The best time to plant onions in Utah is from early March through mid-April, ideally before April 15. Onions should be planted when the soil is dry enough to avoid compaction and when the soil temperature is above 40 °F, with an optimum of around 75 °F. Early planting allows onion seeds to germinate after spring rains and results in better bulb yields because planting too late can lead to hot temperatures that induce bulb development prematurely, reducing bulb size. Onion sets or transplants can also be planted in April for earlier production. For fall planting or overwintering, onions can be sown in August or September with proper protection for winter survival. Key points for Utah onion planting:

• Seed onions from early March to mid-April (best before April 15).
• Soil temperature should be above 40 °F for germination.
• Onions can also be started indoors in January or February and transplanted outdoors in late April or early May.
• Onion sets or transplants can be planted in April.
• Fall planting for overwintering is possible in August or September with mulch or tunnel protection.

This timing best supports onion growth considering Utah's cool climate, soil conditions, and typical seasonal temperatures.