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Vegetable Crop Calendars (A–C)Onion Planting Calendar: Sets vs. Seed
Three ways to start onions, three different results. The method you pick decides how early you plant, how big the bulbs get and whether they store.
Key points
- Sets suit green onions; transplants suit big bulbs; seed offers the widest variety choice.
- Plant as soon as soil is workable, because early leaf growth drives bulb size.
- Count back 10 to 12 weeks for indoor seeding when growing your own transplants.
- Match day-length group to your latitude using local extension advice.
What is the difference between onion sets, transplants and seed?
Nebraska Extension describes sets as small bulbs planted in spring, transplants as seedlings started indoors, and seed as the option for direct sowing in the garden. Each has a job. Sets are convenient, transplants give a strong start toward large bulbs, and seed gives you access to varieties that are not sold as plants.
None of them is wrong. Pick the one that matches what you want from the harvest, not the one that is easiest to buy at the store in March.
Cost and effort differ as well. Seed is the cheapest per plant but demands the most time indoors, while sets cost more and need no equipment. Plants bought in bunches fall between the two.
Which method gives the biggest bulbs?
Nebraska Extension says the logic is simple: more leaf growth means bigger bulbs, so early planting matters. Transplants, which they describe as seedlings started indoors about 10 to 12 weeks before planting, do not bloom the first year and so have more time to size up.
Sets are the weak choice for large storage bulbs. The same source notes they tend to flower in midsummer, which stops bulb growth, so they suit green onions better than storage onions.
Fertility and weeds matter as much as the start method. Onions have shallow roots and short leaves, so they lose badly to weed competition. Mulch lightly once plants are established, and water so the top few inches never dry out completely.
When do you plant onions?
Do not plant by the date on a seed rack. Nebraska Extension's own timing is for Nebraska: seed sown directly from mid to late March once soil can be worked, and transplants set out in early May once the soil is workable and daytime temperatures reach 50 degrees. Your window may be weeks earlier or later.
The method to borrow is this: onions tolerate cool weather, so the signal is soil that crumbles instead of clumping, not warmth. Count back about 10 to 12 weeks from that point to set your indoor seeding date for transplants.
For example, if soil in your garden is workable around the second week of March, a transplant date a few weeks later and an indoor seeding date about ten to twelve weeks before that would give you a calendar, with local frost risk checked each step.
How do you start onion seed indoors?
Sow into flats or deep cells under bright light, keep the medium evenly moist, and trim the tops to a few inches when they flop over, which many growers do to keep seedlings stocky. Cornell's seed starting fact sheet is a good general reference on lights, containers and watering.
Before moving plants out, harden them off for a week or more, easing them into sun and wind. Plant them shallow enough that the base of the plant is barely covered, because deep planting can restrict bulb shape.
Does day length matter when choosing onion varieties?
Yes. Onions begin bulbing in response to day length, and varieties are grouped as short, intermediate or long day. Choosing the wrong group for your latitude can give you tiny bulbs or plants that bolt.
Ask your county extension office which group suits your area and which named varieties do well locally. This question has a local answer, and guessing from a catalog photo is how people end up with a bed of thick-necked disappointments.
If you buy plants or sets, ask where they were grown and for which day-length group. Garden center stock is not always matched to your area.
What should you do with thinnings and extras?
Nebraska Extension suggests thinning direct-sown seedlings and eating the thinnings as green onions. That turns a chore into a bonus, and thinning to even spacing is what lets the remaining plants form real bulbs.
Keep the bed weeded, since onions have shallow roots and compete poorly. Water evenly, and follow your soil test and local recommendations for feeding rather than a generic rule.
Harvest timing and curing are their own topic. For storage onions, wait until tops fall over naturally, then let bulbs dry in a shaded, airy place before storing. Your extension office can describe local curing practice.
How do you build an onion calendar in four steps?
Work from the soil forward rather than from a date on a package. The steps below turn the Nebraska example into a method you can reuse anywhere.
Keep a notebook so each year improves the next.
- Decide your goal: green onions, fresh bulbs or storage bulbs.
- Choose a method that fits that goal: sets for greens, transplants or seed for bulbs.
- Find the date soil becomes workable and set transplant or seed dates around it.
- Back-count 10 to 12 weeks for any indoor seeding.
Timing at a glance
| Item | Window | Caveat |
|---|---|---|
| Onion transplants, Nebraska | Seedlings started indoors 10 to 12 weeks ahead; set out in early May when soil is workable and days reach 50 F | Nebraska guidance; your region and spring will differ, so key off soil workability and your last frost. |
| Onion direct seed, Nebraska | Mid to late March as soon as soil can be worked | Nebraska guidance only; adjust to local soil conditions. |
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.