Getting your onion timing right is the single most important step for a great crop. If you’re wondering when to plant onions in Michigan, you’re already on the right track. Our unique climate, with its cold winters and variable springs, demands a specific schedule. Planting at the correct time ensures your onions develop strong roots and big bulbs before summer’s heat arrives.
This guide gives you the clear, practical steps you need. We’ll cover the best dates, how to choose between seeds, sets, and transplants, and the simple planting techniques that lead to success.
When To Plant Onions In Michigan
For a spring harvest, the planting window in Michigan is early. You need to get them in the ground well before your last frost date. Onions are hardy and can handle cold soil and light frosts. This early start is crucial for bulbing, which is triggered by day length later in the season.
Understanding Your Michigan Planting Zones
Michigan spans USDA Hardiness Zones 4 through 6. This means last frost dates can vary by several weeks across the state. Knowing your zone helps you pinpoint your perfect planting day.
- Zone 6 (Southeast & Lake Michigan coast): Last frost around May 1. Plant late March to mid-April.
- Zone 5 (Most of Lower Peninsula & some Upper Peninsula areas): Last frost around May 15. Plant mid to late April.
- Zone 4 (Northern Lower Peninsula & most of Upper Peninsula): Last frost around May 30. Plant early to mid-May.
A good general rule is to plant as soon as the soil is workable—not frozen, and not a muddy mess. If you can squeeze a handful and it crumbles apart, you’re good to go.
Key Dates for Different Onion Types
Not all onions are started at the same time. The form you plant—seeds, sets (small bulbs), or transplants—changes your calendar.
- Onion Seeds (Started Indoors): Begin these 8-10 weeks before your outdoor planting date. For Zone 5, that means starting seeds indoors in late February or early March.
- Onion Sets & Transplants: These go directly into the garden during your main spring planting window (dates listed above). Sets are the easiest for beginners.
Choosing the Right Onion Variety for Michigan
Onion varieties are classified by the amount of daylight they need to form a bulb: long-day, intermediate-day, and short-day. Michigan’s summer days require a specific type.
You must plant long-day varieties. These need 14-16 hours of daylight to trigger bulbing, which is exactly what our northern summers provide. Planting a short-day variety here will result in very small bulbs.
- Excellent Long-Day Varieties for Michigan: Walla Walla (sweet), Patterson (storage), Red Zeppelin (red storage), Ailsa Craig (exhibition).
- Where to Buy: Always check seed packets or plant tags. Reputable local nurseries and seed catalogs will specify “long-day” for our region.
Step-by-Step Planting Guide
Follow these steps for a perfect start. Having your bed prepared ahead of time makes the process smooth.
1. Prepare Your Soil
Onions need loose, well-draining soil with plenty of nutrients. Work in several inches of compost or aged manure a week or two before planting. A soil pH between 6.0 and 6.8 is ideal. A simple test kit can confirm this.
2. Planting Seeds, Sets, and Transplants
- For Sets & Transplants: Plant them about 1 inch deep. Space sets 4-6 inches apart in rows that are 12-18 inches apart. Gently press the soil around them.
- For Direct-Seeding: Sow seeds ½ inch deep and 1 inch apart in rows. You’ll thin the seedlings later to the proper 4-inch spacing.
The pointed tip of a set or the green shoot of a transplant should be just visible above the soil. Don’t bury them to deeply.
3. Watering and Initial Care
Water your newly planted onions thoroughly. Keep the soil consistently moist but not soggy while they establish roots. A light layer of straw or shredded leaf mulch can help retain moisture and suppress weeds, which compete with shallow onion roots.
Essential Care Through the Season
Consistent care after planting is what turns good starts into a great harvest. Onions have shallow roots systems, so they need your attention.
Watering and Feeding
Onions need about 1 inch of water per week. Water at the base of the plants, not overhead, to prevent foliar disease. They are heavy feeders early on. Side-dress with a nitrogen-rich fertilizer (like blood meal or a balanced vegetable fertilizer) about 3 weeks after planting, and again when they begin to bulb.
Weeding Carefully
Weeds are a major competitor for nutrients. Hand-pull weeds gently to avoid disturbing the onion’s delicate roots. Mulch is your best friend here for minimizing this chore.
Knowing When to Harvest
Harvest time depends on whether you want green onions or storage bulbs. For the spring harvest you’re planning, you’ll likely be picking some as green onions first.
- Green Onions (Scallions): You can start harvesting these as soon as the tops are about 6 inches tall, simply by pulling thinnings.
- Bulb Onions: They are ready when the tops naturally fall over and begin to turn brown. This usually happens in late July or August. Once half the tops are down, stop watering to let the bulbs cure.
Curing and Storing Your Harvest
Proper curing is essential for storage onions to last. After pulling them, let them dry in a warm, airy, shaded spot for 2-3 weeks. A garage or covered porch works well. Once the necks are tight and papery and the outer skins rustle, you can trim the roots and tops and store them in a cool, dry place.
Common Problems and Solutions
Even with perfect timing, a few issues can pop up. Here’s how to handle them.
- Bolting (sending up a flower stalk): Caused by temperature fluctuations. Choose bolt-resistant varieties and plant at the correct time. If a plant bolts, harvest and use it immediately—it won’t store well.
- Small Bulbs: Usually caused by planting the wrong day-length variety, overcrowding, or insufficient water/nutrients during bulbing.
- Pests: Onion thrips and onion maggots are the main culprits. Use floating row covers at planting to prevent maggot flies. For thrips, a strong spray of water or insecticidal soap can help.
FAQ: Planting Onions in Michigan
Can I plant onions in the fall in Michigan?
Yes, but it’s for a very early summer harvest, not spring. Plant hardy multiplier or Egyptian walking onion sets in October so they root before freeze. They’ll be the first thing up in your spring garden.
What is the latest you can plant onions?
For decent bulb development, aim to get long-day varieties in the ground by late May at the absolute latest. Planting after this significantly reduces bulb size because the bulbing daylight trigger passes to quickly.
Should I use onion sets or seeds?
Sets are easier and faster, making them great for beginners. Seeds offer a much wider variety choice and are often more economical, especially for large plantings. Starting seeds indoors does require more time and planning.
Why did my onions not form big bulbs?
The top three reasons are: 1) Planting a short-day or intermediate-day variety in Michigan, 2) Planting to late in the season, or 3) Not providing enough space, water, or fertilizer during the key bulbing period in early summer.
By following this schedule and these tips, you’ll give your Michigan onion crop the best possible start. The reward is a long season of fresh flavors from your garden, from crisp green onions in spring to sturdy storage bulbs that last well into winter.