When to Plant Onions by Zone (Short-Day, Long-Day & Intermediate)

Onions are one of the most rewarding crops you can grow, but they're also one of the most misunderstood. The key thing most people get wrong? Day length. Onions form bulbs in response to daylight hours, not temperature, and planting the wrong type for your latitude means you'll end up with scallions instead of full-sized bulbs.

Here's everything you need to know about when and how to plant onions based on your USDA hardiness zone.

Understanding day-length types

Before we get to planting dates, you need to know which type of onion to grow. This is non-negotiable — it's the single biggest factor in whether you get bulbs or not.

Onion planting dates by zone

Not sure what zone you're in? Use our USDA zone lookup tool to find out.

Three ways to plant onions

From sets (small bulbs)

Onion sets are the easiest method. You buy small, dormant bulbs and push them into the soil. They're widely available at garden centers in spring.

• Pros: Fastest to harvest, easiest for beginners, most tolerant of imperfect conditions
• Cons: Limited variety selection, more prone to bolting (going to flower), smaller maximum bulb size
• Planting depth: 1 inch deep, pointy end up, spaced 4–6 inches apart

From transplants (seedlings)

Transplants are young onion plants, usually sold in bundles of 50–75. They produce the biggest bulbs and offer more variety choices than sets.

• Pros: Largest bulbs, best variety selection, less bolting than sets
• Cons: Need to be planted quickly after purchase, more expensive per plant
• Planting depth: 1 inch deep, spaced 4–6 inches apart in rows 12 inches apart

From seed

Growing from seed gives you the most variety options but requires the most time. Start seeds indoors 8–10 weeks before your outdoor planting date.

• Pros: Widest variety selection, cheapest per plant
• Cons: Longest time to harvest, requires indoor starting setup
• Starting indoors: Sow seeds ¼ inch deep in trays. Keep at 65–75°F. Transplant outdoors when seedlings are pencil-thick. See our indoor seed starting guide for detailed setup instructions.

Soil and site preparation

Onions are heavy feeders. Here is what they need to size up.

Common onion growing mistakes

• Wrong day-length type: This is mistake #1 and it's unfixable. A long-day onion planted in zone 9 will never bulb properly. Double-check your zone and buy accordingly.
• Planting too deep: Onion bulbs should form mostly above the soil line. Plant shallow — just deep enough to anchor the plant.
• Letting them go dry: Onions have shallow roots and need consistent moisture, especially during bulb formation. Water regularly but don't waterlog them.
• Planting too close: Crowded onions stay small. Give each plant 4–6 inches. If you want green onions/scallions, you can plant closer (2 inches).
• Not curing before storage: After harvest, cure onions in a warm, dry, ventilated area for 2–3 weeks. Uncured onions rot in storage.

Companion planting with onions

Onions are excellent companion plants. Their smell deters many common pests:

• Good companions: Carrots, tomatoes, lettuce, peppers, strawberries, beets
• Bad companions: Beans and peas (onions stunt legume growth)

For more companion planting ideas, check out our companion planting chart.

Harvesting and storage

Onions are ready to harvest when the tops fall over and begin to dry. Here's the process:

1. Stop watering when about half the tops have fallen over
2. Wait a week, then pull the bulbs
3. Cure in a warm, dry spot with good airflow for 2–3 weeks
4. Trim roots and tops, then store in a cool, dry place

Properly cured long-day onions can store for 3–6 months. Short-day onions are sweeter but store for only 1–3 months.

Quick reference

Use our seasonal planting calendar to see onion planting dates alongside all your other crops. And if you're planning a spring garden, our spring planting schedule will help you sequence everything.