The successful cultivation of onions in Utah hinges almost entirely on precise timing. Onions are cool-season biennials grown as annuals, requiring a long period of vegetative growth during the cool spring before summer heat and long daylight hours trigger bulb formation. Planting at the wrong time shortens this growth period, resulting in significantly smaller bulbs that may not mature properly. Maximizing the cool spring window allows the plant to develop a substantial green top, which directly correlates with the final size of the storage bulb.
Timing Specifics for Sets, Seeds, and Transplants
The optimal planting date for onions in Utah depends heavily on the method of propagation chosen, as each requires a different lead time. Gardeners must reference their area’s average last spring frost date, which can range from mid-March in warmer Southern Utah (like St. George) to early to mid-April in the central and northern valleys (like Salt Lake City).
Seeds
Starting onions from seed indoors is the most common method for producing large, storage-quality bulbs and requires the longest lead time. Seeds should be sown indoors approximately 10 to 12 weeks before the last expected frost date. For a central Utah gardener with an average last frost around April 7th, this means sowing seeds in late January or early February. This extended indoor period allows the seedlings to reach the pencil-thick stage necessary to thrive once transplanted outdoors.
Sets and Transplants
Onion sets (small, immature bulbs from the previous year) and purchased transplants are planted directly into the garden earlier than the last frost date. These are planted outdoors four to six weeks before the last expected frost. For many areas of Utah, this places the planting window in late February through early April, as soon as the soil is workable. Established onion plants are hardy and can tolerate temperatures dropping as low as 20°F, allowing for this early planting.
Planting sets and transplants early is a calculated risk, as it maximizes foliage growth before the bulbing trigger. Planting too early, especially large sets, can cause the plant to “bolt” (send up a flower stalk), which halts bulb development and makes the onion inedible for storage. To mitigate this risk for storage onions, planting in late April is often recommended.
Selecting the Right Onion Variety for Utah’s Climate
Onion varieties are classified based on the number of daylight hours required to initiate bulbing, a critical factor for Utah gardeners. The state’s northern latitude requires long-day onions, which need approximately 14 to 16 hours of daylight to form a proper bulb. Planting any other type will result in failure to bulb.
Short-day onions, which require only 10 to 12 hours of daylight, are suited for southern states and will only produce scallions or small, immature bulbs in Utah. Intermediate-day onions, needing 12 to 14 hours, may perform well in certain microclimates but are less reliable than true long-day types.
Successful long-day varieties known to thrive in the region include ‘Utah Sweet Spanish’ (a large, globe-shaped heirloom with a mild flavor) and ‘Walla Walla’ (known for its sweetness and colossal size). Other choices are ‘White Sweet Spanish’ and ‘Redwing,’ which offer good storage potential and diverse culinary uses.
Essential Soil Preparation and Early Care
Onions are heavy feeders and require a specific soil environment for successful bulb development. The ideal soil structure is loose, well-drained, and rich in organic matter, such as a sandy loam. Compacted or heavy clay soil restricts the expanding bulb, leading to misshapen or small harvests, so thorough loosening of the bed is necessary before planting.
Onions perform best in a slightly acidic to neutral soil environment (pH 6.0 to 7.0). Before planting, incorporating compost or a complete fertilizer provides the necessary base nutrients. Nitrogen is the most important nutrient for the early growth phase, as it fuels the leaf production that determines the eventual bulb size. Gardeners should side-dress the plants with a high-nitrogen fertilizer in May and June to support the vegetative growth period.
Consistent moisture is crucial immediately after planting, as onions have a shallow root system and are susceptible to drought stress. However, as the bulbs begin to mature and the tops fall over, watering should be stopped to allow the bulbs to cure and prevent rot.