When to start onion seeds

How many weeks before the last frost?

University of New Hampshire Extension gives onions 8 to 10 weeks from seed to transplant and says they are safe to set out 4 weeks before the last frost. That makes them one of the earliest sowings of the year, roughly 12 to 14 weeks before your last frost. With an average last frost of May 10, you would sow indoors February 1 to February 15 and set plants out April 12. Leeks follow the same UNH schedule.

University of Minnesota Extension allows 10 to 12 weeks indoors and starts onions in February in Minnesota, transplanting in May.

Pick the right day-length type first

Bulbing is triggered by day length, so variety matters as much as timing. UMN explains that storage onions grown in Minnesota are generally long-day types, which need 14 or more hours of daylight to form bulbs. Sweet or mild onions are short-day types; they can be grown in the north but usually make small bulbs there. In Florida, the UF/IFAS vegetable gardening guide says bulbing onions must be short-day varieties planted in fall. If you garden somewhere in between, check your state extension service's variety list.

Germination

Onion seed germinates in cold soil, but slowly. Harrington's data (published by Oregon State Extension) recorded seedlings in about 31 days at 41°F, 13 days at 50°F, 7 days at 59°F and 5 days at 68°F, with a 35°F minimum. Iowa State Extension lists 70–75°F indoors, 4 to 20 days to emergence, and 8 to 10 weeks from sowing to planting out.

Unlike most seedlings, onions and leeks don't need individual cells. UMN notes they can be sown in one larger flat and transplanted while still small without harm. UMN's onion guide sows indoor seed three quarters of an inch deep and keeps it evenly moist.

Transplanting

UMN says onion transplants tolerate light frosts and can be planted when temperatures reach about 50°F. At transplanting, loosen the seedlings, trim roots to half an inch and tops to 4 inches, and plant 2 inches deep, 3 to 4 inches apart, in rows 12 to 16 inches apart. Harden them off for several days first, as with any indoor-grown plant.

Direct sowing and sets

You can also direct seed onions as soon as the soil is workable in spring: sow in a 2-inch-wide band a quarter to half inch deep, then thin to 3 to 4 inches apart (UMN). Sets, small bulbs grown the previous year, are another option and go in as soon as the soil can be worked. UMN cautions that most sets sold in Minnesota are short-day types, which bulb poorly that far north.

Common mistakes

• Wrong day-length type. Match the variety to your latitude.
• Starting too late. Onions need 2 to 3 months indoors.
• Too much nitrogen. UMN says excess nitrogen causes late maturity, thick necks that are hard to cure, and poor storage.
• Letting them dry out. Onions are shallow-rooted and need steady moisture.