Colorado · zones 5a–7a
When to plant onions in Colorado.
Sow or transplant between Mar 1 to Apr 14 in a typical year, keyed to Colorado's median last frost around May 9. A second, fall sowing fits Aug 4 to Sep 15. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Colorado season.
1 · Start indoors
Jan 18 to Mar 3
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Mar 1 to Apr 14
Keyed to the ~May 9 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 4 to Sep 15
Counted back from the ~Sep 29 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Colorado spans more than one climate: Zone 6 is the most common zone, but the state runs 5a to 7a. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Colorado growing notes
Colorado's dry air cures a better onion than most states can manage.
The sweet Spanish tradition fits this altitude
Home gardeners across the Rocky Mountain West have leaned on Yellow Sweet Spanish and Ailsa Craig for generations, and Colorado is why: both are bred to bulb reliably at the long summer days this latitude delivers, roughly 37 to 41 degrees north, while staying mild enough to eat fresh instead of only cooked. A mature Sweet Spanish bulb regularly tops a pound in a well-fed Front Range bed, well above what a typical home-garden onion manages.
Set transplants 4 inches apart in a bed that got 2 to 3 inches of compost worked in ahead of time, and feed nitrogen through the first half of the season while the plant is still building leaves. Every leaf the plant grows before the day-length trigger flips becomes one ring in the finished bulb, so a stalled, underfed onion in June simply cannot catch up later no matter how well it gets watered afterward.
Alkaline soil, thin air, and the thrips test
The same calcareous clay that challenges tomatoes suits onions just fine; they tolerate pH up to 8 without complaint. What Colorado's thin, dry air does instead is speed up thrips damage: silvering and streaking on the leaf blades can go from faint to severe in under a week during a hot, low-humidity stretch. Check 5 plants twice a week by holding a leaf over a white index card and flicking it; more than 2 or 3 moving specks per leaf calls for insecticidal soap that same afternoon.
Downy mildew, the fungus that punishes onions in humid states, actually struggles here, since the spores need free moisture sitting on the leaf to infect, and Colorado's air rarely holds it long enough to matter. That is a real advantage for this state's growers: watering at the base any time of day will not invite the disease that punishes overhead watering everywhere else.
Hail during bulbing, then a curing bonus
A hard June hailstorm can shred onion tops right when the plant is building the leaf count that determines final bulb size, and torn foliage cannot photosynthesize its way to a big onion. If hail is forecast during the bulbing weeks, a floating cover or even an overturned laundry basket set over each row for the storm's duration prevents the kind of leaf loss that shows up as a smaller crop in August.
Colorado's payoff comes at harvest: once tops fall over and bulbs are lifted, this state's low humidity and steady breeze cure onions outdoors in as little as 10 days, versus the 2 to 3 weeks growers budget for in muggier climates. Lay bulbs in a single layer out of direct sun with necks fully dry before storage, and a well-cured Sweet Spanish will hold 2 to 3 months in a cool room.
In Colorado: watch for
The pests that find onions here.
The short season here, about 143 frost-free days, keeps onion, bulb in front of onion thrips only briefly. One early row cover usually carries a planting through the worst of it.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: Colorado gives onions about 143 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Colorado: questions
When should I plant onions in Colorado?+
In a typical Colorado year, sow onions or set out transplants between Mar 1 to Apr 14, keyed to the state's median last frost around May 9. There's also a fall window: Aug 4 to Sep 15. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Colorado?+
Start seeds inside about 6 weeks before set-out: roughly Jan 18 to Mar 3 for a typical Colorado season.
What USDA zone is Colorado?+
Colorado spans zones 5a–7a, mostly zone 6. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.