Growing onions successfully in Colorado’s variable climate requires specific attention to variety selection and planting timing. The state’s high altitude, intense solar exposure, and short growing season demand adaptation from traditional gardening methods. Understanding these environmental factors is essential for ensuring the formation of large, healthy bulbs and achieving a productive harvest.
Selecting the Right Onion Types and Timing for Colorado
Onion bulb formation is regulated by photoperiod; the number of daylight hours triggers the switch from leaf growth to bulbing. Due to Colorado’s northern latitude, most of the state requires long-day or intermediate-day varieties, which need 14 to 16 hours of sunlight to develop properly. Planting short-day varieties, which are suited for southern latitudes, will only result in small bulbs or green onions. Popular long-day choices include Copra, Early Yellow Globe, and Walla Walla.
Timing is important because onions are a cool-season crop that must establish significant top growth before the summer solstice triggers bulbing. Starting seeds indoors in January or February is recommended to provide a necessary head start. Planting sets or transplants outdoors should occur in early spring, typically mid-to-late March along the Front Range, as soon as the soil is workable. Planting early allows for maximum vegetative growth before the long summer days initiate bulb enlargement.
Preparing Soil and Planting Methods
Colorado’s native soils are often heavy clay and alkaline, which inhibits the onion’s shallow root system. Optimal growth requires loose, well-drained, and fertile soil to prevent waterlogging and root rot. Amend the planting area by incorporating 1.5 to 2 inches of well-aged compost or organic matter, tilled into the top six to eight inches of soil. This improves drainage and nutrient retention, supporting root development.
While seeds and sets are options, planting transplants is the most reliable method for Colorado’s shorter growing season. Transplants are established seedlings that bypass the lengthy germination stage, reducing the time until harvest. Space plants four to six inches apart to allow for bulb expansion. Plant the sets or transplants shallowly, about one to 1.5 inches deep, ensuring the roots are covered but the neck remains near the soil surface.
Essential Care During the Colorado Growing Season
Onions require consistent moisture, a challenge in Colorado’s arid environment. Since the roots are shallow, the soil must be kept consistently moist but not saturated. Drip irrigation or soaker hoses are effective for delivering steady water directly to the root zone while keeping foliage dry, which prevents fungal issues. Adequate irrigation every five to seven days during warm weather is necessary to support bulb development.
Consistent nutrition is important, as bulb size relates directly to the amount of healthy green top growth established. Onions are heavy feeders, needing a consistent supply of nitrogen until bulbing begins around mid-July. Apply a balanced fertilizer or a nitrogen-heavy side-dressing every two to three weeks until the bulbs start to swell. Garden beds must also be kept meticulously weed-free, as onions compete poorly with fast-growing weeds for water and nutrients.
Managing Onion Thrips
The onion thrip is a common insect pest that thrives in the hot, dry conditions prevalent in the western U.S. Thrips feed by sucking cell contents from the leaves, resulting in silvery streaks and reduced photosynthetic ability. Consistent overhead watering can help physically wash thrips off the plants. For higher infestations, treatment with insecticidal soaps may be necessary. Monitoring for these pests is important, as early bulbing infestations can significantly impact the final harvest size.
Harvesting and Curing Onions for Storage
The onion plant signals readiness for harvest when the green tops begin to yellow and fall over naturally, typically when 75 to 80 percent of the foliage has collapsed. This indicates the plant has finished transferring energy into the bulb. Once the tops begin to fall, immediately stop all watering to initiate a drying-out phase that prepares the onions for long-term storage.
Harvesting involves gently lifting the bulbs from the soil, taking care not to damage the protective outer layers. The subsequent step of curing is crucial for successful storage and involves completely drying the outer skin and neck of the onion. Colorado’s dry air is advantageous for this process, which should be conducted in a warm (75–85°F), dry, and well-ventilated location, out of direct sunlight, for two to four weeks. Once the necks are dry and the outer skins are papery, trim the tops to about one inch and clip the roots. Store cured bulbs in mesh bags or crates in a cool, dark, and dry environment, ideally between 35 and 50 degrees Fahrenheit.