Planting Guides
When to Plant Onions in Denver: Complete Guide + Best Varieties for Zone 5a
Denver, Colorado
USDA Zone 5a
Last Frost: May 10
Last updated: December 8, 2025
Learn when to plant onions in Denver with specific dates for Zone 5a. Compare 6 varieties perfect for Colorado's high altitude climate and 148-day growing season.
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Quick Answer
Plant onions in Denver April 11-25 using long-day varieties. Fall-plant garlic October 15-30.
TL;DR
Plant onions in Denver from April 11-25 for Zone 5a success. Choose long-day varieties like Allium cepa 'Red Wing' and Allium cepa 'Walla Walla' that need 14-16 hours daylight to bulb properly. Fall-plant hardneck garlic like Allium sativum 'Music' in mid-October. Denver's high altitude and 148-day growing season create ideal conditions for storage onions that cure well for winter use.
Frequently Asked Questions
When is the best time to plant onions in Denver?
Plant onions in Denver from April 11-25 for optimal results in Zone 5a. This timing allows 2-3 weeks buffer before the average last frost date of May 9, giving plants time to establish strong root systems during cool weather. Planting earlier than April 11 risks soil being too cold and wet, while planting after April 25 reduces the growing season and results in smaller bulbs.
What onion varieties work best in Denver's high altitude climate?
Long-day varieties only succeed in Denver's 40°N latitude location. Choose Allium cepa 'Red Wing' for excellent storage (8-10 months), Allium cepa 'Walla Walla' for sweet eating onions, or Allium cepa 'Copra' for maximum storage life (10-12 months). Short-day varieties will never bulb at this latitude, producing only green tops. The high altitude and intense UV actually improve storage onion quality by concentrating sulfur compounds.
Can I grow onions from seed in Denver?
Yes, but start seeds indoors February 15-March 1 for transplanting outside April 11-18. Seeds need 10-12 weeks to reach transplant size in Zone 5a's short growing season. Direct seeding outdoors isn't practical because the soil is too cold until late April, leaving insufficient time for bulb development before first frost October 4.
When should I plant garlic in Denver?
Plant hardneck garlic varieties like Allium sativum 'Music' October 15-30 in Denver. This timing allows 4-6 weeks for root establishment before ground freezes, providing the 12-16 weeks of cold vernalization required for proper clove development. Mulch heavily with 4-6 inches of straw before hard freeze to insulate soil and prevent frost heaving.
How do I cure onions in Denver's dry climate?
Denver's low humidity (25-40%) accelerates curing to 2-3 weeks instead of the typical 4 weeks. After harvest, cure in a shaded, well-ventilated area like a garage or shed. Lay in single layers on screens or hang in mesh bags. Check weekly by gently squeezing necks - properly cured onions have completely dry, wiry necks and papery outer skins. The dry climate actually improves storage life by preventing rot.
Why are my onions not forming bulbs in Denver?
Wrong variety selection is the most common cause. Denver's 40°N latitude requires long-day varieties that need 14-16 hours of daylight to trigger bulbing. Short-day or intermediate-day varieties will produce large green tops but never form bulbs at this latitude. Always choose varieties specifically labeled as long-day types like Red Wing, Copra, or Yellow Sweet Spanish for Denver success.
What's the difference between hardneck and softneck garlic for Denver?
Hardneck varieties like Allium sativum 'Music' are ideal for Denver's Zone 5a climate. They require cold vernalization that Denver's winters naturally provide, produce edible scapes in spring, and have superior cold hardiness. Softneck varieties can work but need extra winter protection and may not vernalize properly in milder winters. Hardneck produces fewer (4-8) but larger cloves compared to softneck's many small cloves.
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Nathan Brooks
Nathan is a garden designer in Denver who focuses on water-wise plantings adapted to Colorado's Front Range climate. He grew up in Nebraska, studied environmental science in college, and fell into garden design after spending summers working at a native plant nursery in Fort Collins. Denver's climate is genuinely challenging—low humidity, intense UV, alkaline clay soil, and wild temperature swings that can go from 70°F to snow in 24 hours. Nathan designs gardens that handle all of that without supplemental irrigation once established. He writes about xeric gardening, native plant selection for the Front Range, and the practical realities of high-altitude growing.
