If you're gardening in northern regions and wondering why your onion harvests have been disappointing, you've likely been using the wrong type of onion sets. Long day onion sets are specifically engineered for northern climates where summer days stretch long, and understanding their unique requirements can transform your harvest from sparse to spectacular.
What Exactly Are Long Day Onion Sets?
Long day onion sets are small, partially grown onion bulbs that begin forming their mature bulbs when daylight reaches 14-16 hours per day. Unlike seeds, sets give you a head start in the growing season and are less prone to bolting (premature flowering) than transplants.
These sets aren't just "onions"—they're precision-tuned for specific geographic conditions. When you select the right type for your location, you'll notice dramatic differences in bulb size, sweetness, and storage capability.
| Onion Type | Daylight Requirement | Best USDA Zones | Planting Season | Storage Life |
|---|---|---|---|---|
| Long Day | 14-16 hours | 5 and north | Early spring | 6-8 months |
| Short Day | 10-12 hours | 7 and south | Fall | 3-4 months |
| Day-Neutral | 12-14 hours | 6-7 | Spring or fall | 4-6 months |
This critical distinction explains why southern gardeners often struggle with long day varieties and northern growers fail with short day types. According to the Michigan State University Extension, mismatched onion types account for over 60% of poor onion harvests in home gardens.
When to Plant Long Day Onion Sets: Timing Is Everything
Getting the planting date right separates successful onion growers from frustrated gardeners. Long day onion sets need to establish strong root systems before the critical day length triggers bulb formation.
Plant your sets 4-6 weeks before your last expected frost date, when soil temperatures reach at least 35°F (2°C). In most northern regions, this means planting between late March and mid-April. The Cornell University Cooperative Extension recommends using soil thermometers for precision, as planting in cold, wet soil leads to rot.
Long Day Onion Planting Timeline
- February-March: Prepare soil with compost and balanced fertilizer
- Late March-Early April: Plant sets as soon as soil can be worked
- May: Maintain consistent moisture (1" water weekly)
- June: Daylight triggers bulb formation as days lengthen
- July-August: Reduce watering as bulbs mature
- August-September: Harvest when tops fall over naturally
Step-by-Step Planting Guide for Maximum Yield
Follow these research-backed steps for the best results with your long day onion sets:
Soil Preparation
Onions thrive in loose, well-drained soil with a pH between 6.0-6.8. Two weeks before planting, incorporate 2-3 inches of compost and a balanced organic fertilizer. Avoid fresh manure, which can cause кнопкling (excessive side-shoots).
Planting Technique
Plant sets with the pointed end up, just below the soil surface (about 1" deep). Space them 4-6 inches apart in rows 12-18 inches apart. For larger bulbs, give them more space—6 inches between sets yields significantly bigger onions according to USDA Agricultural Research Service trials.
Watering and Feeding Schedule
Consistent moisture is critical during the first 6-8 weeks. Water deeply once or twice weekly to maintain even soil moisture. When bulbs begin forming (around June), reduce watering to encourage proper curing. Overwatering during bulb formation leads to rot and poor storage quality.
Top Long Day Onion Varieties for Northern Gardens
Not all long day onions perform equally. These research-tested varieties deliver reliable results:
- Yellow Globe - Classic storage onion with excellent flavor, stores 6-8 months
- Sweet Sandwich - Exceptionally sweet for fresh eating, best used within 4 months
- Red Zeppelin - Vibrant color for salads, stores 5-6 months with proper curing
- Ebenezer - White variety perfect for salsas and pickling, stores 4-5 months
- Walla Walla - Super-sweet variety requiring northern latitude for best results
The USDA Agricultural Research Service notes that Yellow Globe consistently outperforms other varieties in northern trials for both yield and storage capability.
Avoiding Common Long Day Onion Problems
Even with perfect timing, northern onion growers face specific challenges:
Bolting Prevention
Bolting (flower stalk formation) ruins bulb development. To prevent this:
- Plant sets at the right time—too early exposes them to cold that triggers bolting
- Choose varieties bred for bolting resistance like Yellow Globe
- Remove flower stalks immediately if they appear
Pest Management
Onion maggots are the most destructive pest in northern regions. The Cornell University Extension recommends:
- Rotating planting locations yearly
- Using floating row covers from planting through June
- Applying beneficial nematodes to soil before planting
Harvesting and Storage: Making Your Onions Last
Proper harvesting and curing determine whether your onions rot in storage or last through winter:
- Wait until at least half the tops have fallen over naturally
- Lift bulbs carefully with a garden fork on a dry day
- Cure in a shaded, well-ventilated area for 2-3 weeks
- Trim roots and tops to 1" after curing completes
- Store in mesh bags at 32-40°F with 65-70% humidity
Following these steps, your long day onions should store for 6-8 months. Check monthly and remove any showing signs of decay. The Michigan State University Extension reports that proper curing reduces storage losses by up to 75% compared to rushed or improper curing methods.
Frequently Asked Questions
Can I grow long day onion sets in containers?
Yes, long day onion sets grow well in containers with proper depth. Use pots at least 8 inches deep with drainage holes. Plant sets 4 inches apart in quality potting mix. Container-grown onions need more frequent watering but can be moved to optimize sun exposure. Expect slightly smaller bulbs than garden-grown onions.
Why are my long day onions not forming bulbs?
This typically happens when you've planted the wrong onion type for your latitude. If you're north of zone 5 and using short day varieties, they won't bulb properly. Other causes include planting too deep, insufficient sunlight, or excessive nitrogen fertilizer. Long day onions require 14+ hours of daylight to initiate bulb formation.
How do I prevent my long day onions from rotting in storage?
Proper curing is essential. After harvest, cure onions in a dry, shaded, well-ventilated area for 2-3 weeks until necks are completely dry and papery. Store only perfect bulbs (no cuts or bruises) in mesh bags at 32-40°F with 65-70% humidity. Check monthly and remove any showing decay. Never wash onions before storage.
Can I save seeds from my long day onions for next year?
Yes, but it requires patience. Onions are biennials, meaning they flower and set seed in their second year. After harvesting, store your best bulbs through winter, then replant in spring. They'll produce flower stalks with seed heads in late summer. Note that hybrid varieties won't produce true-to-type seeds, so select heirloom varieties like Yellow Globe for seed saving.
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