Growing onions successfully in Colorado requires precise timing due to the state’s short, intense growing season and significant variations in altitude and climate. Onion growth is highly sensitive to daylight hours and temperature fluctuations, meaning planting at the wrong moment can result in small bulbs or plants that never bulb at all. Gardeners must accurately gauge their local conditions, particularly the risk of late spring frost, to ensure the crop has enough time to mature before the summer heat and subsequent fall frosts arrive.
Understanding Onion Types and Colorado’s Growing Season
Onion growth in Colorado is primarily determined by photoperiodism, the plant’s response to daylight duration. Onions are categorized into three types based on the number of daylight hours required to trigger bulb formation. Long-day varieties require 14 to 16 hours of sunlight, making them the standard choice for northern Colorado where summer days are long. Intermediate-day varieties, needing 12 to 14 hours of light, are excellent options for the mid-state and Front Range areas. Choosing the wrong variety, such as a short-day onion, will result in a plant that forms only green tops without producing a usable bulb.
Colorado’s last spring frost date varies widely, ranging from mid-April in lower, warmer areas to as late as July in the high mountains. Along the Front Range, the average last frost is typically between early to late May. Onions are a cool-season crop, tolerant of light frost, and benefit from a very early start in the cool spring soil. Planting before the last frost date allows the plant to develop large top growth, which directly correlates to the final size of the bulb.
Practical Planting Methods and Specific Timing
The best time to plant depends on the method chosen: seeds, sets, or transplants. Starting from seed requires beginning indoors approximately 8 to 10 weeks before the expected outdoor transplanting date. This indoor start produces a pencil-sized seedling robust enough for cool early spring temperatures.
Transplants, which are seedlings already started by a nursery, should be moved outdoors approximately three to four weeks before the average last frost date. For many lower-elevation regions, this target date typically falls in mid-April. Planting transplants ensures they have a long period of vegetative growth before the summer solstice triggers bulbing.
Onion sets, which are small, marble-sized dormant bulbs, offer the easiest and earliest planting option outdoors. Sets can be planted as soon as the soil is workable and dry enough, often in mid-to-late March in the Denver metro area. Sets are very cold-tolerant, surviving hard freezes. The soil should be amended with organic material and loosened to a depth of eight inches to accommodate the onion’s shallow root system.
Essential Care and Harvesting
Consistent care is necessary to maximize bulb development during the growing season. Onions are heavy feeders and require a steady supply of nitrogen until bulbing is well underway, usually around mid-July. Applying a nitrogen-rich fertilizer every three to four weeks supports the vigorous green top growth essential for producing large bulbs.
The onion’s shallow root system means consistent soil moisture is crucial, especially during hot summer months. Ensure the soil never completely dries out, as water stress results in small, leathery bulbs. Weed control is equally important, but shallow cultivation is recommended to avoid damaging the delicate roots close to the soil surface.
The harvest window typically opens in late August and continues into September, signaled by the tops of the plants turning yellow and falling over. When 75 to 80 percent of the tops are on the ground, watering must cease immediately to initiate the final maturation and drying of the bulb. Harvested onions should be left to cure in a warm, dry, well-ventilated location out of direct sun until the necks are completely dry and the outer skins are papery.