If you’re planning your garden in the North Star State, knowing when to plant onions in Minnesota is the first step to a great harvest. Getting the timing right is crucial for those big, flavorful bulbs.
Onions are a cool-season crop, which means they can handle a chill. But planting them at the wrong time can lead to small bulbs or early flowering. This guide will walk you through the optimal spring schedule and methods for success in our unique climate.
When To Plant Onions In Minnesota
For most of Minnesota, the target window for planting onion sets or transplants is from late April to mid-May. The exact date hinges on your local soil conditions, not just the calendar.
The perfect signal is when your garden soil is workable. It should be thawed, drained, and dry enough that it doesn’t clump when you squeeze a handful. If it forms a muddy ball, wait a few more days. Planting in cold, wet soil can cause the bulbs to rot.
Understanding Your Onion Types: Day Length is Key
Before you plant, you must choose the right type of onion for our northern latitude. Onions are categorized by the amount of daylight they need to form bulbs.
- Long-Day Onions: These are the only type you should grow in Minnesota. They begin forming bulbs when they receive 14-16 hours of sunlight, which aligns perfectly with our summer days. Popular varieties include ‘Walla Walla’, ‘Ailsa Craig’, and ‘Red Zeppelin’.
- Short-Day Onions: These bulb up with only 10-12 hours of daylight. They are suited for southern states and will not perform well here.
- Day-Neutral Onions: Sometimes called intermediate-day onions, they can work but long-day types are the most reliable for a bountiful crop.
Starting from Seed vs. Sets vs. Transplants
You have three main options for getting your onions started, each with its own timeline.
- Seeds: For the widest variety choice, start seeds indoors very early, around mid-February to early March. They need 10-12 weeks indoors before the last frost. This method requires the most time but is often the most rewarding.
- Sets: These are small, dormant onion bulbs from the previous year. They are the easiest for beginners. You plant them directly in the garden in that late April to mid-May window. Be careful, as they can sometimes bolt (flower) too early.
- Transplants: These are young seedling onions started indoors and sold in bunches. They offer a head start without the work of seeds. Plant these outdoors at the same time as sets. This is a fantastic option for most gardeners.
Step-by-Step Planting Guide
- Prepare the Soil: Onions need loose, fertile, and well-drained soil. Work in several inches of compost or aged manure. A balanced, all-purpose fertilizer mixed into the bed a week before planting will give them a strong start.
- Planting Depth & Spacing: For sets and transplants, plant them so the tip is just barely showing above the soil. Space them 4-6 inches apart in rows that are 12-18 inches apart. If planting from seed directly outdoors (rare for spring in MN due to the short season), sow seeds ½ inch deep and thin later.
- Water Them In: Give your newly planted onions a good drink to settle the soil around their roots. Consistent moisture is key, especially during bulb formation.
Caring for Your Growing Onions
Once planted, a little consistent care will ensure they thrive. Onions have shallow roots, so they don’t compete well with weeds. Keep the bed weeded gently by hand, as hoeing can damage the bulbs.
Water regularly, aiming for about 1 inch per week. Mulching with straw or grass clippings can help retain moisture and suppress weeds. As the bulbs begin to swell in mid-summer, you can gently pull soil away from the tops to allow them to expand more easily.
Common Pests and Problems
Onion thrips are the most common pest. They cause silvery streaks on the leaves. A strong spray of water from the hose can dislodge them, or you can use an insecticidal soap if the infestation is severe.
The main disease to watch for is downy mildew, which appears as fuzzy, purplish growth on leaves. Good air circulation, crop rotation, and avoiding overhead watering are the best preventions. Also, never plant onions in the same spot more than once every three years.
Knowing When to Harvest
Your onions will tell you when they’re ready. In late July or August, you’ll notice the tops start to yellow, flop over, and dry out. This is natural. Once about half the tops have fallen over, it’s time to stop watering.
A week later, gently lift the bulbs with a garden fork on a sunny, dry day. Let them cure right on the garden surface for a week if weather permits, or move them to a covered, airy location like a garage. They need to dry thoroughly for storage.
FAQ: Minnesota Onion Planting
Can I plant onions in the fall in Minnesota?
It’s generally not recommended. Our winters are too severe for overwintering onions unless you use specific, very hardy varieties and heavy mulch—and even then, success is not guaranteed. Spring planting is the reliable method.
What if my onions start to flower?
If a central stalk shoots up, the onion has bolted. This is often caused by temperature fluctuations or stress. The bulb won’t grow much larger and won’t store well. You can still eat it, but use it soon. Choose bolt-resistant varieties and plant at the optimal time to avoid this.
How do I get bigger onions?
Big bulbs start with good soil prep and consistent watering. Also, ensure you’re growing long-day varieties. Giving them enough space to grow is crucial; don’t skimp on that 4-6 inch spacing.
Can I grow onions in containers?
Yes! Choose a container at least 10 inches deep. Use a quality potting mix and ensure it has excellent drainage. You’ll need to water more frequently, as pots dry out faster. Follow the same spacing guidelines for a garden bed.
By following this spring planting guide, you’ll set your onion crop up for success. Remember, the secret is in the timing and choosing the right type for our long summer days. With a little patience and care, you’ll be harvesting a supply of homegrown onions that will last you for months to come.