over and turn brown. That happens in late August or September, depending on the variety and planting date. Do not wait too long, or the bulbs may rot in wet soil.
To harvest, gently lift the bulbs with a garden fork. Do not pull the tops. Lay them in a dry, shady spot for 2-3 weeks to cure. The outer skins will dry and tighten, which helps with storage.
Once cured, cut off the tops and roots. Store onions in a cool, dark, dry place. Good storage varieties like Copra can last 6-8 months. Sweet onions like Walla Walla should be used within a few weeks.
Adjusting For A Cold Spring
Some years, Minnesota has a late spring. Frost can hit in late May or even early June. If that happens, do not panic. You can delay transplanting by a week or two.
If your seedlings are getting too big for their pots, transplant them into larger containers. Use 4-inch pots or even solo cups with drainage holes. Keep them under lights and water carefully.
You can also use row covers or cloches to protect transplants from a late frost. Cover them at night and remove during the day. This buys you a few extra weeks if needed.
Frequently Asked Questions
Can I start onion seeds directly in the garden in Minnesota?
It is possible but not recommended. Direct-sown seeds will not have enough time to mature before winter. The growing season is too short for most varieties. Start seeds indoors for best results.
What is the latest I can start onion seeds in Minnesota?
The latest safe date is around March 20 for southern Minnesota and March 10 for northern areas. Starting later than that means smaller bulbs. For scallions, you can start as late as April 1.
Do I need grow lights for onion seedlings?
Yes, grow lights are essential. A sunny windowsill does not provide enough light for healthy seedlings. Use LED or fluorescent lights placed 2-3 inches above the plants. Keep them on 14-16 hours a day.
How do I know when to transplant onions outdoors?
Transplant after the last frost date for your area. The soil should be workable and at least 50°F. Seedlings should be 6-8 weeks old and have pencil-thick stems. Harden them off first.
Can I start onion seeds in fall for spring planting?
No, onions are biennials. They need a period of cold to flower, but starting them in fall is not practical in Minnesota. The seedlings would not survive the winter. Stick to spring indoor starts.
Final Tips For Success
Growing onions from seed in Minnesota takes planning, but it is worth it. You get more variety and stronger plants. Start with good seeds, provide plenty of light, and watch the calendar.
Keep a garden journal. Write down your start date, transplant date, and harvest date. Next year, you can adjust based on what worked. Over time, you will find the perfect schedule for your garden.
If you have a short growing season, try faster-maturing varieties. Some onions are ready in 90-100 days. Check the seed packet for days to maturity and choose accordingly.
Do not forget to water consistently. Onions need about 1 inch of water per week. In dry spells, water more often. Mulch helps retain moisture and keep the soil cool.
Watch for pests like onion maggots and thrips. Use row covers to protect young plants. Rotate your onion patch each year to prevent soil-borne diseases.
With the right timing and care, you will have a bumper crop of onions. They will store well and add flavor to your meals all winter long. Happy growing.