When to plant onions in my zone? Step-by-step planting calendar and timing guide
Introduction: Why timing matters for onion success
Planting onions at the right time is the single biggest factor between crowded, floppy bulbs and fat, storeable onions. Ask yourself, when to plant onions in my zone? and you will avoid wasted effort and multiple failed crops. Timing controls whether onions bulge before summer heat, survive winter, or only make tops with tiny bulbs.
This guide gives concrete answers, not vague ranges. You will get a zone specific planting calendar, a simple last frost calculation you can do in two minutes, and clear rules for sets, transplants, and seed starting. For example, in USDA zone 5 plant onion sets two to four weeks before your last frost; in zone 8 plan a fall planting for overwintering; in zone 3 start seeds indoors 10 to 12 weeks ahead. You will also learn how to choose short day, long day, or day neutral varieties for your latitude.
Onion types and how they change planting timing
If you searched when to plant onions in my zone? the first decision is day length. Onions are short-day, long-day, or day-neutral, which refers to how many daylight hours trigger bulb formation. Long-day varieties need about 14 to 16 hours, they are best for northern zones, plant them in early spring as soon as soil can be worked. Short-day types begin bulbing at 10 to 12 hours, they suit southern zones, and are often planted in late fall or late winter for spring harvest. Day-neutral types work in middle latitudes, giving more flexible timing.
Seeds versus sets versus transplants changes your calendar. Seeds give the most variety, start indoors 8 to 12 weeks before last frost. Sets are immature bulbs you plant outdoors, they establish fast, plant them about 2 weeks before to 2 weeks after last frost for long-day regions. Transplants offer larger bulbs than sets and can go out a few weeks before last frost. Pick the correct day-length type for your latitude, then choose seed, set, or transplant to fine tune when to plant onions in my zone?
Find your zone and convert it into planting dates
First, find your zone, using the USDA Plant Hardiness Zone Map or your country equivalent. Type your zip code into the map, or contact your local extension for a precise zone. Next, look up your average last frost date. Many extension sites and frost calculators will give a calendar date based on your location.
Now convert zone and frost date into planting dates, step by step.
- Decide seed versus sets or transplants. Seeds started indoors, sow 10 to 12 weeks before your last frost. Sets or transplants go in the ground about 2 to 4 weeks before the last frost.
- Check the onion variety days to maturity. That number tells you how long from transplant or set to harvest.
- Calculate harvest. Planting date plus days to maturity equals estimated harvest date.
Example, when to plant onions in my zone? If you are zone 5, last frost May 15, start seeds indoors around Feb 20, or plant sets around May 1. If the variety needs 120 days, harvest will be late August. Also pick long-day varieties for zones 4 to 7, short-day for zones 8 to 10.
Choose the planting method that fits your zone
If you are asking "when to plant onions in my zone?" choose the method first, then set the date. Seeds, sets, and transplants each have a purpose.
Seeds: start indoors 8 to 12 weeks before last frost for long-day varieties. Best for warm zones if sown in fall so bulbs form during cool winter months. Example, Zone 9, sow in October for a spring harvest.
Sets: plant as soon as soil is workable, usually 2 to 4 weeks before last frost. Best in cold zones with short seasons, for example Zone 4, where sets produce bulbs faster than seeds.
Transplants: move outside 2 to 4 weeks before last frost, after hardening off. Use in cool climates to jump-start growth, or in warm zones plant in fall. Quick rule, seeds 8 to 12 weeks before frost, sets and transplants 2 to 4 weeks before.
Step-by-step planting calendar for common zone groups
If you’re asking "when to plant onions in my zone?" use this simple, calendar-first plan. Pick the row that matches your climate, then follow the exact timing and example months.
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Cold zones, zone 3 to 5: Start seeds indoors 10 to 12 weeks before your last frost, for example late February to mid March if your last frost is mid May. Transplant seedlings 2 to 4 weeks before last frost, or plant onion sets outdoors as soon as the soil is workable, about 4 to 6 weeks before last frost. Choose long-day varieties for best bulb size.
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Temperate zones, zone 6 to 7: Start seeds indoors 8 to 10 weeks before last frost, for example mid March to April. Transplant 4 to 6 weeks before last frost, or plant sets 2 to 4 weeks before. You can also try fall planting in zone 7 with intermediate day-length varieties for earlier spring harvest.
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Warm zones, zone 8 to 10: Plant in fall to overwinter, typically October through January. Start seeds late summer to fall for transplanting in late fall, or set out onion sets in November. Use short-day varieties, because spring planting usually yields poor bulbing in warm climates.
Pro tip, regardless of zone: use well drained soil, keep rows evenly watered, and match variety day-length to your zone for reliable results.
Post-planting care: water, fertility, and thinning
Wondering when to plant onions in my zone? Once they are in the ground, these are the exact care steps that turn transplants or sets into big bulbs.
Watering: water immediately after planting, then keep soil evenly moist. Aim for about one inch of water per week, more in sandy soil, less in clay. For transplants, water every two to three days for the first two weeks if no rain, then taper to weekly deep soaks.
Fertilizing: at planting mix compost or a balanced granular fertilizer into the top two inches of soil. Side dress with a nitrogen boost, for example a light application of a 10 10 10 or blood meal, three to four weeks after planting, then again when bulbs first begin to swell. Cut back on nitrogen once bulbing starts.
Thinning and spacing: thin seedlings to 4 to 6 inches apart for storage onions, or 1 to 2 inches for bunching types. Thin when seedlings are 2 to 3 inches tall, snipping at soil level to avoid root disturbance.
Mulch and signs of establishment: apply one to two inches of straw or shredded leaves to conserve moisture and suppress weeds. Good establishment shows steady green tops, firm bulbs forming at the soil line, and resistance to a gentle tug.
Common problems and quick troubleshooting tips
Bolting: yellow, flowering stalks mean the plant experienced a cold shock or extreme heat. If bolting appears, pull those plants, start fresh, and choose a variety suited to your daylight length. Timing is everything, so check your planting calendar to avoid early cold snaps.
Onion rot: soft necks, brown scale, or a foul smell point to overwatering or poor drainage. Lift and inspect bulbs, discard rotten ones, then improve drainage with raised beds and well drained soil. Plant when soil is warm and not waterlogged.
Pests and small bulbs: thrips, onion maggots, overcrowding, and late planting cause tiny bulbs. Use row cover, rotate crops, space 4 to 6 inches, and follow local guidance on when to plant onions in my zone? for best results.
Conclusion and quick checklist for planting onions in your zone
Keep it simple. Match onion type to your zone, plant sets or transplants 2 to 4 weeks before your last frost in cool climates, or in fall for mild winter zones. Use short-day varieties below 35 latitude, long-day varieties above 40 latitude, and intermediate types in between. Aim for soil temps around 45 to 50 F for reliable root growth.
Quick printable checklist
- Check last frost date for your zone, write it down.
- Choose short-day, intermediate, or long-day onions for your latitude.
- Start seeds indoors 10 to 12 weeks before transplant date, or buy sets.
- Plant 2 to 4 weeks before last frost, soil 45 to 50 F.
- Keep soil moist, side dress with nitrogen at green-up.
- Cure bulbs 2 weeks after top-down, store cool and dry.
Final tip, rotate onions annually, test soil pH, and note exact planting dates for next season success.
