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How to Plant Onions in Colorado

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Onions grow well in Colorado when planted in the spring season. By avoiding excessively cold temperatures, you can maintain a healthy crop of these popular vegetables. Due to the favorable conditions for growing this crop, fresh onions are available in Colorado between July and September and locally grown onions from fall and winter storage can be found between August and April. Whether you’re interested in green onions or dry onions, you can use some simple planting methods to obtain a healthy crop.

Mix organic material into the soil at least 8 inches deep if your soil is heavy in clay or sand. Till all soil to this depth to avoid excessive compaction.

Spread a layer of organic material on the surface of the soil to a depth of about 1 1/2 inches.

Plant onion seeds or transplanted onion plants between the beginning of March and the middle of April. Space the plants evenly throughout the garden, between 3 and 4 inches apart. Seeds should be 1/2 inch to 1 inch deep in the soil.

Water thoroughly after planting and continue frequent watering to keep soil moist. Plants should be watered at least every five to seven days throughout the growing season.

Spread a layer of mulch around the onion plants to prevent weeds.

Plant Onions From An Onion

The onion (Allium cepa, USDA zones 5 through 10), also known as the common onion or bulb onion, is a staple in many kitchens. If you're patient, you can grow onions from seeds; for a faster alternative, try growing them from onions you may already have on hand. These tiny bulbs are cultivated under controlled conditions, and you can use them to grow green onions or, later in the growing season, bulb onions. Add a 2-inch layer of compost if needed and till the soil to a depth of 8 to 10 inches. Where fertilizer is concerned, onions are greedy. Locate your onion bed in an area that receives full sun or at least six hours of direct sunlight per day. Wait four to six weeks before the last expected frost date to plant onion sets or grocery onions outside. Onion Sets: Create a shallow furrow in a prepared bed and plant onion sets, tips up, 4 to 6 inches apart or about 2 inches apart for green onions. Cover it with soil till only the very top where the leaves originate sticks out above the soil. Going forward, onions need about 1 inch of rainfall or irrigation per week. Bulb onions are ready to be pulled when their tops dry out and fall over, usually around mid- to late spring through early summer if planted earlier in the year. Harvest them carefully by gripping the tops and pulling gently.


Plant a cover crop in the area throughout the fall and winter to keep the soil from eroding.

Onion plants in Colorado can be successfully rotated with alfalfa, corn and other small grains.

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