Growing onions in Colorado starts with matching the variety to day length and the available growing season. Elevation and local weather affect planting dates, so use local guidance instead of one date for the whole state.
Choose the right onion type
Colorado State University guidance recommends long-day or day-neutral onions. A mixed assortment containing short-day types is not automatically suitable in its entirety. Read the variety information and choose the packets that fit your location.
Starting from seed also requires enough time to establish plants before bulbing. Check local seed-starting or transplant guidance for your elevation and growing season.
Prepare a sunny, well-drained bed
Use soil-test recommendations to guide fertility. Follow packet directions for sowing and final spacing. Keep competing weeds under control without cultivating so deeply that you injure the onion roots.
Keep watering during bulb growth
Do not stop watering when bulbs first begin to form. Onions need consistent moisture while they are actively growing and enlarging their bulbs. Maintain drainage and avoid swings between very dry and saturated soil.
Near maturity, the tops begin to fall naturally. That is the stage when watering is stopped before harvest according to Utah State University Extension guidance. Do not bend the tops over early to force maturity.
Harvest and cure
Harvest mature bulbs carefully and cure them in a dry, well-ventilated place. Separate damaged bulbs from those intended for storage. Storage life differs by variety, so a sweet onion and a storage onion should not be expected to keep equally long.
Use your seed packet together with regional Extension guidance. The combination helps you match the variety, planting schedule and watering to your own garden.
